3

GEOLOGICA BALCANICA 43 1 ndash 3 Sofia Dec 2014 p 3-26 Diagenesis of Paleogene sandstones in the Padesh strike-slip basin Southwestern Bulgaria Athanas Chatalov Yavor Stefanov Sofia University ldquoSt Kliment Ohridskirdquo 15 Tsar Osvoboditel Blvd 1504 Sofia e-mails chatalovgeauni-sofiabg iavorstefanovyahoocom (Accepted in revised form November 2014)

Abstract Sandstones are a major rock type in the 2500 m thick Palaeogene sedimentary fill of the Padesh strike-slip basin The main diagenetic alterations include mechanical compaction and carbonate cementation while dissolution transformation recrystallization replacement and chemical compaction were of lesser significance during burial These postdepositional processes and their products are discussed in terms of micropetrographic characteristics spatial distributional patterns mechanisms of formation controlling factors and temporal sequence The diagenetic changes are related to eodiagenesis mesodiagenesis and telodiagenesis but most processes occurred during the mesodiagenetic stage The sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of the associated shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime The most peculiar feature is recorded in basal sandstone strata of the Palaeogene succession which display evidence for minor selective dissolution and subsequent kaolinite precipitation in the produced secondary pores as a result of thermal maturation of organic matter in the adjacent black shales Several lines of evidence indicate a hyperthermal character of the Padesh Basin thus resembling other ldquohot-typerdquo sedimentary basins

The present study contributes to deciphering the diagenetic pathway of siliciclastic deposits in strike-slip basins characterized by elevated heat flow and can be applied for the evaluation of their hydrocarbon system potential It also supports previous findings that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Chatalov A Stefanov Y 2014 Diagenesis of Palaeogene sandstones in the Padesh strike-slip basin Southwestern Bulgaria Geologica Balcanica 43 (1 ndash 3) 3-26

Key words sandstones diagenesis strike-slip Padesh Basin Palaeogene SW Bulgaria INTRODUCTION

The spatial and temporal diagenetic alterations in siliciclastic sequences are controlled by several sedi-mentary and environmental parameters (Morad et al 2000 Morad et al 2010b Morad et al 2012) Progres-sive burial is accompanied by physicochemical changes as a result of temperature and pressure increase as well as pore fluid chemistry Thus during the postdepo-sitional history sandstones undergo modifications of their porosity mineralogy and chemical composition in various and sometimes complex diagenetic systems (Bjoslashrlykke 2014) Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs

(Morad et al 2010b Taylor et al 2010) and advances in understanding the timing and burial depths of diagenetic processes have led to substantial improvements in the prediction of sandstone reservoir quality (Ajdukiewicz Lander 2010 Morad et al 2012) Another aspect in the study of sandstone diagenesis is the relationship between the type of depositional basin and the diagenetic alterations of its siliciclastic sedimentary fill (Morad et al 2000 Morad et al 2010b) Hyperthermal basins and in particular some strike-slip basins are illustrative in this respect being characterized by elevated values of thermal flow that has direct influence on the postdepositional evolution during burial

4

In this article diagenetic changes of Palaeogene sandy sediments accumulated in the Padesh strike-slip basin are described and discussed The recognized post-depositional alterations are considered in the context of their micropetrographic characteristics spatial occur-rence controlling factors mechanisms of formation and temporal sequence The drawn conclusions are based on microscopical observations and complemen-tary geochemical data The new results are combined with those obtained from the study of associated shales (Stefanov Chatalov 2008) and coal (Shishkov et al 1989) aiming to elucidate the specific thermal evolution of Padesh Basin Reassessment of many fundamental facts and interpretations from a previous work (Stefanov 2011) is also made in the light of recent progress in the field of sandstone diagenesis and therefore a new synthesis of all data is presented here

Strike-slip basins are the most tectonically active type of sedimentary basins (Nilsen Sylvester 1995) They form in a wide variety of geodynamical settings and are characterized by specific basin geometries rapid sedimentation and subsidence rates and complex structural and burial history (Allen Allen 2013) Strike-slip basins can be divided into ldquohotrdquo and ldquocoldrdquo types based on whether the mantle has been involved in their formation (Nilsen Sylvester 1995) Both types have been extensively studied in terms of tectonics stra-tigraphy of the sedimentary fill provenance of the clastic material and depositional systems (Biddle Christie-Blick 1985 Ridgway DeCelles 1993 Nilsen Sylvester 1995 Crowell 2003 Wysocka Swierczew-ska 2003 Allen Allen 2013) However a comprehen-sive liteterature review reveals that only few researchers have focused on the diagenetic alterations of siliciclastic sediments (Welton Link 1982 Hyde et al 1988 Sachsenhofer et al 1998 Kim et al 2007 Gier et al 2008) Thus the present study can be an important contribution to unravelling the diagenetic pathway in strike-slip basins which provide almost ideal conditions for petroleum accumulation and are amongst the richest oil regions in many countries (Mann 2012)

GEOLOGICAL BACKGROUND

The Padesh Basin belongs to a group of sedimentary basins in Southwestern Bulgaria characterized by MiddlendashLate Eocene to Early Oligocene depositional evolution (Zagorchev 2001) The accumulated sediments have total thickness of about 2500 m (Stefanov et al 2007) They overlie unconformably the basement consisting of Precambrian Palaeozoic and Triassic rocks (Zagorchev 2001) while the 50-60 m thick Neogene cover comprises poorly consolidated siliciclastic deposits (Milovanov et al 2009) (Fig 1) The Padesh Basin was referred to as type area for the Upper Eoecene and Lower Oligocene in Southwestern Bulgaria on the basis of its complete sedimentary fill and good exposures (Zagorchev et al 1989) The basin features are well studied in terms of stratigraphy

tectonics petrology and mineral resources (see Stefanov 2011 and references therein)

According to Zagorchev (2001) the Padesh Basin represents a postdepositional graben delimited by two longitudinal normal fault zones which are complicated by normal and strike-slip faults with later manifestation An alternative origin was proposed by Ivanov (1998) who claimed that the basin acquired wedge-shaped configuration and asymmetric topography as an effect of movements along regional strike-slip faults Later on Kounov (2002) explained the formation of Late EocenendashEarly Oligocene basins in Southwestern Bulgaria with the extension evolutionary stage of OsogovondashLisets dome and the resulting peripheral detachment faults Several lines of sedimentological evidence unequivocally attest that the Padesh Basin is a typical example of strike-slip basin (Stefanov Chatalov 2008 Stefanov et al 2008 Stefanov 2011) and the same view has been recently adopted by Vangelova Vangelov (2013)

The Paleogene succession consists of predominant gravels and sandstones subordinate shales carbonates volcano-sedimentary and volcanic rocks and minor amount of coal (Belmustakov 1948 Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) The proposed litho-stratigraphic subdivision comprises four formations (Suhostrel Komatinitsa Logodazh and Padesh) and several formal units of lower rank (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989) Thus the Suhostrel Formation is subdivided into four members (Elska Pilyovo Solashka and Debochitsa) and the Padesh Formation includes three lithostratigraphic markers (Milina Tuffsandstone Ovnarska Limestone and Ravnashka Effusive) While the oldest deposits have been for long defined as Upper Eocene (eg Belmustakov 1948 Zagorchev Popov 1968) the chronostratigraphic range of Suhostrel Formation has recently been extended to the Lower and Middle Eocene (YpresianndashBartonian) by Vatsev et al (2003) and Vatsev et al (2011) on the basis of determined foraminiferal taxa (Fig 2) Published interpretations of the sedimentary facies depositional regime and climate conditions are controversial in many respects (Zagorchev Popov 1968 Zagorchev et al 1989 Zagorchev 2001 Vatsev et al 2003 Vatsev et al 2011 Vangelova Vangelov 2013) In a compre-hensive sedimentological study by Stefanov (2011) the following depositional environments were identified alluvial fan braided river lacustrine (oxic and anoxic) and shallow marine as well as formation of syntectonic violin breccia

PREVIOUS SEDIMENTOLOGICAL RESEARCH

Macroscopic descriptions of sedimentary and volcano-sedimentary rocks of the Padesh Basin were presented in several papers which treated mainly regional and

5

Fig 1 Geological map of the Padesh Basin with shown location of the studied Paleogene stratigraphic sections (after Stefanov et al 2007) Legend 1 ndash Quaternary sediments 2 ndash Neogene sediments 3 ndash Oligocene volcanic rocks 4 ndash Padesh Formation (PriabonianndashRupelian) 5 ndash Logodazh Formation (BartonianndashPriabonian) 6 ndash Komatinitsa Formation (Bartonian) 7-10 ndash Suhostrel Formation 7 ndash Debochitsa Member (Lutetian) 8 ndash Solashka Member (Lutetian) 9 ndash Pilyovo Member (Ypresian) 10 ndash Elska Member (Ypresian) 11 ndash Triassic sedimentary rocks 12 ndash Permian sedimentary rocks 13 ndash Struma Diorite Formation (Lower Palaeozoic) 14 ndash Frolosh Formation (VendianndashLower Palaeozoic) 15 ndash Precambrian metamorphic rocks 16 ndash faults а ndash proven b ndash supposed c ndash thrust stratigraphic problems (see Stefanov 2011 and references therein) Stefanov Chatalov (2007) interpreted the formation of Priabonian pisoids and ooids (Logodazh Formation) with Mg-calcite primary mineralogy in a brackish-water setting In two

subsequent papers Stefanov et al (2007 2008) discussed in detail the composition of clastic rocks and analyzed their provenance respectively The clay mineralogy of associated shales was also studied and linked with the sediment burial history (Stefanov

6

Fig 2 Summarized stratigraphic-lithologic log of the Palaeogene succession (after Stefanov et al 2007) with shown occurrence of some diagenetic products and outlined distributional trends of diagenetic processes The chronostratigraphic data are from Vatsev et al (2011) Chatalov 2008) Other results were obtained with respect to sedimentary structures microfacies of carbonate rocks depositional environments climate regime and basin type and moreover the main diagenetic alterations in sandstones shales and limestones were outlined (Stefanov 2011)

MATERIAL AND METHODS

The geographic location of studied stratigraphic sections in the Padesh Basin is shown in Figure 1 The distribution of collected samples from sandstones by lithostratigraphic units (Suhostrel Fm Logodazh Fm

4

and Padesh Fm) and classification groups was presented in a previous paper together with stratigraphic-lithologic logs of four Palaeogene sections (Stefanov et al 2007 Table 1 Figs 3 4 6)

Standard thin sections for observation with transmitted light microscopy were prepared from 87 samples Most of them were stained following the procedures proposed by Evamy (1963) and Dickson (1965) to identify the carbonate minerals forming cement matrix detrital grains and alteration products and to determine semiquantitatively the Fe2+ content The authigenic kaolinite recognized in thin sections was further examined by means of scanning electrone microscope JEOL JSM-5510 (Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) on fresh rock surfaces after vacuum coating with gold dust Microprobe chemical analyses of carbonate cements were performed using uncovered carbon-coated double polished thin sections The scanning electron microscope JEOL JSM 35 CF equipped with Tracor Northern TN-2000 X-ray microanalyzer (Eurotest Control JSC Sofia) was operating under the following conditions 20 kеV accelerating voltage 2times10-9 Aring beam current and 2 microm spot size The used JEOL standards were diopside for Mg and Ca Fe2O3 for Fe and MnO2 for Mn as the detection limit was 100 ppm

DIAGENETIC PROCESSES AND PRODUCTS

The Paleogene sandstones of Padesh Basin are defined as texturally immature to submature deposits having relatively low mineralogical maturity (Stefanov et al 2007 Fig 6) They belong to the groups of feldspathic arenites and lithic arenites following the classification scheme proposed by Folk еt al (1970) In this study the whole spectrum of diagenetic processes in sandstones is established ie mechanical compaction cementation dissolution transformation recrystallization replace-ment and chemical compaction Their products are described below as the respective mechanisms of formation and conditions of the postdepositional setting are discussed in detail The distributional trends of some diagenetic processes and the vertical occurrence of some diagenetic products are shown in a summary stratigraphic-lithologic log of the Palaeogene succession (Fig 2)

Mechanical compaction

This irreversible diagenetic process involves decrease of the bulk sediment volume by grain packing and porosity reduction in the subsurface as a result of effective stress ie the difference between lithostatic pressure and pore pressure Mechanical compaction is probably the dominant mechanism leading to porosity loss in sandstones (Lundegard 1992) Its magnitude during the postdepositional alteration of sediments is controlled by the composition size shape and sorting of the constituent grains and the burial history (Pittman Larese 1991 Lander Walderhang 1999 Paxton et al

2002 Chuhan et al 2003 Fawad et al 2011 Bjoslashrlykke 2014 Rossi Alaminos 2014) During progressive burial clastic sediments undergo initially rearrangement (slippage rotation) of the detrital grains which subsequently gives way to grain deformation eg bending flattening fracturing and breakage (Schmidt McDonald 1979) Intergranular volume (IGV) is an indicator of compaction measured as the sum () of intergranular porosity intergranular cement and depositional matrix (Paxton et al 2002)

The most common effects of mechanical compaction in the Palaeogene sandstones include bending of flexible mica flakes (Fig 3a) rearrangement of detrital grains in the rock texture resulting in increased number of long contacts (Figs 3b 6g Stefanov et al 2007 Plate VIc) and ductile deformation of carbonate intraclasts and lithics derived from softer rock types eg shales marlstones phyllites and various schists (Fig 3b Stefanov et al 2007 Plate VIc VIIa b) Brittle grain fracturing shows lesser occurrence (Fig 3c) being more typical for some rigid lithic fragments eg granitoids gneisses amphibolites sandstones and unstable mineral grains eg hornblende sphene pyroxene (Stefanov et al 2007 Plates VIId VIIIa c) Weak plastic deformation of elongated detrital feldspar grains is very rarely observed (Fig 3c)

The highest degree of mechanical compaction is recognized in matrix-free sandstones containing small amount of post-compaction carbonate cement (Fig 3a b 6a) ie these rocks have the lowest IGV value (lt15-20) Sandstones with clay andor carbonate matrix were also strongly compacted as the primary microporosity in the fine-grained groundmass was obliterated during burial (Smosna 1989) Less intensive compaction affected the rocks with relatively high amount of matrix (Fig 5e Stefanov et al 2007 Plates VId VIIe) compared to those with low matrix content (Figs 4d 6g Stefanov et al 2007 Plates IIIc IVd VIc) because in the former type the effective stress is distributed over a larger number of grain contacts resulting in a low overall compressibility (Mondol et al 2007) The lowest degree of mechanical compaction is observed in sandstones with pre-compaction carbonate cement which stabilized the rock texture in the early diagenesis preventing further compaction (Pittman Larese 1991) This is illustrated by the highest recorded IGV values (35-40 ) great number of floating grains and point contacts (Figs 3d f 4a-b f 5f) and well preserved ie undeformed ductile grains and skeletal fragments (Fig 4a Stefanov Chatalov 2007 Fig 4a c) In some arenites the presence of abundant mica flakes andor ductile lithics (especially derived from sedimentary and metamorphic rocks) squeezed between rigid grains promoted advanced compaction (Pittman Larese 1991 Smosna Bruner 1997 Worden et al 2000 Mansurbeg et al 2013 Rossi Alaminos 2014) locally resulting in the formation of pseudomatrix (Fig 3b Stefanov et al 2007 Plate VIc) Other depositional

7

8

Fig 3 a ndash Bended biotite and muscovite flakes between rigid detrital grains due to advanced mechanical compaction Suhostrel Formation b ndash Ferroan dolomite (blue) filling intergranular pores and locally replacing (red arrows) lithics derived from marlstones and limestones The post-compaction origin of the dolomite cement is emphasized by its patchy distribution in the rock texture the great number of long contacts and the presence of ductily deformed grains forming pseudomatrix Note pressure-dissolution contacts between some quartz grains (green arrows) Thin-section stained after Evamy (1963) Suhostrel Formation c ndash Rare cases of plastically deformed detrital feldspar (fs) and brittle grain fracturing (arrow) as a result of mechanical compaction Logodazh Formation d ndash Strongly displaced framework grains by poikilotopic calcite cement suggesting early phreatic carbonate precipitation Logodazh Formation e ndash Displacive effect of eodiagenetic calcite cement recognized by pulling apart of two rigid formerly fractured detrital grains (asterisks) Logodazh Formation f ndash Abundant blocky calcite with planar crystal boundaries formed by pre-compaction cementation Logodazh Formation Note All microphotographs except b (in plane-polarized light) are in cross-polarized light

characteristics of the sandstones also controlled the process of mechanical compaction for example the predominantly poor sorting angular morphology and

coarse size of the sand grains favoured more intensive compaction (Chuhan et al 2003 Fawad et al 2011 Rossi Alaminos 2014)

4

In terms of vertical distribution in the Palaeogene succession mechanical compaction was conspicuously more intensive in the rocks of Suhostrel Formation as a result of their greater burial depth and moreover shows some trend of increasing degree downsection (Fig 2) In comparison the sandstones of Logodazh Formation and Padesh Formation were less affected by mechanical compaction which is related to their shallow burial depth and the locally abundant precipitation of early carbonate cements (see below)

Cementation

Precipitation of mineral cements takes place as supersaturated pore fluids flow from areas of higher solubility to areas of lower solubility due to variations in pH pore water composition andor temperature (Bjoslashrlykke 1988) Cementation plays a major role in reducing the porosity in sandstones but unlike compaction it can be a reversible diagenetic process because some cement phases may dissolve during burial Three types of intergranular intragranular and fracture cements by composition (calcite ferroan dolomite and kaolinite) were identified in the sandstones of Padesh Basin (Stefanov et al 2007) Among them calcite has much broader distribution and greater abundance in the Paleogene section while minor amounts of dolomite and kaolinite show only local occurrence (Fig 2)

Calcite

Calcite cement in sandstones is formed over the entire diagenetic realm from near-surface conditions to deep burial ie showing neither rigorous temperature dependence nor strong spatial localization (Morad 1998 Milliken 2005) This broad distributional pattern is well illustrated by the vertical occurrence of intergranular and fracture calcite cements in sandstones of the studied Palaeogene succession Both textural types display medium to coarse crystalline size indicating relatively slow growth controlled by the flow rate of the pore fluids and the permeability of the sediments With respect to crystal morphology and distribution in the rock texture the intergranular cement has invariably blocky pattern and occasionally poikilotopic appearance (Figs 3d f 4a-b 5c e) This calcite shows low or slightly elevated Fe2+ contents (Table 1) that might be attributed to variable redox potential of the diagenetic milieu (ie oxic and suboxic zones) and amount of dissolved Fe in the circulating fluids Such conclusion is supported by a lacking trend in the vertical distribution of ferroan and non-ferroan calcite cements in the section Moreover the absence of zoned crystals (Figs 4a-b 5f) reflects precipitation from mineral-forming solutions with homogeneous composition

The sandstones of Logodazh Formation and Padesh Formation contain pre-compaction calcite cement Its eodiagenetic origin ie related to shallow burial of the sediments is attested by the low degree of

mechanical compaction with high IGV values reaching 35-40 (Figs 3d-f 4a-b 5f) The loose packing of framework grains is reflected by a great number of point contacts andor floating distribution in the rock texture This fabric pattern is enhanced by the displacive effect of calcite cementation that also resulted in expansion of mica flakes and pulling apart of rigid formerly fractured detrital grains (Fig 3e) while ductile lithics and mica flakes remained undeformed (Fig 4a) Similar microfabric characteristics are common in pedogenic calcretes and dolocretes (Richter 1985 Braithwaite 1989 Chatalov 2006 Armenteros 2010) but have been reported in sandstones unaffected by vadose diagenesis (Burley Kantorowicz 1986 Buczynski Chafetz 1987 Garcia et al 1998 Morad et al 1998 Anjos et al 2000) The displacive growth requires supersaturated solutions with respect to calcite and differential stress caused by the mineral growth (Armenteros 2010) The relatively rapid pre-compaction cementation was probably favoured by the predominantly angular morphology and low sphericity of the detrital grains (Heald Renton 1966)

The sandstones of Suhostrel Formation were only locally cemented by intergranular carbonate and micropetrographic observations reveal the sole presence of post-compaction calcite The latter has definitely mesodiagenetic origin ie related to deep burial of the sediments which is recognized by the high degree of mechanical compaction with low IGV values of 15-20 or less The close packing of framework grains is reflected by scarce point contacts opposed to a great number of long contacts and additional evidence for chemical compaction (Figs 5b 6a Stefanov et al 2007 Plates IVa VIa e VIIc) No expansion of mica flakes andor displacement of rigid grains are observed while the ductile grains are clearly deformed (Fig 3a) The anomalously high IGV values in some sandstones are due to the advanced replacement of framework grains by calcite (Fig 5c) or the presence of associated blocky cement and syntaxial overgrowths around detrital carbonate grains in calcilithites (Fig 4c)

Calcite-filled fractures occur exclusively in the sandstones of Suhostrel Formation The veins are of the syntaxial type (Bons et al 2012) and intersect all other textural elements indicating post-compaction origin during deep burial Three types of blocky calcite are distinguished with respect to Fe-Mn contents as neither of them displays zoned crystal pattern Most common are weakly ferroan (Fig 4d) and ferroan cements which reflect variable presence of dissolved Fe in the circulating fluids The third type of calcite showing pale brown colour in plane light is observed in one sample (Fig 4e) and the chemical analysis recorded slightly elevated contents of both Mn and Fe with the MnFe ratio varying from 21 to 31 (Table 1)

The interpretation of carbonate cements in sandstones should focus on the nature of precipitating fluids source of ions temperature range timing and diagenetic setting Solving these problems requires

9

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

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Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

5

Fig 1 Geological map of the Padesh Basin with shown location of the studied Paleogene stratigraphic sections (after Stefanov et al 2007) Legend 1 ndash Quaternary sediments 2 ndash Neogene sediments 3 ndash Oligocene volcanic rocks 4 ndash Padesh Formation (PriabonianndashRupelian) 5 ndash Logodazh Formation (BartonianndashPriabonian) 6 ndash Komatinitsa Formation (Bartonian) 7-10 ndash Suhostrel Formation 7 ndash Debochitsa Member (Lutetian) 8 ndash Solashka Member (Lutetian) 9 ndash Pilyovo Member (Ypresian) 10 ndash Elska Member (Ypresian) 11 ndash Triassic sedimentary rocks 12 ndash Permian sedimentary rocks 13 ndash Struma Diorite Formation (Lower Palaeozoic) 14 ndash Frolosh Formation (VendianndashLower Palaeozoic) 15 ndash Precambrian metamorphic rocks 16 ndash faults а ndash proven b ndash supposed c ndash thrust stratigraphic problems (see Stefanov 2011 and references therein) Stefanov Chatalov (2007) interpreted the formation of Priabonian pisoids and ooids (Logodazh Formation) with Mg-calcite primary mineralogy in a brackish-water setting In two

subsequent papers Stefanov et al (2007 2008) discussed in detail the composition of clastic rocks and analyzed their provenance respectively The clay mineralogy of associated shales was also studied and linked with the sediment burial history (Stefanov

6

Fig 2 Summarized stratigraphic-lithologic log of the Palaeogene succession (after Stefanov et al 2007) with shown occurrence of some diagenetic products and outlined distributional trends of diagenetic processes The chronostratigraphic data are from Vatsev et al (2011) Chatalov 2008) Other results were obtained with respect to sedimentary structures microfacies of carbonate rocks depositional environments climate regime and basin type and moreover the main diagenetic alterations in sandstones shales and limestones were outlined (Stefanov 2011)

MATERIAL AND METHODS

The geographic location of studied stratigraphic sections in the Padesh Basin is shown in Figure 1 The distribution of collected samples from sandstones by lithostratigraphic units (Suhostrel Fm Logodazh Fm

4

and Padesh Fm) and classification groups was presented in a previous paper together with stratigraphic-lithologic logs of four Palaeogene sections (Stefanov et al 2007 Table 1 Figs 3 4 6)

Standard thin sections for observation with transmitted light microscopy were prepared from 87 samples Most of them were stained following the procedures proposed by Evamy (1963) and Dickson (1965) to identify the carbonate minerals forming cement matrix detrital grains and alteration products and to determine semiquantitatively the Fe2+ content The authigenic kaolinite recognized in thin sections was further examined by means of scanning electrone microscope JEOL JSM-5510 (Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) on fresh rock surfaces after vacuum coating with gold dust Microprobe chemical analyses of carbonate cements were performed using uncovered carbon-coated double polished thin sections The scanning electron microscope JEOL JSM 35 CF equipped with Tracor Northern TN-2000 X-ray microanalyzer (Eurotest Control JSC Sofia) was operating under the following conditions 20 kеV accelerating voltage 2times10-9 Aring beam current and 2 microm spot size The used JEOL standards were diopside for Mg and Ca Fe2O3 for Fe and MnO2 for Mn as the detection limit was 100 ppm

DIAGENETIC PROCESSES AND PRODUCTS

The Paleogene sandstones of Padesh Basin are defined as texturally immature to submature deposits having relatively low mineralogical maturity (Stefanov et al 2007 Fig 6) They belong to the groups of feldspathic arenites and lithic arenites following the classification scheme proposed by Folk еt al (1970) In this study the whole spectrum of diagenetic processes in sandstones is established ie mechanical compaction cementation dissolution transformation recrystallization replace-ment and chemical compaction Their products are described below as the respective mechanisms of formation and conditions of the postdepositional setting are discussed in detail The distributional trends of some diagenetic processes and the vertical occurrence of some diagenetic products are shown in a summary stratigraphic-lithologic log of the Palaeogene succession (Fig 2)

Mechanical compaction

This irreversible diagenetic process involves decrease of the bulk sediment volume by grain packing and porosity reduction in the subsurface as a result of effective stress ie the difference between lithostatic pressure and pore pressure Mechanical compaction is probably the dominant mechanism leading to porosity loss in sandstones (Lundegard 1992) Its magnitude during the postdepositional alteration of sediments is controlled by the composition size shape and sorting of the constituent grains and the burial history (Pittman Larese 1991 Lander Walderhang 1999 Paxton et al

2002 Chuhan et al 2003 Fawad et al 2011 Bjoslashrlykke 2014 Rossi Alaminos 2014) During progressive burial clastic sediments undergo initially rearrangement (slippage rotation) of the detrital grains which subsequently gives way to grain deformation eg bending flattening fracturing and breakage (Schmidt McDonald 1979) Intergranular volume (IGV) is an indicator of compaction measured as the sum () of intergranular porosity intergranular cement and depositional matrix (Paxton et al 2002)

The most common effects of mechanical compaction in the Palaeogene sandstones include bending of flexible mica flakes (Fig 3a) rearrangement of detrital grains in the rock texture resulting in increased number of long contacts (Figs 3b 6g Stefanov et al 2007 Plate VIc) and ductile deformation of carbonate intraclasts and lithics derived from softer rock types eg shales marlstones phyllites and various schists (Fig 3b Stefanov et al 2007 Plate VIc VIIa b) Brittle grain fracturing shows lesser occurrence (Fig 3c) being more typical for some rigid lithic fragments eg granitoids gneisses amphibolites sandstones and unstable mineral grains eg hornblende sphene pyroxene (Stefanov et al 2007 Plates VIId VIIIa c) Weak plastic deformation of elongated detrital feldspar grains is very rarely observed (Fig 3c)

The highest degree of mechanical compaction is recognized in matrix-free sandstones containing small amount of post-compaction carbonate cement (Fig 3a b 6a) ie these rocks have the lowest IGV value (lt15-20) Sandstones with clay andor carbonate matrix were also strongly compacted as the primary microporosity in the fine-grained groundmass was obliterated during burial (Smosna 1989) Less intensive compaction affected the rocks with relatively high amount of matrix (Fig 5e Stefanov et al 2007 Plates VId VIIe) compared to those with low matrix content (Figs 4d 6g Stefanov et al 2007 Plates IIIc IVd VIc) because in the former type the effective stress is distributed over a larger number of grain contacts resulting in a low overall compressibility (Mondol et al 2007) The lowest degree of mechanical compaction is observed in sandstones with pre-compaction carbonate cement which stabilized the rock texture in the early diagenesis preventing further compaction (Pittman Larese 1991) This is illustrated by the highest recorded IGV values (35-40 ) great number of floating grains and point contacts (Figs 3d f 4a-b f 5f) and well preserved ie undeformed ductile grains and skeletal fragments (Fig 4a Stefanov Chatalov 2007 Fig 4a c) In some arenites the presence of abundant mica flakes andor ductile lithics (especially derived from sedimentary and metamorphic rocks) squeezed between rigid grains promoted advanced compaction (Pittman Larese 1991 Smosna Bruner 1997 Worden et al 2000 Mansurbeg et al 2013 Rossi Alaminos 2014) locally resulting in the formation of pseudomatrix (Fig 3b Stefanov et al 2007 Plate VIc) Other depositional

7

8

Fig 3 a ndash Bended biotite and muscovite flakes between rigid detrital grains due to advanced mechanical compaction Suhostrel Formation b ndash Ferroan dolomite (blue) filling intergranular pores and locally replacing (red arrows) lithics derived from marlstones and limestones The post-compaction origin of the dolomite cement is emphasized by its patchy distribution in the rock texture the great number of long contacts and the presence of ductily deformed grains forming pseudomatrix Note pressure-dissolution contacts between some quartz grains (green arrows) Thin-section stained after Evamy (1963) Suhostrel Formation c ndash Rare cases of plastically deformed detrital feldspar (fs) and brittle grain fracturing (arrow) as a result of mechanical compaction Logodazh Formation d ndash Strongly displaced framework grains by poikilotopic calcite cement suggesting early phreatic carbonate precipitation Logodazh Formation e ndash Displacive effect of eodiagenetic calcite cement recognized by pulling apart of two rigid formerly fractured detrital grains (asterisks) Logodazh Formation f ndash Abundant blocky calcite with planar crystal boundaries formed by pre-compaction cementation Logodazh Formation Note All microphotographs except b (in plane-polarized light) are in cross-polarized light

characteristics of the sandstones also controlled the process of mechanical compaction for example the predominantly poor sorting angular morphology and

coarse size of the sand grains favoured more intensive compaction (Chuhan et al 2003 Fawad et al 2011 Rossi Alaminos 2014)

4

In terms of vertical distribution in the Palaeogene succession mechanical compaction was conspicuously more intensive in the rocks of Suhostrel Formation as a result of their greater burial depth and moreover shows some trend of increasing degree downsection (Fig 2) In comparison the sandstones of Logodazh Formation and Padesh Formation were less affected by mechanical compaction which is related to their shallow burial depth and the locally abundant precipitation of early carbonate cements (see below)

Cementation

Precipitation of mineral cements takes place as supersaturated pore fluids flow from areas of higher solubility to areas of lower solubility due to variations in pH pore water composition andor temperature (Bjoslashrlykke 1988) Cementation plays a major role in reducing the porosity in sandstones but unlike compaction it can be a reversible diagenetic process because some cement phases may dissolve during burial Three types of intergranular intragranular and fracture cements by composition (calcite ferroan dolomite and kaolinite) were identified in the sandstones of Padesh Basin (Stefanov et al 2007) Among them calcite has much broader distribution and greater abundance in the Paleogene section while minor amounts of dolomite and kaolinite show only local occurrence (Fig 2)

Calcite

Calcite cement in sandstones is formed over the entire diagenetic realm from near-surface conditions to deep burial ie showing neither rigorous temperature dependence nor strong spatial localization (Morad 1998 Milliken 2005) This broad distributional pattern is well illustrated by the vertical occurrence of intergranular and fracture calcite cements in sandstones of the studied Palaeogene succession Both textural types display medium to coarse crystalline size indicating relatively slow growth controlled by the flow rate of the pore fluids and the permeability of the sediments With respect to crystal morphology and distribution in the rock texture the intergranular cement has invariably blocky pattern and occasionally poikilotopic appearance (Figs 3d f 4a-b 5c e) This calcite shows low or slightly elevated Fe2+ contents (Table 1) that might be attributed to variable redox potential of the diagenetic milieu (ie oxic and suboxic zones) and amount of dissolved Fe in the circulating fluids Such conclusion is supported by a lacking trend in the vertical distribution of ferroan and non-ferroan calcite cements in the section Moreover the absence of zoned crystals (Figs 4a-b 5f) reflects precipitation from mineral-forming solutions with homogeneous composition

The sandstones of Logodazh Formation and Padesh Formation contain pre-compaction calcite cement Its eodiagenetic origin ie related to shallow burial of the sediments is attested by the low degree of

mechanical compaction with high IGV values reaching 35-40 (Figs 3d-f 4a-b 5f) The loose packing of framework grains is reflected by a great number of point contacts andor floating distribution in the rock texture This fabric pattern is enhanced by the displacive effect of calcite cementation that also resulted in expansion of mica flakes and pulling apart of rigid formerly fractured detrital grains (Fig 3e) while ductile lithics and mica flakes remained undeformed (Fig 4a) Similar microfabric characteristics are common in pedogenic calcretes and dolocretes (Richter 1985 Braithwaite 1989 Chatalov 2006 Armenteros 2010) but have been reported in sandstones unaffected by vadose diagenesis (Burley Kantorowicz 1986 Buczynski Chafetz 1987 Garcia et al 1998 Morad et al 1998 Anjos et al 2000) The displacive growth requires supersaturated solutions with respect to calcite and differential stress caused by the mineral growth (Armenteros 2010) The relatively rapid pre-compaction cementation was probably favoured by the predominantly angular morphology and low sphericity of the detrital grains (Heald Renton 1966)

The sandstones of Suhostrel Formation were only locally cemented by intergranular carbonate and micropetrographic observations reveal the sole presence of post-compaction calcite The latter has definitely mesodiagenetic origin ie related to deep burial of the sediments which is recognized by the high degree of mechanical compaction with low IGV values of 15-20 or less The close packing of framework grains is reflected by scarce point contacts opposed to a great number of long contacts and additional evidence for chemical compaction (Figs 5b 6a Stefanov et al 2007 Plates IVa VIa e VIIc) No expansion of mica flakes andor displacement of rigid grains are observed while the ductile grains are clearly deformed (Fig 3a) The anomalously high IGV values in some sandstones are due to the advanced replacement of framework grains by calcite (Fig 5c) or the presence of associated blocky cement and syntaxial overgrowths around detrital carbonate grains in calcilithites (Fig 4c)

Calcite-filled fractures occur exclusively in the sandstones of Suhostrel Formation The veins are of the syntaxial type (Bons et al 2012) and intersect all other textural elements indicating post-compaction origin during deep burial Three types of blocky calcite are distinguished with respect to Fe-Mn contents as neither of them displays zoned crystal pattern Most common are weakly ferroan (Fig 4d) and ferroan cements which reflect variable presence of dissolved Fe in the circulating fluids The third type of calcite showing pale brown colour in plane light is observed in one sample (Fig 4e) and the chemical analysis recorded slightly elevated contents of both Mn and Fe with the MnFe ratio varying from 21 to 31 (Table 1)

The interpretation of carbonate cements in sandstones should focus on the nature of precipitating fluids source of ions temperature range timing and diagenetic setting Solving these problems requires

9

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

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Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

6

Fig 2 Summarized stratigraphic-lithologic log of the Palaeogene succession (after Stefanov et al 2007) with shown occurrence of some diagenetic products and outlined distributional trends of diagenetic processes The chronostratigraphic data are from Vatsev et al (2011) Chatalov 2008) Other results were obtained with respect to sedimentary structures microfacies of carbonate rocks depositional environments climate regime and basin type and moreover the main diagenetic alterations in sandstones shales and limestones were outlined (Stefanov 2011)

MATERIAL AND METHODS

The geographic location of studied stratigraphic sections in the Padesh Basin is shown in Figure 1 The distribution of collected samples from sandstones by lithostratigraphic units (Suhostrel Fm Logodazh Fm

4

and Padesh Fm) and classification groups was presented in a previous paper together with stratigraphic-lithologic logs of four Palaeogene sections (Stefanov et al 2007 Table 1 Figs 3 4 6)

Standard thin sections for observation with transmitted light microscopy were prepared from 87 samples Most of them were stained following the procedures proposed by Evamy (1963) and Dickson (1965) to identify the carbonate minerals forming cement matrix detrital grains and alteration products and to determine semiquantitatively the Fe2+ content The authigenic kaolinite recognized in thin sections was further examined by means of scanning electrone microscope JEOL JSM-5510 (Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) on fresh rock surfaces after vacuum coating with gold dust Microprobe chemical analyses of carbonate cements were performed using uncovered carbon-coated double polished thin sections The scanning electron microscope JEOL JSM 35 CF equipped with Tracor Northern TN-2000 X-ray microanalyzer (Eurotest Control JSC Sofia) was operating under the following conditions 20 kеV accelerating voltage 2times10-9 Aring beam current and 2 microm spot size The used JEOL standards were diopside for Mg and Ca Fe2O3 for Fe and MnO2 for Mn as the detection limit was 100 ppm

DIAGENETIC PROCESSES AND PRODUCTS

The Paleogene sandstones of Padesh Basin are defined as texturally immature to submature deposits having relatively low mineralogical maturity (Stefanov et al 2007 Fig 6) They belong to the groups of feldspathic arenites and lithic arenites following the classification scheme proposed by Folk еt al (1970) In this study the whole spectrum of diagenetic processes in sandstones is established ie mechanical compaction cementation dissolution transformation recrystallization replace-ment and chemical compaction Their products are described below as the respective mechanisms of formation and conditions of the postdepositional setting are discussed in detail The distributional trends of some diagenetic processes and the vertical occurrence of some diagenetic products are shown in a summary stratigraphic-lithologic log of the Palaeogene succession (Fig 2)

Mechanical compaction

This irreversible diagenetic process involves decrease of the bulk sediment volume by grain packing and porosity reduction in the subsurface as a result of effective stress ie the difference between lithostatic pressure and pore pressure Mechanical compaction is probably the dominant mechanism leading to porosity loss in sandstones (Lundegard 1992) Its magnitude during the postdepositional alteration of sediments is controlled by the composition size shape and sorting of the constituent grains and the burial history (Pittman Larese 1991 Lander Walderhang 1999 Paxton et al

2002 Chuhan et al 2003 Fawad et al 2011 Bjoslashrlykke 2014 Rossi Alaminos 2014) During progressive burial clastic sediments undergo initially rearrangement (slippage rotation) of the detrital grains which subsequently gives way to grain deformation eg bending flattening fracturing and breakage (Schmidt McDonald 1979) Intergranular volume (IGV) is an indicator of compaction measured as the sum () of intergranular porosity intergranular cement and depositional matrix (Paxton et al 2002)

The most common effects of mechanical compaction in the Palaeogene sandstones include bending of flexible mica flakes (Fig 3a) rearrangement of detrital grains in the rock texture resulting in increased number of long contacts (Figs 3b 6g Stefanov et al 2007 Plate VIc) and ductile deformation of carbonate intraclasts and lithics derived from softer rock types eg shales marlstones phyllites and various schists (Fig 3b Stefanov et al 2007 Plate VIc VIIa b) Brittle grain fracturing shows lesser occurrence (Fig 3c) being more typical for some rigid lithic fragments eg granitoids gneisses amphibolites sandstones and unstable mineral grains eg hornblende sphene pyroxene (Stefanov et al 2007 Plates VIId VIIIa c) Weak plastic deformation of elongated detrital feldspar grains is very rarely observed (Fig 3c)

The highest degree of mechanical compaction is recognized in matrix-free sandstones containing small amount of post-compaction carbonate cement (Fig 3a b 6a) ie these rocks have the lowest IGV value (lt15-20) Sandstones with clay andor carbonate matrix were also strongly compacted as the primary microporosity in the fine-grained groundmass was obliterated during burial (Smosna 1989) Less intensive compaction affected the rocks with relatively high amount of matrix (Fig 5e Stefanov et al 2007 Plates VId VIIe) compared to those with low matrix content (Figs 4d 6g Stefanov et al 2007 Plates IIIc IVd VIc) because in the former type the effective stress is distributed over a larger number of grain contacts resulting in a low overall compressibility (Mondol et al 2007) The lowest degree of mechanical compaction is observed in sandstones with pre-compaction carbonate cement which stabilized the rock texture in the early diagenesis preventing further compaction (Pittman Larese 1991) This is illustrated by the highest recorded IGV values (35-40 ) great number of floating grains and point contacts (Figs 3d f 4a-b f 5f) and well preserved ie undeformed ductile grains and skeletal fragments (Fig 4a Stefanov Chatalov 2007 Fig 4a c) In some arenites the presence of abundant mica flakes andor ductile lithics (especially derived from sedimentary and metamorphic rocks) squeezed between rigid grains promoted advanced compaction (Pittman Larese 1991 Smosna Bruner 1997 Worden et al 2000 Mansurbeg et al 2013 Rossi Alaminos 2014) locally resulting in the formation of pseudomatrix (Fig 3b Stefanov et al 2007 Plate VIc) Other depositional

7

8

Fig 3 a ndash Bended biotite and muscovite flakes between rigid detrital grains due to advanced mechanical compaction Suhostrel Formation b ndash Ferroan dolomite (blue) filling intergranular pores and locally replacing (red arrows) lithics derived from marlstones and limestones The post-compaction origin of the dolomite cement is emphasized by its patchy distribution in the rock texture the great number of long contacts and the presence of ductily deformed grains forming pseudomatrix Note pressure-dissolution contacts between some quartz grains (green arrows) Thin-section stained after Evamy (1963) Suhostrel Formation c ndash Rare cases of plastically deformed detrital feldspar (fs) and brittle grain fracturing (arrow) as a result of mechanical compaction Logodazh Formation d ndash Strongly displaced framework grains by poikilotopic calcite cement suggesting early phreatic carbonate precipitation Logodazh Formation e ndash Displacive effect of eodiagenetic calcite cement recognized by pulling apart of two rigid formerly fractured detrital grains (asterisks) Logodazh Formation f ndash Abundant blocky calcite with planar crystal boundaries formed by pre-compaction cementation Logodazh Formation Note All microphotographs except b (in plane-polarized light) are in cross-polarized light

characteristics of the sandstones also controlled the process of mechanical compaction for example the predominantly poor sorting angular morphology and

coarse size of the sand grains favoured more intensive compaction (Chuhan et al 2003 Fawad et al 2011 Rossi Alaminos 2014)

4

In terms of vertical distribution in the Palaeogene succession mechanical compaction was conspicuously more intensive in the rocks of Suhostrel Formation as a result of their greater burial depth and moreover shows some trend of increasing degree downsection (Fig 2) In comparison the sandstones of Logodazh Formation and Padesh Formation were less affected by mechanical compaction which is related to their shallow burial depth and the locally abundant precipitation of early carbonate cements (see below)

Cementation

Precipitation of mineral cements takes place as supersaturated pore fluids flow from areas of higher solubility to areas of lower solubility due to variations in pH pore water composition andor temperature (Bjoslashrlykke 1988) Cementation plays a major role in reducing the porosity in sandstones but unlike compaction it can be a reversible diagenetic process because some cement phases may dissolve during burial Three types of intergranular intragranular and fracture cements by composition (calcite ferroan dolomite and kaolinite) were identified in the sandstones of Padesh Basin (Stefanov et al 2007) Among them calcite has much broader distribution and greater abundance in the Paleogene section while minor amounts of dolomite and kaolinite show only local occurrence (Fig 2)

Calcite

Calcite cement in sandstones is formed over the entire diagenetic realm from near-surface conditions to deep burial ie showing neither rigorous temperature dependence nor strong spatial localization (Morad 1998 Milliken 2005) This broad distributional pattern is well illustrated by the vertical occurrence of intergranular and fracture calcite cements in sandstones of the studied Palaeogene succession Both textural types display medium to coarse crystalline size indicating relatively slow growth controlled by the flow rate of the pore fluids and the permeability of the sediments With respect to crystal morphology and distribution in the rock texture the intergranular cement has invariably blocky pattern and occasionally poikilotopic appearance (Figs 3d f 4a-b 5c e) This calcite shows low or slightly elevated Fe2+ contents (Table 1) that might be attributed to variable redox potential of the diagenetic milieu (ie oxic and suboxic zones) and amount of dissolved Fe in the circulating fluids Such conclusion is supported by a lacking trend in the vertical distribution of ferroan and non-ferroan calcite cements in the section Moreover the absence of zoned crystals (Figs 4a-b 5f) reflects precipitation from mineral-forming solutions with homogeneous composition

The sandstones of Logodazh Formation and Padesh Formation contain pre-compaction calcite cement Its eodiagenetic origin ie related to shallow burial of the sediments is attested by the low degree of

mechanical compaction with high IGV values reaching 35-40 (Figs 3d-f 4a-b 5f) The loose packing of framework grains is reflected by a great number of point contacts andor floating distribution in the rock texture This fabric pattern is enhanced by the displacive effect of calcite cementation that also resulted in expansion of mica flakes and pulling apart of rigid formerly fractured detrital grains (Fig 3e) while ductile lithics and mica flakes remained undeformed (Fig 4a) Similar microfabric characteristics are common in pedogenic calcretes and dolocretes (Richter 1985 Braithwaite 1989 Chatalov 2006 Armenteros 2010) but have been reported in sandstones unaffected by vadose diagenesis (Burley Kantorowicz 1986 Buczynski Chafetz 1987 Garcia et al 1998 Morad et al 1998 Anjos et al 2000) The displacive growth requires supersaturated solutions with respect to calcite and differential stress caused by the mineral growth (Armenteros 2010) The relatively rapid pre-compaction cementation was probably favoured by the predominantly angular morphology and low sphericity of the detrital grains (Heald Renton 1966)

The sandstones of Suhostrel Formation were only locally cemented by intergranular carbonate and micropetrographic observations reveal the sole presence of post-compaction calcite The latter has definitely mesodiagenetic origin ie related to deep burial of the sediments which is recognized by the high degree of mechanical compaction with low IGV values of 15-20 or less The close packing of framework grains is reflected by scarce point contacts opposed to a great number of long contacts and additional evidence for chemical compaction (Figs 5b 6a Stefanov et al 2007 Plates IVa VIa e VIIc) No expansion of mica flakes andor displacement of rigid grains are observed while the ductile grains are clearly deformed (Fig 3a) The anomalously high IGV values in some sandstones are due to the advanced replacement of framework grains by calcite (Fig 5c) or the presence of associated blocky cement and syntaxial overgrowths around detrital carbonate grains in calcilithites (Fig 4c)

Calcite-filled fractures occur exclusively in the sandstones of Suhostrel Formation The veins are of the syntaxial type (Bons et al 2012) and intersect all other textural elements indicating post-compaction origin during deep burial Three types of blocky calcite are distinguished with respect to Fe-Mn contents as neither of them displays zoned crystal pattern Most common are weakly ferroan (Fig 4d) and ferroan cements which reflect variable presence of dissolved Fe in the circulating fluids The third type of calcite showing pale brown colour in plane light is observed in one sample (Fig 4e) and the chemical analysis recorded slightly elevated contents of both Mn and Fe with the MnFe ratio varying from 21 to 31 (Table 1)

The interpretation of carbonate cements in sandstones should focus on the nature of precipitating fluids source of ions temperature range timing and diagenetic setting Solving these problems requires

9

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

4

and Padesh Fm) and classification groups was presented in a previous paper together with stratigraphic-lithologic logs of four Palaeogene sections (Stefanov et al 2007 Table 1 Figs 3 4 6)

Standard thin sections for observation with transmitted light microscopy were prepared from 87 samples Most of them were stained following the procedures proposed by Evamy (1963) and Dickson (1965) to identify the carbonate minerals forming cement matrix detrital grains and alteration products and to determine semiquantitatively the Fe2+ content The authigenic kaolinite recognized in thin sections was further examined by means of scanning electrone microscope JEOL JSM-5510 (Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) on fresh rock surfaces after vacuum coating with gold dust Microprobe chemical analyses of carbonate cements were performed using uncovered carbon-coated double polished thin sections The scanning electron microscope JEOL JSM 35 CF equipped with Tracor Northern TN-2000 X-ray microanalyzer (Eurotest Control JSC Sofia) was operating under the following conditions 20 kеV accelerating voltage 2times10-9 Aring beam current and 2 microm spot size The used JEOL standards were diopside for Mg and Ca Fe2O3 for Fe and MnO2 for Mn as the detection limit was 100 ppm

DIAGENETIC PROCESSES AND PRODUCTS

The Paleogene sandstones of Padesh Basin are defined as texturally immature to submature deposits having relatively low mineralogical maturity (Stefanov et al 2007 Fig 6) They belong to the groups of feldspathic arenites and lithic arenites following the classification scheme proposed by Folk еt al (1970) In this study the whole spectrum of diagenetic processes in sandstones is established ie mechanical compaction cementation dissolution transformation recrystallization replace-ment and chemical compaction Their products are described below as the respective mechanisms of formation and conditions of the postdepositional setting are discussed in detail The distributional trends of some diagenetic processes and the vertical occurrence of some diagenetic products are shown in a summary stratigraphic-lithologic log of the Palaeogene succession (Fig 2)

Mechanical compaction

This irreversible diagenetic process involves decrease of the bulk sediment volume by grain packing and porosity reduction in the subsurface as a result of effective stress ie the difference between lithostatic pressure and pore pressure Mechanical compaction is probably the dominant mechanism leading to porosity loss in sandstones (Lundegard 1992) Its magnitude during the postdepositional alteration of sediments is controlled by the composition size shape and sorting of the constituent grains and the burial history (Pittman Larese 1991 Lander Walderhang 1999 Paxton et al

2002 Chuhan et al 2003 Fawad et al 2011 Bjoslashrlykke 2014 Rossi Alaminos 2014) During progressive burial clastic sediments undergo initially rearrangement (slippage rotation) of the detrital grains which subsequently gives way to grain deformation eg bending flattening fracturing and breakage (Schmidt McDonald 1979) Intergranular volume (IGV) is an indicator of compaction measured as the sum () of intergranular porosity intergranular cement and depositional matrix (Paxton et al 2002)

The most common effects of mechanical compaction in the Palaeogene sandstones include bending of flexible mica flakes (Fig 3a) rearrangement of detrital grains in the rock texture resulting in increased number of long contacts (Figs 3b 6g Stefanov et al 2007 Plate VIc) and ductile deformation of carbonate intraclasts and lithics derived from softer rock types eg shales marlstones phyllites and various schists (Fig 3b Stefanov et al 2007 Plate VIc VIIa b) Brittle grain fracturing shows lesser occurrence (Fig 3c) being more typical for some rigid lithic fragments eg granitoids gneisses amphibolites sandstones and unstable mineral grains eg hornblende sphene pyroxene (Stefanov et al 2007 Plates VIId VIIIa c) Weak plastic deformation of elongated detrital feldspar grains is very rarely observed (Fig 3c)

The highest degree of mechanical compaction is recognized in matrix-free sandstones containing small amount of post-compaction carbonate cement (Fig 3a b 6a) ie these rocks have the lowest IGV value (lt15-20) Sandstones with clay andor carbonate matrix were also strongly compacted as the primary microporosity in the fine-grained groundmass was obliterated during burial (Smosna 1989) Less intensive compaction affected the rocks with relatively high amount of matrix (Fig 5e Stefanov et al 2007 Plates VId VIIe) compared to those with low matrix content (Figs 4d 6g Stefanov et al 2007 Plates IIIc IVd VIc) because in the former type the effective stress is distributed over a larger number of grain contacts resulting in a low overall compressibility (Mondol et al 2007) The lowest degree of mechanical compaction is observed in sandstones with pre-compaction carbonate cement which stabilized the rock texture in the early diagenesis preventing further compaction (Pittman Larese 1991) This is illustrated by the highest recorded IGV values (35-40 ) great number of floating grains and point contacts (Figs 3d f 4a-b f 5f) and well preserved ie undeformed ductile grains and skeletal fragments (Fig 4a Stefanov Chatalov 2007 Fig 4a c) In some arenites the presence of abundant mica flakes andor ductile lithics (especially derived from sedimentary and metamorphic rocks) squeezed between rigid grains promoted advanced compaction (Pittman Larese 1991 Smosna Bruner 1997 Worden et al 2000 Mansurbeg et al 2013 Rossi Alaminos 2014) locally resulting in the formation of pseudomatrix (Fig 3b Stefanov et al 2007 Plate VIc) Other depositional

7

8

Fig 3 a ndash Bended biotite and muscovite flakes between rigid detrital grains due to advanced mechanical compaction Suhostrel Formation b ndash Ferroan dolomite (blue) filling intergranular pores and locally replacing (red arrows) lithics derived from marlstones and limestones The post-compaction origin of the dolomite cement is emphasized by its patchy distribution in the rock texture the great number of long contacts and the presence of ductily deformed grains forming pseudomatrix Note pressure-dissolution contacts between some quartz grains (green arrows) Thin-section stained after Evamy (1963) Suhostrel Formation c ndash Rare cases of plastically deformed detrital feldspar (fs) and brittle grain fracturing (arrow) as a result of mechanical compaction Logodazh Formation d ndash Strongly displaced framework grains by poikilotopic calcite cement suggesting early phreatic carbonate precipitation Logodazh Formation e ndash Displacive effect of eodiagenetic calcite cement recognized by pulling apart of two rigid formerly fractured detrital grains (asterisks) Logodazh Formation f ndash Abundant blocky calcite with planar crystal boundaries formed by pre-compaction cementation Logodazh Formation Note All microphotographs except b (in plane-polarized light) are in cross-polarized light

characteristics of the sandstones also controlled the process of mechanical compaction for example the predominantly poor sorting angular morphology and

coarse size of the sand grains favoured more intensive compaction (Chuhan et al 2003 Fawad et al 2011 Rossi Alaminos 2014)

4

In terms of vertical distribution in the Palaeogene succession mechanical compaction was conspicuously more intensive in the rocks of Suhostrel Formation as a result of their greater burial depth and moreover shows some trend of increasing degree downsection (Fig 2) In comparison the sandstones of Logodazh Formation and Padesh Formation were less affected by mechanical compaction which is related to their shallow burial depth and the locally abundant precipitation of early carbonate cements (see below)

Cementation

Precipitation of mineral cements takes place as supersaturated pore fluids flow from areas of higher solubility to areas of lower solubility due to variations in pH pore water composition andor temperature (Bjoslashrlykke 1988) Cementation plays a major role in reducing the porosity in sandstones but unlike compaction it can be a reversible diagenetic process because some cement phases may dissolve during burial Three types of intergranular intragranular and fracture cements by composition (calcite ferroan dolomite and kaolinite) were identified in the sandstones of Padesh Basin (Stefanov et al 2007) Among them calcite has much broader distribution and greater abundance in the Paleogene section while minor amounts of dolomite and kaolinite show only local occurrence (Fig 2)

Calcite

Calcite cement in sandstones is formed over the entire diagenetic realm from near-surface conditions to deep burial ie showing neither rigorous temperature dependence nor strong spatial localization (Morad 1998 Milliken 2005) This broad distributional pattern is well illustrated by the vertical occurrence of intergranular and fracture calcite cements in sandstones of the studied Palaeogene succession Both textural types display medium to coarse crystalline size indicating relatively slow growth controlled by the flow rate of the pore fluids and the permeability of the sediments With respect to crystal morphology and distribution in the rock texture the intergranular cement has invariably blocky pattern and occasionally poikilotopic appearance (Figs 3d f 4a-b 5c e) This calcite shows low or slightly elevated Fe2+ contents (Table 1) that might be attributed to variable redox potential of the diagenetic milieu (ie oxic and suboxic zones) and amount of dissolved Fe in the circulating fluids Such conclusion is supported by a lacking trend in the vertical distribution of ferroan and non-ferroan calcite cements in the section Moreover the absence of zoned crystals (Figs 4a-b 5f) reflects precipitation from mineral-forming solutions with homogeneous composition

The sandstones of Logodazh Formation and Padesh Formation contain pre-compaction calcite cement Its eodiagenetic origin ie related to shallow burial of the sediments is attested by the low degree of

mechanical compaction with high IGV values reaching 35-40 (Figs 3d-f 4a-b 5f) The loose packing of framework grains is reflected by a great number of point contacts andor floating distribution in the rock texture This fabric pattern is enhanced by the displacive effect of calcite cementation that also resulted in expansion of mica flakes and pulling apart of rigid formerly fractured detrital grains (Fig 3e) while ductile lithics and mica flakes remained undeformed (Fig 4a) Similar microfabric characteristics are common in pedogenic calcretes and dolocretes (Richter 1985 Braithwaite 1989 Chatalov 2006 Armenteros 2010) but have been reported in sandstones unaffected by vadose diagenesis (Burley Kantorowicz 1986 Buczynski Chafetz 1987 Garcia et al 1998 Morad et al 1998 Anjos et al 2000) The displacive growth requires supersaturated solutions with respect to calcite and differential stress caused by the mineral growth (Armenteros 2010) The relatively rapid pre-compaction cementation was probably favoured by the predominantly angular morphology and low sphericity of the detrital grains (Heald Renton 1966)

The sandstones of Suhostrel Formation were only locally cemented by intergranular carbonate and micropetrographic observations reveal the sole presence of post-compaction calcite The latter has definitely mesodiagenetic origin ie related to deep burial of the sediments which is recognized by the high degree of mechanical compaction with low IGV values of 15-20 or less The close packing of framework grains is reflected by scarce point contacts opposed to a great number of long contacts and additional evidence for chemical compaction (Figs 5b 6a Stefanov et al 2007 Plates IVa VIa e VIIc) No expansion of mica flakes andor displacement of rigid grains are observed while the ductile grains are clearly deformed (Fig 3a) The anomalously high IGV values in some sandstones are due to the advanced replacement of framework grains by calcite (Fig 5c) or the presence of associated blocky cement and syntaxial overgrowths around detrital carbonate grains in calcilithites (Fig 4c)

Calcite-filled fractures occur exclusively in the sandstones of Suhostrel Formation The veins are of the syntaxial type (Bons et al 2012) and intersect all other textural elements indicating post-compaction origin during deep burial Three types of blocky calcite are distinguished with respect to Fe-Mn contents as neither of them displays zoned crystal pattern Most common are weakly ferroan (Fig 4d) and ferroan cements which reflect variable presence of dissolved Fe in the circulating fluids The third type of calcite showing pale brown colour in plane light is observed in one sample (Fig 4e) and the chemical analysis recorded slightly elevated contents of both Mn and Fe with the MnFe ratio varying from 21 to 31 (Table 1)

The interpretation of carbonate cements in sandstones should focus on the nature of precipitating fluids source of ions temperature range timing and diagenetic setting Solving these problems requires

9

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

8

Fig 3 a ndash Bended biotite and muscovite flakes between rigid detrital grains due to advanced mechanical compaction Suhostrel Formation b ndash Ferroan dolomite (blue) filling intergranular pores and locally replacing (red arrows) lithics derived from marlstones and limestones The post-compaction origin of the dolomite cement is emphasized by its patchy distribution in the rock texture the great number of long contacts and the presence of ductily deformed grains forming pseudomatrix Note pressure-dissolution contacts between some quartz grains (green arrows) Thin-section stained after Evamy (1963) Suhostrel Formation c ndash Rare cases of plastically deformed detrital feldspar (fs) and brittle grain fracturing (arrow) as a result of mechanical compaction Logodazh Formation d ndash Strongly displaced framework grains by poikilotopic calcite cement suggesting early phreatic carbonate precipitation Logodazh Formation e ndash Displacive effect of eodiagenetic calcite cement recognized by pulling apart of two rigid formerly fractured detrital grains (asterisks) Logodazh Formation f ndash Abundant blocky calcite with planar crystal boundaries formed by pre-compaction cementation Logodazh Formation Note All microphotographs except b (in plane-polarized light) are in cross-polarized light

characteristics of the sandstones also controlled the process of mechanical compaction for example the predominantly poor sorting angular morphology and

coarse size of the sand grains favoured more intensive compaction (Chuhan et al 2003 Fawad et al 2011 Rossi Alaminos 2014)

4

In terms of vertical distribution in the Palaeogene succession mechanical compaction was conspicuously more intensive in the rocks of Suhostrel Formation as a result of their greater burial depth and moreover shows some trend of increasing degree downsection (Fig 2) In comparison the sandstones of Logodazh Formation and Padesh Formation were less affected by mechanical compaction which is related to their shallow burial depth and the locally abundant precipitation of early carbonate cements (see below)

Cementation

Precipitation of mineral cements takes place as supersaturated pore fluids flow from areas of higher solubility to areas of lower solubility due to variations in pH pore water composition andor temperature (Bjoslashrlykke 1988) Cementation plays a major role in reducing the porosity in sandstones but unlike compaction it can be a reversible diagenetic process because some cement phases may dissolve during burial Three types of intergranular intragranular and fracture cements by composition (calcite ferroan dolomite and kaolinite) were identified in the sandstones of Padesh Basin (Stefanov et al 2007) Among them calcite has much broader distribution and greater abundance in the Paleogene section while minor amounts of dolomite and kaolinite show only local occurrence (Fig 2)

Calcite

Calcite cement in sandstones is formed over the entire diagenetic realm from near-surface conditions to deep burial ie showing neither rigorous temperature dependence nor strong spatial localization (Morad 1998 Milliken 2005) This broad distributional pattern is well illustrated by the vertical occurrence of intergranular and fracture calcite cements in sandstones of the studied Palaeogene succession Both textural types display medium to coarse crystalline size indicating relatively slow growth controlled by the flow rate of the pore fluids and the permeability of the sediments With respect to crystal morphology and distribution in the rock texture the intergranular cement has invariably blocky pattern and occasionally poikilotopic appearance (Figs 3d f 4a-b 5c e) This calcite shows low or slightly elevated Fe2+ contents (Table 1) that might be attributed to variable redox potential of the diagenetic milieu (ie oxic and suboxic zones) and amount of dissolved Fe in the circulating fluids Such conclusion is supported by a lacking trend in the vertical distribution of ferroan and non-ferroan calcite cements in the section Moreover the absence of zoned crystals (Figs 4a-b 5f) reflects precipitation from mineral-forming solutions with homogeneous composition

The sandstones of Logodazh Formation and Padesh Formation contain pre-compaction calcite cement Its eodiagenetic origin ie related to shallow burial of the sediments is attested by the low degree of

mechanical compaction with high IGV values reaching 35-40 (Figs 3d-f 4a-b 5f) The loose packing of framework grains is reflected by a great number of point contacts andor floating distribution in the rock texture This fabric pattern is enhanced by the displacive effect of calcite cementation that also resulted in expansion of mica flakes and pulling apart of rigid formerly fractured detrital grains (Fig 3e) while ductile lithics and mica flakes remained undeformed (Fig 4a) Similar microfabric characteristics are common in pedogenic calcretes and dolocretes (Richter 1985 Braithwaite 1989 Chatalov 2006 Armenteros 2010) but have been reported in sandstones unaffected by vadose diagenesis (Burley Kantorowicz 1986 Buczynski Chafetz 1987 Garcia et al 1998 Morad et al 1998 Anjos et al 2000) The displacive growth requires supersaturated solutions with respect to calcite and differential stress caused by the mineral growth (Armenteros 2010) The relatively rapid pre-compaction cementation was probably favoured by the predominantly angular morphology and low sphericity of the detrital grains (Heald Renton 1966)

The sandstones of Suhostrel Formation were only locally cemented by intergranular carbonate and micropetrographic observations reveal the sole presence of post-compaction calcite The latter has definitely mesodiagenetic origin ie related to deep burial of the sediments which is recognized by the high degree of mechanical compaction with low IGV values of 15-20 or less The close packing of framework grains is reflected by scarce point contacts opposed to a great number of long contacts and additional evidence for chemical compaction (Figs 5b 6a Stefanov et al 2007 Plates IVa VIa e VIIc) No expansion of mica flakes andor displacement of rigid grains are observed while the ductile grains are clearly deformed (Fig 3a) The anomalously high IGV values in some sandstones are due to the advanced replacement of framework grains by calcite (Fig 5c) or the presence of associated blocky cement and syntaxial overgrowths around detrital carbonate grains in calcilithites (Fig 4c)

Calcite-filled fractures occur exclusively in the sandstones of Suhostrel Formation The veins are of the syntaxial type (Bons et al 2012) and intersect all other textural elements indicating post-compaction origin during deep burial Three types of blocky calcite are distinguished with respect to Fe-Mn contents as neither of them displays zoned crystal pattern Most common are weakly ferroan (Fig 4d) and ferroan cements which reflect variable presence of dissolved Fe in the circulating fluids The third type of calcite showing pale brown colour in plane light is observed in one sample (Fig 4e) and the chemical analysis recorded slightly elevated contents of both Mn and Fe with the MnFe ratio varying from 21 to 31 (Table 1)

The interpretation of carbonate cements in sandstones should focus on the nature of precipitating fluids source of ions temperature range timing and diagenetic setting Solving these problems requires

9

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

4

In terms of vertical distribution in the Palaeogene succession mechanical compaction was conspicuously more intensive in the rocks of Suhostrel Formation as a result of their greater burial depth and moreover shows some trend of increasing degree downsection (Fig 2) In comparison the sandstones of Logodazh Formation and Padesh Formation were less affected by mechanical compaction which is related to their shallow burial depth and the locally abundant precipitation of early carbonate cements (see below)

Cementation

Precipitation of mineral cements takes place as supersaturated pore fluids flow from areas of higher solubility to areas of lower solubility due to variations in pH pore water composition andor temperature (Bjoslashrlykke 1988) Cementation plays a major role in reducing the porosity in sandstones but unlike compaction it can be a reversible diagenetic process because some cement phases may dissolve during burial Three types of intergranular intragranular and fracture cements by composition (calcite ferroan dolomite and kaolinite) were identified in the sandstones of Padesh Basin (Stefanov et al 2007) Among them calcite has much broader distribution and greater abundance in the Paleogene section while minor amounts of dolomite and kaolinite show only local occurrence (Fig 2)

Calcite

Calcite cement in sandstones is formed over the entire diagenetic realm from near-surface conditions to deep burial ie showing neither rigorous temperature dependence nor strong spatial localization (Morad 1998 Milliken 2005) This broad distributional pattern is well illustrated by the vertical occurrence of intergranular and fracture calcite cements in sandstones of the studied Palaeogene succession Both textural types display medium to coarse crystalline size indicating relatively slow growth controlled by the flow rate of the pore fluids and the permeability of the sediments With respect to crystal morphology and distribution in the rock texture the intergranular cement has invariably blocky pattern and occasionally poikilotopic appearance (Figs 3d f 4a-b 5c e) This calcite shows low or slightly elevated Fe2+ contents (Table 1) that might be attributed to variable redox potential of the diagenetic milieu (ie oxic and suboxic zones) and amount of dissolved Fe in the circulating fluids Such conclusion is supported by a lacking trend in the vertical distribution of ferroan and non-ferroan calcite cements in the section Moreover the absence of zoned crystals (Figs 4a-b 5f) reflects precipitation from mineral-forming solutions with homogeneous composition

The sandstones of Logodazh Formation and Padesh Formation contain pre-compaction calcite cement Its eodiagenetic origin ie related to shallow burial of the sediments is attested by the low degree of

mechanical compaction with high IGV values reaching 35-40 (Figs 3d-f 4a-b 5f) The loose packing of framework grains is reflected by a great number of point contacts andor floating distribution in the rock texture This fabric pattern is enhanced by the displacive effect of calcite cementation that also resulted in expansion of mica flakes and pulling apart of rigid formerly fractured detrital grains (Fig 3e) while ductile lithics and mica flakes remained undeformed (Fig 4a) Similar microfabric characteristics are common in pedogenic calcretes and dolocretes (Richter 1985 Braithwaite 1989 Chatalov 2006 Armenteros 2010) but have been reported in sandstones unaffected by vadose diagenesis (Burley Kantorowicz 1986 Buczynski Chafetz 1987 Garcia et al 1998 Morad et al 1998 Anjos et al 2000) The displacive growth requires supersaturated solutions with respect to calcite and differential stress caused by the mineral growth (Armenteros 2010) The relatively rapid pre-compaction cementation was probably favoured by the predominantly angular morphology and low sphericity of the detrital grains (Heald Renton 1966)

The sandstones of Suhostrel Formation were only locally cemented by intergranular carbonate and micropetrographic observations reveal the sole presence of post-compaction calcite The latter has definitely mesodiagenetic origin ie related to deep burial of the sediments which is recognized by the high degree of mechanical compaction with low IGV values of 15-20 or less The close packing of framework grains is reflected by scarce point contacts opposed to a great number of long contacts and additional evidence for chemical compaction (Figs 5b 6a Stefanov et al 2007 Plates IVa VIa e VIIc) No expansion of mica flakes andor displacement of rigid grains are observed while the ductile grains are clearly deformed (Fig 3a) The anomalously high IGV values in some sandstones are due to the advanced replacement of framework grains by calcite (Fig 5c) or the presence of associated blocky cement and syntaxial overgrowths around detrital carbonate grains in calcilithites (Fig 4c)

Calcite-filled fractures occur exclusively in the sandstones of Suhostrel Formation The veins are of the syntaxial type (Bons et al 2012) and intersect all other textural elements indicating post-compaction origin during deep burial Three types of blocky calcite are distinguished with respect to Fe-Mn contents as neither of them displays zoned crystal pattern Most common are weakly ferroan (Fig 4d) and ferroan cements which reflect variable presence of dissolved Fe in the circulating fluids The third type of calcite showing pale brown colour in plane light is observed in one sample (Fig 4e) and the chemical analysis recorded slightly elevated contents of both Mn and Fe with the MnFe ratio varying from 21 to 31 (Table 1)

The interpretation of carbonate cements in sandstones should focus on the nature of precipitating fluids source of ions temperature range timing and diagenetic setting Solving these problems requires

9

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

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Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

Table 1 Representative chemical analyses of calcite and dolomite cements (weight )

lithostratigraphic unit and cement type CaCO3 MgCO3 FeCO3 MnCO3 Padesh Fm

intergranular 9987 nd nd nd

Logodash Fm 9815 071 094 047 Suhostrel Fm 9605 059 242 080 Logodash Fm 5263 3621 1061 037

Suhostrel Fm fracture 9667 094 074 203 9671 082 097 183

nd ndash not detected

isotope analyses and other complementary data from cathodoluminescence geochemistry and fluid inclusion microthermometry (Morad 1998) Because these meth-ods were not implemented in the present study only some implications regarding the origin of described calcite cements can be made on the basis of mainly micropetrographic observations Thus the lack of fibrous bladed microcrystalline pendant and meniscus cements precludes marine and vadose diagenetic envi-ronments for the calcite precipitation Source marine water is also ruled out by the low Mg and elevated Fe-Mn contents (Table 1) of the blocky calcite (see Boles 1998 Morad 1998) Furthermore vadose cementation is unlikely for the lack of diagnostic pedogenic features (eg rhizocretions glaebules alveolar fabrics rhombic crystals) and the good preservation of depositional structures (cf Tucker Wright 1990 Spoumltl Wright 1992 Beckner Mozley 1998 Hall et al 2004) such as parallel lamination and cross-bedding (Stefanov 2011)

The eodiagenetic pre-compaction calcite cemen-tation in sandstones of the Logodazh Formation and Padesh Formation can be attributed to phreatic pre-cipitation by groundwater in sediments with high primary permeability This conclusion is supported by the overall homogeneous textural and chemical distri-butional patterns coarse crystal size and occurrence of poikilotopic calcite which is commonly regarded to be of meteoric origin (Folk 1974 Beckner Mozley 1998 Garcia et al 1998 Morad et al 1998 Ufnar et al 2004 Al-Ramadan et al 2005 Van den Bril Swennen 2008) Low saturation levels of phreatic pore waters with respect to calcite typically enhance the precipitation of poikilotopic crystals (Beckner Mozley 1998) with a very low nucleation rate and slow mineral growth (Tucker Wright 1990) Indeed burial recrystallization of early micriticmicrosparitic cements may result in the formation of poikilotopic calcite but micritemicrospar relics are often recognized in those cases (Saigal Bjoslashrlykke 1987 Hall et al 2004 Calvo et al 2011 Bojanowski et al 2014) and such evidence was not found in this study The mechanisms of groundwater carbonate precipitation are mostly evaporation evapotranspiration CO2 degassing and the

common ion effect (Alonso-Zarza Wright 2010) In this context the inferred semi-arid climate conditions during deposition of the clastic fluvial facies of Logodazh Formation and Padesh Formation (Stefanov et al 2008) are in accordance with the established intensive eodiagenetic carbonate cementation (see Garcia et al 1998 Morad 1998 Morad et al 2010b)

The origin of mesodiagenetic post-compaction calcite cement in sandstones of the Suhostrel Formation should be explained in a different way Local internal supply of Ca and C could have been related to decay of plant remains (Morad 1998) alteration of tuffaceous material andor volcanic lithics (Morad De Ros 1994) and dissolution of detrital Ca-plagioclase (Boles 1998) For example the sandstones of Solashka Member and Debochitsa Member contain abundant plant detritus while minor amount of volcaniclastic rocks occurs in the Solashka Member (Zagorchev Popov 1968 Ivanov Chernyavska 1971 Zagorchev et al 1989 Stefanov et al 2007) Moreover enrichment of plagioclase grains and feldspar-bearing lithics is typical for the rocks of Suhostrel Formation (Stefanov et al 2007 Table 1 Fig 6) in which calcite commonly replaces plagioclase (Fig 5b f) On the other hand sandstones consisting of detrital carbonates may undergo intensive early cementation restricting mechanical compaction (Milliken et al 1998 Al-Ramadan et al 2005 Mansurbeg et al 2009) Such litharenites containing abundant extrabasinal carbonate rock fragments (ie calcithites) occur in the Solashka Member (Stefanov et al 2007 Fig 6 Plate VIf) In these deposits the sand-sized detrital grains derived from Triassic limestones and dolostones served as nuclei for calcite cements The latter originated probably by partial dissolution of the lithics and subsequent precipitation of dissolved carbonate in the form of blocky calcite and syntaxial overgrowths (Fig 4c) However all above mentioned internal sources must have been volumetrically insufficient for the post-compaction carbonate cementation One possible external source was linked to the diagenetic modification of adjacent shales (Hower et al 1976 Boles Franks 1979) because such rocks have the potential to supply chemical components for carbonate

10

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

Fig 4 a ndash Ferroan calcite cement (blue) with pre-compaction origin evidenced by the presence of predominantly point contacts and undeformed shale lithic grains (arrow) Thin-section stained after Dickson (1965) Logodazh Formation b ndash Calcite cement with low Fe content (pink) predating significant compaction as revealed by the floating detrital grains and high IGV value of 35-40 Thin-section stained after Evamy (1963) Logodazh Formation c ndash Detrital mono- and polycrystalline carbonate grains in calcilithite with locally developed syntaxial overgrowths (arrow) Suhostrel Formation d ndash Weakly ferroan calcite cement filling former fracture Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Veinlet of blocky calcite with slightly elevated Fe-Mn content Suhostrel Formation f ndash Ferroan dolomite cement (blue) with pre-compaction origin which is recognized by the loose packing of framework grains and displacement effect in expanded biotite flake (arrow) The latter is partly replaced by Fe and Ti oxides Thin-section stained after Evamy (1963) Logodazh Formation Note All microphotographs except c (in cross-polarized light) are in plane-polarized light

11

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

cementation of porous sandstones Thus a major effect from the smectite-to-illite transformation is the release of interlayer Ca2+ which can migrate toward associated sandy deposits through a high rate of fluid flow (Morad et al 2000 Worden Morad 2003) as a result of progressive compaction of the shales (Bjoslashrlykke 1993) Another factor promoting the transfer of dissolved components is the concomitant release of water phase during the transformation process (Hower et al 1976 Boles Franks 1979 Lynch 1997 Thyne 2001) Because the latter takes place most intensively in the temperature interval 70-100degC (Pollastro 1993) this creates favourable conditions for calcite cementation of adjacent sandstones during burial An important side effect from the illitization of smectite is the resulting acidity of the pore fluids (Hower et al 1976 Bjoslashrlykke 1984) which in turn favours the dissolution of carbonate available in the shales and therefore provides additional supply of carbonate ions (Milliken Land 1993 Wintsch Kvale 1994) The suggested influence of clay diagenesis on the late calcite precipitation in the Paleogene sandstones is supported by XRD data for advanced smectite-to-illite transformation in the interbedded shales of Suhostrel Formation as a result of elevated temperatures during burial (Stefanov Chatalov 2008) This hypothesis is also in accordance with micropetrographic observations showing that the post-compaction carbonate cementation predated the formation of secondary porosity as an effect of chemical reactions related to organic matter maturation (see below)

The fracture-filling calcite was precipitated from burial fluids as the elevated Fe-Mn contents reflect formation in a reducing diagenetic environment (Milliken et al 1998 Traveacute Calvet 2001 Parcerisa et al 2005 Cantarero et al 2014) Moreover the blocky calcite texture is evidence for crystal growth competition during a single fluid precipitation event (Bons et al 2012) Possible sources for this latest diagenetic carbonate cement include the interbedded lacustrine limestones and earlier formed calcite cements in the sandstones of Suhostrel Formation but also the Triassic carbonate basement of the Paleogene succession as the most feasible driving mechanism was dissolution by chemical compaction (Choquette James 1987 Souza Silva 1998 Heydari 2000 Morad et al 2010a Lacroix et al 2011) Fracturing and fluid flow along pressure gradients may result in mesodiagenetic carbonate cementation in the intergranular pore space of sandstones and along fractures (Morad 1998) This conclusion is conformable to the tectonically active setting of formation of the Padesh Basin where conduits (fractures faults) were likely created favouring the migration of fluids enriched in dissolved carbonate

Ferroan dolomite

The intergranular dolomite cement also displays coarse crystal size blocky morphology and lack of Fe2+ zoning (Fig 4f) It is characterized by elevated iron content

(Table 1) defining the mineral as ferroan dolomite ie with gt2 mole FeCO3 (Tucker Wright 1990) The precipitation occurred in open pore space between framework grains which is proved by the lack of traces from replacement of former cement phases or rock matrix (see Morad 1998) The ferroan dolomite in sandstones of the Logodazh Formation is pre-compaction cement as revealed by the high IGV value great number of point contacts and displacement effect in expanded mica flakes (Fig 4f) Similar micropetrographic characteristics were used in many studies to explain the early origin of dolomite cements including ferroan phases (Boles 1998 Garcia et al 1998 Hesse Abid 1998 Morad et al 1998 Schmid et al 2006 Zamanzadeh et al 2009) The eodiagenetic character of the dolomite precipitation can be related to episodic fluctuations in the dilution of pore waters that resulted in shifting between dolomite and calcite equilibrium fields The ferroan dolomite was formed in the suboxic zone during dry periods of increase in the MgCa ratio of the circulating solutions caused by evaporation possibly coupled with water-sediment interaction (Morad et al 1998) In comparison the ferroan dolomite in sandstones of the Suhostrel Formation is post-compaction cement which is recognized by its patchy occurrence in the rock texture the high degree of mechanical compaction and the coexistence with deformed ductile grains locally forming pseudomatrix (Fig 3b) These fabric characteristics resemble other examples of late ferroan dolomite cementation described in sandstones (Milliken 1998 Ochoa Arribas 2005 Estupintildean et al 2007 Sliaupa et al 2008 Khalifa Morad 2012) One possible source was related to the diagenetic modification of adjacent shales and in particular the concurrent illitization of smectite resulting in certain release of Ca Mg and Fe (Hower et al 1976 Boles Franks 1979 McHargue Price 1982 Kantorowicz 1985 Milliken 1998) Dissolution of unstable ferromagnesian minerals may have also contributed to the internal supply of metal cations (Macaulay et al 1993 Anjos et al 2000) According to Surdam et al (1989) Fe can be incorporated in the pore fluids as a result of thermal destabilization of iron organometal complexes which are derived through reactions involving smectite minerals iron oxides and organic matter

Kaolinite

The observed kaolinite in feldspathic litharenites of the Suhostrel Formation was precipitated from solutions in free pore space of the sandstones (Stefanov et al 2007) The micropetrographic evidence for such interpretation corresponds to four criteria for authigenic clay formation proposed by Wilson Pittman (1977) composition morphology inner fabric and distribution in the rock texture The colour homogeneity lack of admixtures and inclusions plus thе monomineralic character of kaolinite aggregates are related to the first

12

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

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Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

criterion implying cement origin Secondly the SEM images reveal distinctive morphologies including pseudohexagonal crystals with platy or blocky appearance that form typical stacked ldquobookletsrdquo and locally outline ldquovermiformrdquo aggregates (Fig 5a) With respect to inner fabric the kaolinite crystals are notably larger than allogenic clay particles in the sandstone matrix and clay products formed by the replacement of feldspar grains (see below) Last but not least the kaolinite fills intergranular and intragranular pores and therefore is defined as cement phase In this context where the authigenic mineral is abundant in the interior of feldspar grains (Fig 5b) no traces from replacement are recognized eg fabric elements such as cleavage andor twins (cf Wilson Pittman 1977 Morad Aldahan 1987a Milliken 2003 Mansurbeg et al 2008) Also relic patches of calcite cement are not observed in the kaolinite-filled intergranular pores (Fig 5c d) thus precluding replacement process (cf Ketzer et al 2003 Playagrave et al 2010) Most importantly however the kaolinite aggregates display sharp boundaries with the adjoining textural components and hence clearly indicate cementation (Milliken 2003 Loucks 2005 Estupintildean et al 2010 Playagrave et al 2010 Shalaby et al 2014)

Besides the apparent origin of the kaolinite as a cement phase there is solid evidence that the precipitation occurred by occlusion of secondary pores Thus the former voids correspond to some criteria for identification of secondary porosity in sandstones proposed by Schmidt McDonald (1979) and subsequently elaborated by other authors (Shanmugam 1984 Burley Kantorovicz 1986) For example the kaolinite aggregates outline mostly irregular sectors with sharp boundaries and no traces from replacement within the feldspar grains as the latter have apparently undergone partial leaching ie formation of intragranular pores prior to the kaolinite precipitation (Fig 5b) It is noteworthy that the calcite replacing detrital feldspar in a patchy manner was also partly dissolved and therefore predated the kaolinite formation (cf Muchez et al 1992) Similarly where the kaolinite occupies intergranular pores it bounds on jagged crystals of calcite cement (Fig 5c) andor peripherally corroded feldspar grains (Fig 5d) that have been partly leached before the kaolinite precipitation The limited development of secondary porosity in the sandstones is emphasized by the minor total occurrence of authigenic kaolinite (le5) and only partly dissolved textural components along with the abundance of associated feldspar grains and calcite cement wholly unaffected by dissolution

Clarifying the diagenetic conditions that have favoured precipitation of kaolinite seems directly related to its vertical distribution in the Palaeogene succession As noted above this clay mineral fills secondary pores only in feldspathic lithic arenites from lower levels of the section (Fig 2) Thus it can be inferred that the kaolinite formation was restricted within strata that

reached maximum burial depth Such vertical trend is additionally demonstrated by the greater amount of kaolinite (~5) in sandstones of the Elska Member compared to its negligible content (lt1) in samples taken from the Solashka Member as the latter unit has higher stratigraphic position In the same context secondary porosity in sandstones may be particularly related to mesodiagenesis or even enhanced with increasing burial depth as mainly carbonate and silicate phases are dissolved (Schmidt McDonald 1979 Morad et al 2000 Wilkinson et al 2001 Mansurbeg et al 2008 Estupintildean et al 2010 Shalaby et al 2014) Two major factors controlling the formation of secondary porosity can be the downward migration of meteoric waters (Garcia et al 1998 Morad et al 2000 Lanson et al 2002 Ketzer et al 2003 El-Ghali et al 2006 Wilkinson et al 2006) and the deep percolation of pore fluids containing organic acids (Surdam et al 1984) andor СО2 (Schmidt McDonald 1979) Applying the first hypothesis for the recorded dissolution of detrital feldspar and carbonate cement is not supported by any micropetrographic evidence If eodiagenetic (ie shallow burial) origin is assumed more abundant distribution of secondary porosity in the Paleogene succession or at least a lack of positive relationship between the volume of produced secondary pores and increasing burial depth should be expected considering the uniform vertical distribution of detrital feldspar and carbonate cement Also other alteration products suggesting eodiagenetic conditions such as expanded kaolinitized micas pronounced vermicular fabric of kaolinite or replacement of kaolinite by early carbonate cements (McAulay et al 1994 Osborne et al 1994 Morad et al 1998 Ketzer et al 2003 Wilkinson et al 2004 El-Ghali et al 2006) are not found in the sandstones of Suhostrel Formation Moreover the amount of secondary porosity should increase upsection if meteoric water was the main controlling factor not during the shallow burial of sediments but after the uplift and exposure of consolidated rocks on the surface (Bjoslashrlykke et al 1989 Garcia et al 1998 Morad et al 2000 Franccedila et al 2003 Ketzer et al 2003 Bertier et al 2008) This alternative telodiagenetic origin is further precluded by the coexistence of dickite and kaolinite (see below) because the former mineral provides irrefutable evidence for mesodiagenetic conditions of formation Therefore the first hypothesis cannot explain the occurrence of kaolinite cement within a relatively narrow vertical interval of the Palaeogene succession the more so as in its lowermost part In support a very important conclusion is that the sandstones with recognized secondary porosity were buried at maximum depth of 2500ndash3000 m (Stefanov 2011) Such a great distance from the surface should have been beyond the range of meteoric waters because they can hardly penetrate down to more than 2000 m (Morad et al 2000) Furthermore some authors have suggested (eg Giles Marshall 1986 Worden and Morad 2003) that meteoric fluids reach equilibrium

13

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

Fig 5 a ndash Authigenic kaolinite ldquobookletsrdquo locally forming ldquovermiformrdquo aggregates Single dickite crystals with blocky morphology and larger size (arrow) were formed by transformation of kaolinite during burial SEM image Suhostrel Formation b ndash Kaolinite (kl) filling secondary pores in the interior of detrital feldspar grain (fs) Patchy calcite (cc) replacing the feldspar was also partly leached before the kaolinite precipitation Note the distinct convex-concave contacts between the adjacent feldspar and quartz grains (arrows) indicative of pressure dissolution Suhostrel Formation c ndash Kaolinite (kl) bounding on jagged coarse crystals of partly dissolved intergranular ferroan calcite cement (cc) The anomalously high IGV value is due to advanced replacement of framework grains by calcite (arrows) Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Kaolinite-filled (kl) secondary pore formed by peripheral dissolution of detrital plagioclase grain (pl) and leaching of intergranular calcite cement (cc) The kaolinite displays sharp boundaries with the adjoining textural components and hence indicates cementation but not replacement Suhostrel Formation e ndash Irregular recrystallization of micritic matrix (mc) into microspar (ms) and pseudospar (ps) Suhostrel Formation f ndash Pre-compaction calcite cement (cc) corroding the periphery of detrital quartz (qz) with the replaced part of grain being recognized by produced characteristic halo ie corona-like fabric Thin-section stained after Dickson (1965) Logodazh Formation Note All microphotographs of thin-sections except e (in plane-polarized light) are in cross-polarized light

14

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

with respect to the potential reactive minerals as early as in the beginning of their descending percolation

Ruling out the first hypothesis makes feasible the assumption that the main control on the formation of secondary porosity was related to the postdepositional modification of organic matter in adjacent shales Such rocks having thickness of several tens of meters and elevated organic content (Pilyovo Member) overlie the sandstones of Elska Member and are covered by the rocks of Solashka Member It is well known that during thermal maturation of kerogene in the interval 80-120deg С the most intensive generation of organic (mainly carboxyl) acids takes place (Surdam et al 1989) The anions of these acids enhance the solubility of carbonate and alumosilicate (mostly feldspar) minerals by forming complex compounds with Si Al Ca and Mg (Surdam et al 1984 Welch Ullman 1993 Yang et al 2015) Within the range of the mentioned temperature interval (approximately at Tgt100deg C) thermal degradation (decarboxylation) of organic matter begins leading to additional release of CO2 The effect from generation of that gas ie formation of carbonic acid might similarly cause dissolution of carbonates and alumosilicates in adjacent sandstones The described mode of formation of secondary porosity was established in many sandstone reservoirs (Franks Forester 1984 Burley Kantorowicz 1986 Hesse Abid 1998 Marfil et al 2003 Mansurbeg et al 2008 Estupintildean et al 2010 Li et al 2014) and was especially well investigated in the Rotliegend oil-bearing strata of Northern Europe and the North Sea (Scotchman et al 1989 Platt 1993 Schoumlner Gaupp 2005) In this study besides the proximity to organic-rich shales and appropriate temperature conditions another favourable factor was the lack of carbonate in the rocks of Pilyovo Member that did not allow the acid fluids generated by kerogene maturation to be neutralized (Franks Forester 1984 Giles Marshall 1986 Bjoslashrlykke Jahren 2012 Ehrenberg et al 2012) Also the presence of mainly sapropel organic matter in the black shales presumably provided moderate release of СО2 thus restricting the formation of volumetrically little porosity in the underlying and overlying sandstones Another possible limiting factor particularly for the feldspar dissolution was the defficiency in anions of the produced organic acids that resulted in relatively low mobilization of Al from the crystal lattice of feldspars (for details see MacGowan Surdam 1990) As for the calcite cement favourable conditions for its large-scale leaching likewise were not created because the pattern of carbonate dissolution depends directly on the ratio between the same anions and рСО2 (Surdam et al 1989) Some other factors could have also influenced the formation of secondary porosity in the feldspathic litharenites of Suhostrel Formation For example the dissolution of K-feldspars (ie release of K) was probably favoured by the progressive illitization of smectite (ie utilization of K) in the adjacent black shales (Boles Franks 1979 Awwiler 1993 Lynch

1997) in view of the fact that the smectite-to-illite transformation overlaps to some extent with the temperature interval of kerogen maturation Secondly the reduction of Fe during this transformation is compensated by the parallel oxidation of C contained in the organic matter which ultimately results in the generation of additional СО2 and H+ ie carbonic acid capable of dissolving feldspars and carbonates (Crossey et al 1986 Lynch et al 1997) In third place the clay dewatering during the smectite alteration into illite may have promoted the exchange of chemical elements in particular the soluble anions of organic acids between the sandstones and the shales by creating increased fluid flow (Lynch 1997) On the other hand some authors suggested that СО2 can be produced as a byproduct from inorganic reactions between carbonates and alumosilicates in the sandstones (Hutcheon Abercrombie 1990 Hutcheon et al 1993) thus leading to dissolution of the carbonates at temperatures gt100deg C (Smith Ehrenberg 1989) To sum up the obtained results conform to the inference of Taylor et al (2010) that secondary porosity related to dissolution of framework grains andor cements in sandstones is most commonly volumetrically minor However the available petrographic data are in contradiction with the statement of Yuan et al (2015) that selective dissolution of feldspars rather than carbonates takes place by organic CO2 from thermal evolution of organic matter

There are grounds to conclude that the formation of secondary porosity and its subsequent occlusion by kaolinite cement were closely linked thus confirming the postulation of Bjoslashrlykke Jahren (2012) that mineral dissolution and precipitation must be balanced in the burial diagenetic environment It is known that Si4+ and Al3+ necessary for kaolinite crystallization are supplied by relatively fresh water in acid milieu and therefore the main source of both elements was related to the feldspar dissolution (Boles Franks 1979) Other possible sources of Si were the smectite-to-illite transformation (Lynch 1997) and pressure dissolution of quartz and feldspar that can favour kaolinite formation at burial depths in the interval 2000-3500 m (Sarkisyan 1972) Although Al may also originate from leaching of other alumosilicates in sandstones eg micas no affirmative petrographic evidence was found in this study The low mobility of Al probably promoted its rapid incorporation into the crystal lattice of kaolinite ie the formation of secondary porosity could have occurred without large-scale export of the released components out of the diagenetic system Nevertheless the precipitation of kaolinite as a direct result of feldspar dissolution cannot take place in a fully closed system but requires a fluid flow to remove K+ Na+ Ca2+ and silica (Bjoslashrlykke 1998) Such semi-closed pattern of the system was predetermined by a relatively low rate of pore fluid flow (Nedkvitne Bjoslashrlykke 1992) as the composition of newly precipitated mineral (ie kaolinite) is similar to the one of some of the dissolved

15

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

solid phases (ie feldspar) This interpretation explains also the limited formation of secondary pores in which the kaolinite virtually occupied the same volume as the leached minerals (Surdam et al 1989) thus producing so called redistributional secondary porosity sensu Giles de Boer (1990) In addition the relatively rapid precipitation of kaolinite is implied by the lack of evidence for mechanical compaction effects postdating the selective dissolution of feldspar and calcite

Dissolution

Complete dissolution of parts of or whole mineral grains also known under the term congruent dissolution involves the transfer of material from solid state into pore fluids leaving a void within the host rock (Milliken 2005) The only observed manifestation of this destructive diagenetic process in the Palaeogene sandstones was related to the leaching of carbonate cements (ie decementation) carbonate replacement phases and framework feldspar grains in feldspathic litharenites of the Suhostrel Formation The particular controlling factors and formation mechanisms of the minor amount of produced secondary porosity were discussed in the previous section

Transformation

The performed SEM observations of kaolinite-bearing sandstones revealed the presence of scarce dickite crystals The latter show distinct blocky morphology being thicker (5-7 μm) than the associated kaolinite crystals (Fig 5a) and such differences in size between the two minerals have been recorded in many other studies (Morad et al 1998 Ketzer et al 2003 El-Ghali et al 2006 Bertier et al 2008 Mansurbeg et al 2008 Estupintildean et al 2010 Khalifa Morad 2012) Dickite is the stable polytype of kaolinite formed by transformation with increasing temperature and burial depth although the precise mechanism of alteration is still controversial (Ehrenberg et al 1993 Beaufort et al 1998 Lanson et al 2002 Cuadros et al 2014) The common coexistence of both minerals over a large temperature range implies a kinetic control on the kaolinite-to-dickite reaction rate and additional dependence on other parameters for example waterrock ratio acidity of formation waters or αK+αH+ ratio (Beaufort et al 1998 Morad et al 2000 Lanson et al 2002 Fialips et al 2003 Martiacuten-Martiacuten et al 2007 De Bona et al 2008) Apparently the mesodiagenetic formation of dickite in the Palaeogene sandstones was restricted by their relatively low porosity and permeability ie low waterrock ratio Also the limited or inhibited influence of organic-rich acids after the generation of secondary porosity and kaolinite precipitation could not favour the extensive transformation of kaolinite to dickite

Recrystallization

This postdepositional process is marked by changes of the crystal shape and size due to thermodynamic instability while the gross mineralogy remains the same Recrystallization fabrics are represented in the studied sandstones by microspar and pseudospar developed within micritic matrix (Fig 5e) and calcimudstone lithoclasts Also recrystallization has affected some intergranular calcite cements as revealed by their correspondence to criteria for the recognition of pseudospar (Bathurst 1975) for example curved crystal boundaries and low percent of enfacial junctions The associated vermiform and booklet aggregates of kaolinite with various thicknesses of the individual plates (Fig 5a) likewise suggest recrystallization related to progressive burial during which transition from poorly ordered to well-ordered kaolinite occurred (McAulay et al 1994 Osborne et al 1994 Wilkinson et al 2004 El-Ghali et al 2006 Wilkinson et al 2006)

Replacement

This allochemical diagenetic process involves the selective incongruent dissolution of one detrital or authigenic mineral and simultaneous precipitation of another authigenic mineral as a result of ion exchange between the host solid material and circulating solutions along thin films (Milliken 2005) Replacement reactions take place when the solubility of the parent mineral is exceeded under a given set of pH Eh temperature and salinity conditions The mechanism and kinetics of mineral replacement by dissolution-precipitation processes have been subjects of discussion and controversy in the geological literature (Maliva Siever 1988 Putnis 2009 Xia et al 2009)

Several authigenic minerals corresponding to different criteria for the recognition of replacement fabrics (see Boggs 2009 Table 81) are observed in the studied sandstones The most common mineral is calcite replacing quartz (Fig 5c f) feldspar (Figs 5b 6a) femic minerals (Fig 5c Stefanov et al 2007 Table VIIIe) lithic fragments (Fig 6b) and clay matrix The alteration has affected both the interior and peripheral sectors of detrital grains causing corrosion in the latter case Corona-like fabric typical for calcretesdolocretes (eg Khalaf 2007) is distinguished around some quartz grains as a result of incomplete replacement (Fig 5c f) The calcitization occurred more or less concurrently with the precipitation of intergranular calcite cement which is evidenced by remnants of strongly replaced clastic grains in abnormally oversized pores (Fig 5c) In some cases the process was apparently controlled by the composition of attacked mineral phases for example the plagioclases demonstrate greater susceptibility to replacement (Fig 5b 6a) compared to the K-feldspars and the volcanic lithics are preferentially replaced (Fig 6b) relative to other rock fragments Another secondary

16

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

Fig 6 a ndash Feldspar grain (dashed line) partly replaced by calcite The low amount of intergranular calcite suggests strong mechanical compaction predating the mesodiagenetic carbonate cementation Suhostrel Formation b ndash Calcite (arrows) replacing the interior and peripheral sectors of volcanic rock fragment Logodazh Formation c ndash Detrital biotite expanded along cleavage planes by authigenic non-ferroan calcite (red) Note the lack of carbonate elsewhere in the rock texture indicating alteration on a local scale Thin-section stained after Evamy (1963) Suhostrel Formation d ndash Plagioclase grain replaced by claysericite during the telodiagenetic stage (hypergenesis) Blocky calcite cement is in red Thin-section stained after Evamy (1963) Suhostrel Formation e ndash Chlorite (green) replacing longitudinally biotite flake (dark brown) Suhostrel Formation f ndash Chlorite aggregates (arrows) developed within detrital feldspar grain Suhostrel Formation g ndash Evidence for chemical compaction including convex-concave contacts (yellow arrows) and triple junctions (blue arrow) between quartz grains Suhostrel Formation Note All microphotographs excepth c and e (in plane-polarized light) are in cross-polarized light

17

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

carbonate mineral is ferroan dolomite that locally replaces lithic grains derived from limestones and marlstones (Fig 3b)

A specific fabric is observed in some biotite flakes where authigenic calcite was precipitated between the mica cleavage planes (Fig 6c) The lack of any carbonate elsewhere in the rock texture clearly indicates diagenetic alteration on a local scale According to Boles Johnson (1984) biotite causes pH increase by attracting H+ to the mica surface and preferential growth of carbonate in the adjacent pore space that largely expands the phyllosilicate grain (see also Claeys Mount 1991) Later on Alcantar et al (2003) proved experimentally that calcite is precipitated between the cleavage planes from pore fluids with abundant Ca2+ even when the bulk solution is undersaturated with respect to CaCO3

Chloritization of detrital biotite (Fig 6e) feldspar (Fig 6f) amphibole and rock fragments is also a common replacement process in the Palaeogene sandstones The ions needed for formation of the authigenic chlorite were derived from the dissolution of heavy minerals volcanic rock fragments feldspar grains and matrix hematite as well as from clay diagenetic reactions and destabilization of organometallic complexes (see Anjos et al 2003 Worden Morad 2003 and references therein) Some researchers have demonstrated that chloritization of biotite occurs during deep burial and moreover contributes to the K+ supply for smectite transformation into illite (Claeys Mount 1991 Deschamps et al 2012) According to Morad Aldahan (1987b) the chloritization of feldspar is preceded by some dissolution of the replaced grain

Other minor products of replacement include Fe and Ti oxides within ferromagnesian minerals (Fig 4f) ferroan carbonate cements and authigenic pyrite The calcitization of detrital dolomite grains and the formation of abundant claysericite within plagioclase grains (Fig 6d) are particularly attributed to chemical reactions during weathering (cf Beacutetard et al 2009 Borrelli et al 2014)

Chemical compaction

Factors that control chemical compaction include stress fluid pressure temperature primary mineral com-position pore-fluid chemistry and time-temperature history although there is still much controversy over their relative importance (eg Renard et al 1997 Sheldon et al 2003 Bjoslashrlykke 2014) The mechanism commonly used to describe this late diagenetic process is pressure dissolution at grain boundaries with subsequent diffusion of solutes towards the pore space Mechanical compaction determines the IGV values before the onset of chemical compaction (Bjoslashrlykke Jahren 2012)

Pressure dissolution is poorly manifested in the studied rocks being significant only in the sandstones of Elska Member and Solashka Member (Fig 2) By analogy with the mechanical compaction this process

has mostly affected cement-free sandstones containing small amount of clay matrix (see Renard et al 1997) ie with low IGV values Convex-concave contacts between adjoining detrital grains commonly accom-panied by triple junctions at 120deg with straight-line boundaries (see Henares et al 2014) are typical microfabrics (Figs 3b 5b 6g Stefanov et al 2007 Plate IIIc) while sutured grain boundaries and stylolites were not formed due to the insufficient burial depth

DIAGENETIC SEQUENCE

The differentiation of three diagenetic zones ie eogenetic mesogenetic and telogenetic was proposed by Choquette Pray (1970) originally for carbonate rocks but was later applied also for sandstones (Schmidt Macdonald 1979 Morad 1998 Morad et al 2000 Worden Morad 2003) In this scheme the individual zones and associated diagenetic processes roughly correspond to three temporal stages (eodiage-netic mesodiagenetic and telodiagenetic) Eodiagenesis (early diagenesis) includes all processes that occur at or near the sediment surface where the composition of interstitial waters is controlled by the depositional environment and climate regime Diagenetic modi-fications take place in low temperature and pressure conditions at depths ranging from a few metres to hundreds of metres The upper temperature limit of mesodiagenesis (burial diagenesis) is defined as 70degC (Morad et al 2000) which is equivalent to about 2000 m burial depth in areas with normal geothermal gradient of 25-30degCkm (Allen Allen 2013) Shallow mesodia-genesis corresponds to temperatures reaching approx-imately 100 degC while deep mesodiagenesis extends to the onset of metamorphism (sim200 degC) The main factors that influence mesodiagenetic changes comprise basin thermal history primary mineralogy and fabric distribution of eodiagenetic alterations exchange of material with neighbouring lithologies formation water chemistry and presence of petroleum-related fluids (Worden Morad 2003) Telodiagenesis (uplift-related diagenesis hypergenesis) occurs when formerly buried deposits are uplifted close to or exposed on the surface where they are subjected to subaerial influence by meteoric water

The eodiagenetic alteration of the Palaeogene sandy deposits was characterized by initial mechanical compaction phreatic precipitation of calcite and ferroan dolomite cements carbonate replacement of detrital grains and probably some oxidationndashreduction reactions (Fig 7) Most diagenetic processes occurred during the succeeding mesodiagenetic stage Further mechanical compaction was accompanied andor postdated by formation of intergranular carbonate cements Meanwhile intensive modification of shales including compaction and temperature driven mineral transformations influenced the calcite and ferroan dolomite cementation In particular the mesodiagenetic alteration of organic-rich shales favoured the generation

18

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

Fig 7 Diagenetic sequence for the Palaeogene sandstones of Padesh Basin Dashed lines signify lower rate or only supposed development of the postdepositional processes Duration of the three diagenetic stages (after Morad et al 2000) is not to scale

of minor amount of secondary porosity and subsequent precipitation of kaolinite cement It is important to mention that the thermal maturation of kerogene in those black shales and the selective dissolution in adjacent sandstones postdated the carbonate cemen-tation Other mesodiagenetic processes include recrystallization of calcite and kaolinite transformation of kaolinite to dickite and some replacement reactions eg chloritization calcitization and dolomitization The latest processes of deep mesodiagenesis were chemical compaction and calcite precipitation in fractures The former may have continued during uplift at a slower rate as long as the temperature was higher than 70-80degC (see Bjoslashrlykke Jahren 2012) The final telodiagenetic stage was marked by oxidation of clastic grains authigenic pyrite and ferroan carbonate cements calcite replacement of detrital dolomite and formation of claysericite alteration products after plagioclase

IMPLICATIONS FROM THE BURIAL HISTORY

The most deeply buried Palaeogene deposits of the Padesh Basin comprise the siliciclastic rocks of Elska Member and Pilyovo Member which are exposed at present in the southern part of the basin Sandstones of the former unit show petrographic evidence for minor selective dissolution and kaolinite precipitation as a result of thermal maturation of kerogene in the adjacent black shales at temperatures exceeding 100deg C This conclusion about the palaeothermal conditions is further supported by the presence of dickite and observed effects of chemical compaction in the same sandstones because the kaolinite transformation to dickite indicates Tgt90-100degC (Ehrenberg et al 1993 McAulay et al 1994) and pressure dissolution becomes dominant in siliciclastic rocks at Tgt80-100degC (Bjoslashrlykke 2014) The total thickness of the Palaeogene succession

19

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

(Stefanov et al 2007) and the Neogene sedimentary cover (Milovanov et al 2009) suggests that the maximum burial depth of the basal strata of Suhostrel Formation was in the interval 2500ndash3000 m taking into account approximate corrections for decompaction and erosion (Stefanov 2011) Therefore it is clear that assuming a normal geothermal gradient and an average surface temperature of 15degC cannot explain the above mentioned diagenetic changes at the inferred burial depth Furthermore the short-range to long-range ordered illitesmectite (ie R=1 and Rge3) with 10-20 expandable layers recorded in the shales of Pilyovo Member (Stefanov Chatalov 2008) implies even higher maximum paleotemperatures (Stefanov 2011) Thus application of the semiquantitative illite-smectite geothermometer indicates temperatures of the R=1Rge3 transition mostly in the interval 140-180degС (Pollastro 1993 Abid et al 2004 Vaacutezquez et al 2014) Similar value is obtained from the vitrinite reflectance (Ro=112) determined in associated bituminous coal of the Pilyovo Member by Shishkov et al (1989) because using the time-independent approach and empirical formula Tmax=(lnRo+168)00124 for ldquonormalrdquo burial conditions from Barker Pawlewicz (1994) yields paleotemperature of 145degС The calculated geothermal gradient of the Padesh Basin based on the illite-smectite geothermometer is 50-66degСkm for maximum depth of 2500 m and 42-55degСkm for maximum depth of 3000 m respectively Apparently all these values reflect increased heat flow typical of hyperthermal sedimentary basins similarly to the ldquohotrdquo Pannonian Basin in Central Europe and some strike-slip basins in California (Allen Allen 2013)

CONCLUSIONS

The present study of sandstone diagenesis in the Padesh Basin complements previously obtained results re-garding the composition and provenance of the siliciclastic rocks The inferred diagenetic history is consistent with the specific depositional evolution of this Palaeogene strike-slip basin characterized by rapid sedimentation high subsidence rate and active tectonic regime The recognized diagenetic alterations are related to three temporal stages (eodiagenesis meso-diagenesis and telodiagenesis) which had different relative impact on the postdepositional modification of sediments The main diagenetic processes were mechanical compaction and cementation while the influence of dissolution recrystallization transfor-mation replacement and chemical compaction was of lesser significance In general the sandstone diagenesis was controlled by the depositional facies climate conditions primary mineralogy and fabric distribution and composition of interbedded shales chemistry of the pore fluids sedimentation and subsidence rates burial depth and basin thermal regime A very important factor during the mesodia-genetic stage was the high geothermal gradient as an effect of elevated heat flow

In this context the newly acquired data are in accordance with former studies of the associated shales and coal indicating a hyperthermal nature of the Padesh Basin Additional work should focus on the detailed investigation of carbonate cements with the aim to better constrain their timing temperature range and origin of the pore fluids

This contribution is among the few accomplished researches on siliciclastic diagenesis in strike-slip basins The obtained results about the burial history can be used in future studies of ldquohotrdquo sedimentary basins of similar geodynamic type and can be particularly applied for the evaluation of their hydrocarbon system potential Also the presented sedimentological evidence conclusively supports the long-standing hypothesis that thermal maturation of kerogene in carbonate-free organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature

Acknowledgements

The authors are grateful to the following colleagues for their participation in the sample preparation and analytical work B Popova (Thin section laboratory Faculty of Geology and Geography Sofia University ldquoSt Kliment Ohridskirdquo) N Dimitrov (SEM laboratory Faculty of Chemistry and Pharmacy Sofia University ldquoSt Kliment Ohridskirdquo) and H Stanchev (SEM laboratory Eurotest Control JSC Sofia)

Marlena Yaneva (Geological Institute Sofia) is thanked for the critical review of manuscript

REFERENCES

Abid IA Hesse R Harper JD 2004 Variations in mixed-layer illitesmectite diagenesis in the rift and post-rift sediments of the Jeanne drsquoArc Basin Grand Banks offshore Newfoundland Canada Canadian Journal of Earth Sciences 41 401-429

Ajdukiewicz JM Lander RH 2010 Sandstone reservoir quality prediction the state of the art American Associ-ation of Petroleum Geologists Bulletin 94 1083-1091

Alcantar N Israelachvili J Boles J 2003 Forces and ionic transport between mica surfaces Implication for pressure solution Geochimica et Cosmochimica Acta 67 1289-1304

Allen PA Allen JR 2013 Basin Analysis Principles and Application to Petroleum Play Assessment (3rd Edition) WileyndashBlackwell Oxford 632 pp

Alonso-Zarza AM Wright VP 2010 Calcretes In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Facies environments and processes Developments in Sedimentology 61 225-268

Al-Ramadan K Morad S Proust JN Al-Aasm I 2005 Distribution of diagenetic alterations in siliciclastic shore-face deposits within a sequence stratigraphic framework evidence from the Upper Jurassic Boulonnais NW France Journal of Sedimentary Research 75 943-959

Anjos SM C De Ros LF Souza RS Silva CMA Sombra CL 2000 Depositional and diagenetic controls

20

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

on the reservoir quality of Lower Cretaceous Pendecircncia sandstones Potiguar rift basin Brazil American Association of Petroleum Geologists Bulletin 84 1719-1742

Anjos SMC De Ros LF Silva CMA 2003 Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin offshore eastern Brazil In Worden RH Morad S (Eds) Clay Mineral Cements in Sandstones International Association of Sedimentologists Special Publications 34 291-316

Armenteros I 2010 Diagenesis of carbonates in continental settings In Alonso-Zarza AM Tanner LH (Eds) Carbonates in continental settings Geochemistry diage-nesis and applications Developments in sedimentology 62 62-151

Awwiler DN 1993 Illitesmectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the Texas Gulf Coast Paleocene-Eocene Journal of Sedimentary Petrology 63 501-512

Barker CE Pawlewicz MJ 1994 Calculation of vitrinite reflectance from thermal histories and peak temperatures A comparison of methods In Mukhopadhyay PK Dow WG (Eds) Vitrinite reflectance as a maturity parameter applications and limitations American Chemical Society Symposium Series 570 216-229

Bathurst RGC 1975 Carbonate sediments and their diagenesis (2nd edition) Elsevier AmsterdamndashLondonndashNew York 627 pp

Beaufort D Cassagnabere A Petit S Lanson B Berger G Lacharpagne JC Johansen H 1998 Kaolinite-to-dickite reaction in sandstone reservoirs Clay Minerals 33 297-316

Beckner J Mozley PS 1998 Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation Albuquerque Basin New Mexico USA In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 27-51

Belmustakov E 1948 Geology of the south part of Piyanets region ndash Southwestern Bulgaria Review of the Bulgarian Geological Society 20 1-63 (in Bulgarian with French abstract)

Bertier P Swennen R Lagrou D Laenen B Kemps R 2008 Palaeo-climate controlled diagenesis of the Westphalian C amp D fluvial sandstones in the Campine Basin (northeast Belgium) Sedimentology 55 1375-1417

Beacutetard F Caner L Gunnell Y Bourgeon G 2009 Illite neoformation in plagioclase during weathering evidence from semi-arid Northeast Brazil Geoderma 152 53-62

Biddle KT Christie-Blick N (Eds) 1985 Strike-slip deformation basin formation and sedimentation Society of Economic Paleontologists and Mineralogists Special Publications 37 386 pp

Bjoslashrlykke K 1984 Formation of secondary porosiry how important is it In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 277-286

Bjoslashrlykke K 1988 Sandstone diagenesis in relation to preservation destruction and creation of porosity In Chillingar GV Wolf KH (Eds) Diagenesis I Developments in Sedimentology 41 555-588

Bjoslashrlykke K 1993 Fluid flow in sedimentary basins Sedimentary Geology 86 137-158

Bjoslashrlykke K 1998 Clay mineral diagenesis in sedimentary basins ndash a key to the prediction of rock properties Examples

from the North Sea basin Clay Minerals 33 15-34 Bjoslashrlykke K 2014 Relationships between depositional

environments burial history and rock properties Some principal aspects of diagenetic process in sedimentary basins Sedimentary Geology 301 1-14

Bjoslashrlykke K Jahren J 2012 Open or closed geochemical systems during diagenesis in sedimentary basins Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs American Association of Petroleum Geologists Bulletin 96 2193-2214

Bjoslashrlykke K Ramm M Saigal GC 1989 Sandstone diagenesis and porosity modification during basin evolution Geologische Rundschau 78 243-268

Boggs S 2009 Petrology of Sedimentary Rocks (2nd edition) Cambridge University Press Cambridge 600 pp

Bojanowski MJ Barczuk A Wetzel A 2014 Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit Sudetes Mountains Poland) Sedimentology 61 1211-1239

Boles JR 1998 Carbonate cementation in Tertiary sandstones San Joaquin Basin California In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 261-284

Boles J R Franks SG 1979 Clay diagenesis in Wilcox sandstones of southwest Texas implications of smectite diagenesis on sandstone cementation Journal of Sedimentary Petrology 49 55-70

Boles JR Johnson KS 1984 Influence of mica surfaces on pore-water pH Chemical Geology 43 303-317

Bons PD Elburg MA Gomez-Rivas E 2012 A review of the formation of tectonic veins and their microstructures Journal of Structural Geology 43 33-62

Borrelli L Perri F Critelli S Gullagrave G 2014 Characterization of granitoid and gneissic weathering profiles of the Mucone basin (Calabria southern Italy) Catena 113 325-340

Braithwaite CJR 1989 Displacive calcite and grain breakage in sandstones Journal of Sedimentary Petrology 59 258-266

Buczynski C Chafetz HS 1987 Siliciclastic grain breakage and displacement due to carbonate crystal growth an example from the Leuders Formation (Permian) of north-central Texas USA Sedimentology 34 837-843

Burley SD Kantorowicz JD 1986 Thin-section and SEM textural criteria for the recognition of cement-dissolution porosity in sandstones Sedimentology 33 587-604

Calvo R Ayalon A Bein A Sass E 2011 Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel) Journal of Geochemical Exploration 108 88-98

Cantarero I Zafra CJ Traveacute A Martiacuten-Martiacuten JD Baqueacutes V Playagrave E 2014 Fracturing and cementation of shallow burried Miocene proximal alluvial fan deposits Marine and Petroleum Geology 55 87-99

Chatalov A 2006 Calcrete paleosols in the Upper Bunt-sandstein from the Iskur River gorge Northwestern Bulgaria Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 239 1-35

Choquette PW James NP 1987 Diagenesis 12 Limestones - The deep burial environment Geoscience Canada 14 3-35

Choquette PW Pray LC 1970 Geologic nomenclature and

21

classification of porosity in sedimentary carbonates American Association of Petroleum Geologists Bulletin 54 207-250

Chuhan FA Kjeldstad A Bjoslashrlykke K Hoslasheg K 2003 Experimental compression of loose sands relevance to porosity reduction during burial in sedimentary basins Canadian Geotechnical Journal 40 995-1011

Claeys PF Mount JF 1991 Diagenetic origin of carbonate sulfide and oxide inclusions in biotites of the Great Valley Group (Cretaceous) Sacramento Valley California Journal of Sedimentary Petrology 61 719-731

Crossey LJ Surdam RS Lahann RW 1986 Application of organicinorganic diagenesis to porosity prediction In Gautier D (Ed) Roles of organic matter in sediment diagenesis Society of Economic Paleontologists and Mineralogists Special Publications 38 147-155

Crowell J (Ed) 2003 Evolution of the Ridge Basin Southern California an interplay of sedimentation and tectonics Geological Society of America Special Papers 367 247 pp

Cuadros J Vega R Toscano A Arroyo X 2014 Kaolinite transformation into dickite during burial diagenesis American Mineralogist 99 681-695

De Bona J Dani N Ketzer JM De Ros LF 2008 Dickite in shallow oil reservoirs from Reconcavo Basin Brazil diagenetic implications for basin evolution Clay Minerals 43 213-233

Deschamps R Kohler E Gasparrini M Durand O Euzen T Nader F 2012 Impact of mineralogy and diagenesis on reservoir quality of the Lower Cretaceous Upper Mannville Formation (Alberta Canada) Oil amp Gas Science and Technology Revue IFP Energies Nouvelles 67 31-58

Dickson JAD 1965 A modified staining technique for carbonates in thin section Nature 205 p 587

Ehrenberg SN Aagaard P Wilson MJ Fraser AR Duthie DML 1993 Depth-dependent transformation of kaolinite to dickite in sandstones of the Norwegian continental shelf Clay Minerals 28 325-352

Ehrenberg SN Walderhaug O Bjoslashrlykke K 2012 Carbonate porosity creation by mesogenetic dissolution reality or illusion American Association of Petroleum Geologists Bulletin 96 217-233

El-Ghali MAK Mansurbeg H Morad S Al-Aasm I Ramseyer K 2006 Distribution of diagenetic alterations in glaciogenic arenites within depositional facies and sequence stratigraphic framework evidence from the Upper Ordovician of the Murzuk Basin SW Libya Sedimentary Geology 190 323-351

Estupintildean J Marfil R Delgado A Permanyer A 2007 The impact of carbonate cements on the reservoir quality in the Napo Formation sandstones (Cretaceous Oriente Basin Ecuador) Geologica Acta 5 89-107

Estupintildean J Marfil R Scherer M Permanyer A 2010 Reservoir sandstones of the Cretaceous Napo Formation U and T members in the Oriente Basin Ecuador links between diagenesis and sequence stratigraphy Journal of Petroleum Geology 33 221-246

Evamy BD 1963 The application of chemical staining tech-nique to a study of dedolomitization Sedimentology 2 164-170

Fawad M Mondol NH Jahren J Bjoslashrlykke K 2011 Mechanical compaction and ultrasonic velocity of sands with different texture and mineralogical composition Geophysical Prospecting 59 697-720

Fialips C-I Majzlan J Beaufort D Navrotsky A 2003 New thermochemical evidence on the stability of dickite

vs kaolinite American Mineralogist 88 837-845 Folk RL 1974 The natural history of crystalline calcium

carbonate effect of magnesium content and salinity Journal of Sedimentary Petrology 44 40-53

Folk RL Andrews PB Lewis DW 1970 Detrital sedimentary rock classification and nomenclature for use in New Zealand New Zealand Journal of Geology and Geophysics 13 937-968

Franccedila AB Arauacutejo LM Maynard JB Potter PE 2003 Secondary porosity formed by deep meteoric leaching Botucatu eolianite southern South America American Association of Petroleum Geologists Bulletin 87 1073-1082

Franks SG Forester RW 1984 Relationships between secondary porosity pore-fluid chemistry and carbon dioxide Texas Gulf Coast In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 63-79

Garcia AGV Morad S De Ros LF Al-Aasm IS 1998 Palaeogeographical palaeoclimatic and burial history controls on the diagenetic evolution of reservoir sandstones Evidence from the Lower Cretaceous Serraria sandstones in the Sergipe-Alagoas Basin NE Brazil In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentlogists Special Publications 26107-140

Gier S Worden RH Johns WD Kurzweil H 2008 Diagenesis and reservoir quality of Miocene sandstones in the Vienna Basin Austria Marine and Petroleum Geology 25 681-695

Giles MR de Boer RB 1990 Origin and significance of redistributional secondary porosity Marine and Petroleum Geology 7 378-397

Giles MR Marshall JD 1986 Constraints on the develop-ment of secondary porosity in the subsurface re-evaluation of processes Marine and Petroleum Geology 3 243-255

Hall JS Mozley P Davis MJ Roy DN 2004 Environments of formation and controls on spatial distribution of calcite cementation in PliondashPleistocene fluvial deposits New Mexico USA Journal of Sedimentary Research 74 643-653

Heald MT Renton JJ 1966 Experimental study on sandstone cementation Journal of Sedimentary Petrology 36 977-991

Henares S Caracciolo C Cultrone G Fernaacutendez J Viseras C 2014 The role of diagenesis and depositional facies on pore system evolution in a Triassic outcrop analogue (SE Spain) Marine and Petroleum Geology 51 136-151

Hesse R Abid IA 1998 Carbonate cementation ndash the key to reservoir properties of four sandstone levels (Cretaceous) in the Hibernia Oilfield Jeanne drsquoArc Basin Newfoundland Canada In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 363-393

Heydari E 2000 Porosity loss fluid flow and mass transfer in limestone reservoirs application to the Upper Jurassic Smackover Formation Mississippi American Association of Petroleum Geologists Bulletin 84 100-118

Hower J Eslinger EV Hower ME Perry EA 1976 Mechanism of burial metamorphism in argillaceous sediment 1 Mineralogical and chemical evidence Bulletin of the Geological Society of America 87 725-737

Hutcheon I Abercrombie H 1990 Carbon dioxide in clastic rocks and silicate hydrolysis Geology 18 541-544

22

Hutcheon I Shevalier M Abercrombie H 1993 pH buffering by metastable mineral-fluid equilibria and evolution of carbon dioxide fugacity during burial diagenesis Geochimica et Cosmochimica Acta 57 1017-1027

Hyde RS Miller HG Hiscott RN Wright JA 1988 Basin architecture and thermal maturation in the strike-slip Deer Lake Basin Carboniferous of Newfoundland Basin Research 1 85-105

Ivanov Z 1998 Tetctonics of Bulgaria Habilitation thesis Sofia University ldquoSt Kliment Ohridskirdquo Sofia 634 pp (in Bulgarian)

Ivanov R Chernyavska S 1971 On the age of Paleogene volcanism in Western Bulgaria based on data from geological-petrological and palinological studies I Suho-strel-Padesh Paleogene Proceedings of the Geological Institute Series Stratigraphy and Lithology 20 71-86 (in Bulgarian with Russian and English abstracts)

Kantorowicz JD 1985 The origin of authigenic ankerite from the Ninian Field UK North Sea Nature 315 214-216

Ketzer JM Morad S Nystuen JP De Ros LF 2003 The role of the Cimmerian unconformity (Early Cretaceous) in the kaolinitization and related reservoir-quality evolution in Triassic sandstones of the Snorre Field North Sea In Worden RH Morad S (Eds) Clay mineral cements in sandstones International Association of Sedimentologists Special Publications 34 361-382

Khalaf FI 2007 Occurrences and genesis of calcrete and dolocrete in the Mio-Pleistocene fluvial sequence in Kuwait northeast Arabian Peninsula Sedimentary Geology 199 129-139

Khalifa M Morad S 2012 Impact of structural setting on diagenesis of fluvial and tidal sandstones The Bahi Formation Upper Cretaceous NW Sirt Basin North Central Libya Marine and Petroleum Geology 38 211-231

Kim JC Lee YI Hisada K 2007 Depositional and compositional controls on sandstone diagenesis the Tetori Group (Middle Jurassic-Early Cretaceous) Central Japan Sedimentary Geology 195 183-202

Kounov A 2002 Thermotectonic evolution of Kraishte Western Bulgaria PhD Thesis Swiss Federal Institute of Technology Zuumlrich 219 pp

Lacroix B Buatier M Labaume P Traveacute A Dubois M Charpentier D Ventalon S Convert-Gaubier D 2011 Microtectonic and geochemical characterization of thrusting in a foreland basin Example of the south-Pyrenean orogenic wedge (Spain) Journal of Structural Geology 33 1359-1377

Lander RH Walderhang O 1999 Predicting porosity through simulating sandstone composition and quartz cementation American Association of Petroleum Geologists Bulletin 84 433-449

Lanson B Beaufort D Berger G Bauer A Cassagnabere A Meunier A 2002 Authigenic kaolin and illitic minerals during burial diagenesis of sandstones a review Clay Minerals 37 1-22

Li Q Jiang Z Liu K Zhang C You X 2014 Factors controlling reservoir properties and hydrocarbon accumulation of lacustrine deep-water turbidites in the Huimin Depression Bohai Bay Basin East China Marine and Petroleum Geology 57 327-344

Loucks RG 2005 Revisiting the importance of secondary dissolution pores in Tertiary sandstones along the Texas Gulf Coast Gulf Coast Association of Geological

Societies Transactions 55 448-455 Lundegard PD 1992 Sandstone porosity loss ndash a lsquobig

picturersquo view of the importance of compaction Journal of Sedimentary Petrology 62 250-260

Lynch FL 1997 Frio Shale mineralogy and the stoichiometry of the smectite-to-illite reaction the most important reaction in clastic sedimentary diagenesis Clays and Clay Minerals 45 618-631

Lynch FL Mack LE Land LS 1997 Burial diagenesis of illitesmectite in shales and the origins of authigenic quartz and secondary porosity in sandstones Geochimica et Cosmochimica Acta 61 1995-2006

Macaulay CI Haszeldine RS Fallick AE 1993 Distribution chemistry isotopic composition and origin of diagenetic carbonates Magnus Sandstone North sea Journal of Sedimentary Petrology 63 33-43

MacGowan DB Surdam RC 1990 Carboxylic acid anions in formation waters San Joaquin Basin and Louisiana Gulf Coast USA Implications for clastic diagenesis Applied Geochemistry 5 687-701

Maliva RG Siever R 1988 Diagenetic replacement controlled by force of crystallization Geology 16 688-691

Mann P 2012 Comparison of structural styles and giant hydrocarbon occurrences within four active strike-slip regions California Southern Caribbean Sumatra and East China In Gao D (Ed) Tectonics and sedimentation Implications for petroleum systems American Association of Petroleum Geologists Memoirs 100 43-93

Mansurbeg H Caja MA Marfil RM Remacha SE Garcia D Martiacuten-Crespo T El-Ghali MAK Nystuen JP 2009 Diagenetic evolution and porosity destruction of turbiditic hybrid arenites and siliciclastic sandstones of foreland basins Evidence from the Eocene Hecho Group Pyrenees Spain Journal of Sedimentary Research 79 711-735

Mansurbeg H Morad S Salem A Marfil R El-ghali MAK Nystuen JP Caja MA Amorosi A Garcia D La Iglesia A 2008 Diagenesis and reservoir quality evolution of Palaeocene deep-water marine sandstones the Shetland-Faroes Basin British continental shelf Marine and Petroleum Geology 25 514-543

Mansurbeg H Morad S Plink-Bjoumlrklund P El-Ghali MAK Caja MA Marfil R 2013 Diagenetic alterations related to falling stage and lowstand systems tracts of shelf slope and basin floor sandstones (Eocene Central Basin Spitsbergen) In Morad S Ketzer M de Ros LF (Eds) Linking diagenesis to sequence stra-tigraphy International Association of Sedimentologists Special Publications 45 353-378

Marfil R Delgado A Rossi C La Iglesia A Ramseyer K 2003 Origin and diagenetic evolution of kaolin in reservoir sandstones and associated shales of the Jurassic and Cretaceous Salam Field Western Desert (Egypt) In Worden RH Morad S (Eds) Clay mineral cements in sandstones International Association of Sedimentologists Special Publications 34 319-342

Martiacuten-Martiacuten JD Goacutemez-Gras D Sanfeliu T Thiry M Ruiz-Cruz MD Franco F 2007 Extensive dickitization of the Permo-Triassic fluvial sandstones from the Eastern Iberian Ranges Spain Clays and Clay Minerals 55 481-490

McAulay GE Burley SD Fallick AE Kusznir NJ 1994 Palaeohydrodynamic fluid flow regimes during diagenesis of the Brent Group in the HuttonndashNW Hutton

23

reservoir constraints from oxygen isotope studies of authigenic kaolin and reserves flexural modeling Clay Minerals 29 609-629

McHargue TR Price RC 1982 Dolomite from clay in argillaceous marine associated carbonates Journal of Sedimentary Petrology 52 873-886

Milliken KL 1998 Carbonate diagenesis in non-marine foreland sandstones at the western edge of the Alleghanian overthrust belt southern Appalachians In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 87-105

Milliken KL 2003 Microscale distribution of kaolinite in Breathitt Formation sandstones (middle Pennsylvanian) implications for mass balance In Worden RH Morad S (Eds) Clay mineral cements in sandstones International Association of Sedimentologists Special Publications 34 343-360

Milliken ML 2005 Late diagenesis and mass transfer in sandstone-shale sequences In Mackenzie FT (Ed) Sediments diagenesis and sedimentary rocks Treatise on Geochemistry 7 ElsevierndashPergamon Oxford 159-190

Milliken KL Land LS 1993 The origin and fate of silt sized carbonate in subsurface Miocene-Oligocene mudstones south Texas Gulf Coast Sedimentology 40 107-124

Milliken KL McBride EF Cavazza W Cibin U Fontana D Picard MD Zuffa GG 1998 Geochemical history of calcite precipitation in Tertiary sandstones northern Apennines Italy In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 213-239

Milovanov P Petrov I Zhelev V Sinyovski D Marinova A Klimov I Valev V Ichev M Ilieva E 2009 Geological map of the Republic of Bulgaria 150 000 Map sheet Delchevo and Simitli Apis 50 Sofia

Mondol NH Bjoslashrlykke K Jahren J K Hoslasheg K 2007 Experimental mechanical compaction of clay mineral aggregates ndash changes in physical properties of mudstones during burial Marine and Petroleum Geology 24 289-311

Morad S 1998 Carbonate cementation in sandstones distribution patterns and geochemical evolution In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentlogists Special Publications 26 1-26

Morad S De Ros LF Nystuen JP Bergan M 1998 Carbonate diagenesis and porosity evolution in sheet-flood sandstones Evidence from the Middle and Lower Lunde Members (Triassic) in the Snorre Field Norwegian North Sea In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 53-85

Morad S Al-Aasm IS Sirat M Sattar MM 2010a Vein calcite in Cretaceous carbonate reservoirs of Abu Dhabi Record of origin of fluids and diagenetic conditions Journal of Geochemical Exploration 106 156-170

Morad S Al-Ramadan K Ketzer JM de Ros LF 2010b The impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy American Association of Petroleum Geologists Bulletin 94 1267-1309

Morad S Aldahan AA 1987a A SEM study of diagenetic kaolitization and illitization of detrital feldspars in sandstones Clay Minerals 22 237-243

Morad S Aldahan AA 1987b Diagenetic chloritization of feldspars in sandstones Sedimentary Geology 51 155-164

Morad S De Ros LF 1994 Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group Murchison Field North Viking Graben (northern North Sea ndash comment) Sedimentary Geology 93 135-141

Morad S Ketzer JK De Ros LF 2000 Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks implications for mass transfer in sedimentary basins Sedimentology 47 (Suppl 1) 95-120

Morad S Ketzer JK De Ros LF 2012 Linking diagenesis to sequence stratigraphy An integrated tool for understanding and predicting reservoir quality distribution In Morad S Ketzer JK De Ros LF (Eds) Linking diagenesis to sequence stratigraphy International Association of Sedimentlogists Special Publications 45 1-36

Muchez P Viaene W Dusar M 1992 Diagenetic control on secondary porosity in flood plain deposits an example of the Lower Triassic of northeastern Belgium Sedimentary Geology 78 285-298

Nedkvitne T Bjoslashrlykke K 1992 Secondary porosity in the Brent Group (Middle Jurassic) Huldra Field North sea implications for predicting lateral continuity of sandstones Journal of Sedimentary Petrology 62 23-34

Nilsen TH Sylvester AG 1995 Strike-slip basins In Busby CJ Ingersoll RV (Eds) Tectonics of sedimentary Basins Blackwell Cambridge 425-457

Ochoa M Arribas J 2005 Diagenetic paths in the margin of a Triassic Basin NW zone of the Iberian Chain Spain International Journal of Earth Sciences 94 256-266

Osborne M Haszeldine RS Fallick AE 1994 Variation in kaolin morphology with growth temperature in isotopically mixed pore-fluids Brent Group UK North Sea Clay Minerals 29 591-608

Parcerisa D Goacutemez-Gras D Traveacute A 2005 A model of early calcite cementation in alluvial fans evidence from the Burdigalian sandstones and limestones of the Vallegraves-Penedegraves half-graben (NE Spain) Sedimentary Geology 178 197-217

Paxton ST Szabo JO Ajdukiewicz JM Klimentidis RE 2002 Construction of an intergranular volume compaction curve for evaluating and predicting compaction and porosity loss in rigid-grain sandstone reservoirs American Association of Petroleum Geologists Bulletin 86 2047-2067

Pittman ED Larese RE 1991 Compaction of lithic sands experimental results and applications American Association of Petroleum Geologists Bulletin 75 1279-1299

Platt JD 1993 Controls on the clay mineral distribution and chemistry in the early Permian Rotliegend of Germany Clay Minerals 28292-416

Playagrave E Traveacute A Caja MA Salas R 2010 Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well Lower Cretaceous Valencia Trough Spain) Geofluids 10 314-333

Pollastro RM 1993 Considerations and applications of the illitesmectite geothermometer in hydrocarbon-bearing rocks of Miocene to Mississippian age Clays and Clay Minerals 41119-133

Putnis A 2009 Mineral replacement reactions Reviews in Mineralogy and Geochemistry 70 87-124

24

Renard F Ortoleva P Gratier JP 1997 Pressure solution in sandstones Influence of clays and dependence on temperature and stress Tectonophysics 280 257-266

Richter DK 1985 Die Dolomite der Evaporit- und der Dolcrete-Playasequenz im mittleren Keuper bei Coburg (NE-Bayern) Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 170 87-128

Ridgway KD DeCelles PG 1993 Petrology of mid-Cenozoic strike-slip basins in an accretionary orogen St Elias Mountains Yukon Territory Canada In Johnsson MJ Basu A (Eds) Processes controlling the composition of clastic sediments Geological Society of America Special Papers 284 67-89

Rossi C Alaminos A 2014 Evaluating the mechanical compaction of quartzarenites The importance of sorting (Llanos foreland basin Colombia) Marine and Petroleum Geology 56 222-238

Sachsenhofer RF Rantitsch G Hasenhuumlttl C Russegger B Jelen B 1998 Smectite to illite diagenesis in Early Miocene sediments from the hyperthermal western Pannonian Basin Clay Minerals 33 523-537

Saigal GC Bjoslashrlykke K 1987 Carbonate cements in clastic reservoir rocks from offshore Norway ˗ relationships between isotopic composition textural development and burial depth In Marshall JD (Ed) Diagenesis of sedimentary sequences Geological Society London Special Publications 36 313-324

Sarkisyan SG 1972 Origin of authigenic clay minerals and their significance in petroleum geology Sedimentary Geology 7 1-22

Schmid S Worden RH Fisher QJ 2006 Sedimentary facies and the context of dolocrete in the Lower Triassic Sherwood Sandstone Group Corrib Field west of Ireland Sedimentary Geology 187 205-227

Schmidt V McDonald DA 1979 Texture and recognition of secondary porosity in sandstones In Scholle PA Schluger PR (Eds) Aspects of diagenesis Society of Economic Paleontologists and Mineralogists Special Publications 26 209-225

Schoumlner R Gaupp R 2005 Contrasting red bed diagenesis the southern and northern margin of the Central European Basin International Journal of Earth Sciences 94 897-916

Scotchman IC Jones LH Miller RS 1989 Clay diagenesis and oil migration in Brent Group sandstones of northwestern Hutton Field UK North Sea Clay Minerals 24 339-374

Shalaby MR Hakimi MH Abdullah WH 2014 Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin northern Western Desert Egypt Geological Journal 49 239-255

Shanmugam G 1984 Secondary porosity in sandstones basic contributions of Chepikov and Savkevich American Association of Petroleum Geologists Bulletin 68 106-107

Sheldon HAWheeler JWorden RH Cheadle MJ 2003 An analysis of the roles of stress temperature and pH in chemical compaction of sandstones Journal of Sedimentary Research 73 64-71

Shishkov G Valcheva S Minchev D Salabasheva V Stefanova Y 1989 Petrology of the coal basins and deposits in Bulgaria Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 78 livre 1 ndash Geacuteologie II 122-137 (in Bulgarian with English and Russian abstracts)

Sliaupa S Cyziene J Molenaar N Musteikyte D 2008 Ferroan dolomite cement in Cambrian sandstones burial

history and hydrocarbon generation of the Baltic sedimentary basin Acta Geologica Polonica 58 27-41

Smith JT Ehrenberg SN 1989 Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium Marine and Petroleum Geology 6 129-135

Smosna R 1989 Compaction law for Cretaceous sandstones of Alaskarsquos North slope Journal of Sedimentary Petrology 59 572-584

Smosna R Bruner K 1997 Depositional controls over po-rosity development in lithic sandstones of the Appalachian Basin reducing exploration risk In Kupecz JA Gluyas J Bloch S (Eds) Reservoir quality prediction in sandstones and carbonates American Association of Petroleum Geologists Memoirs 69 249-265

Souza RS Silva CM 1998 Origin and timing of carbonate cementation of the Namorado Sandstone (Cretaceous) Albacora Field Brazil implications for oil recovery In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 309-325

Spoumltl C Wright VP 1992 Groundwater dolocretes from the Upper Triassic of the Paris Basin France a case study of an arid continental diagenetic facies Sedimentology 39 1119-1136

Stefanov Y 2011 Sedimentology of Paleogene rocks of the Padesh Basin Southwestern Bulgaria PhD thesis Sofia University ldquoSt Kliment Ohridskirdquo Sofia 183 pp (in Bulgarian)

Stefanov Y Chatalov A 2007 Formation of high-Mg calcite pisoids and ooids in brackish-water environment an example from the Priabonian sequence in the Padesh basin Southwestern Bulgaria Comptes rendus de lrsquoAcadeacutemie bulgare des Sciences 60 291-297

Stefanov Y Chatalov A 2008 Mineral composition of the clay fraction in the Paleogene shales from the Padesh Basin southwestern Bulgaria In Proceedings ldquo60 years Geology in the Sofia University ldquoSt Kliment Ohridskirdquo Sofia University Publishing House 145-150 (in Bulgarian)

Stefanov Y Chatalov A Yaneva M 2007 Petrographic composition and provenance analysis of the Paleogene clastic rocks from the Padesh Basin I Petrographic composition of the clastic rocks Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 100 livre 1 ndash Geacuteologie 283-335 (in Bulgarian with English abstract)

Stefanov Y Chatalov A Yaneva M 2008 Petrographic composition and provenance analysis of the Paleogene clastic rocks from the Padesh Basin II Provenance analysis Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 101 livre 1 ndash Geacuteologie 57-77 (in Bulgarian with English abstract)

Surdam RC Boese SW Crossey LJ 1984 The chemistry of secondary porosity In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 129-149

Surdam RC Crossey LJ Hagen ES Heasler HP 1989 Organic-inorganic interactions and sandstone diagenesis American Association of Petroleum Geologists Bulletin 73 1-23

Taylor TR Giles MR Hathon LA Diggs TN Braunsdorf NR Birbiglia GV Kittridge MG Macaulay CI Espejo IS 2010 Sandstone diagenesis and reservoir quality prediction models myths and reality American Association of Petroleum Geologists

25

reservoir constraints from oxygen isotope studies of authigenic kaolin and reserves flexural modeling Clay Minerals 29 609-629

McHargue TR Price RC 1982 Dolomite from clay in argillaceous marine associated carbonates Journal of Sedimentary Petrology 52 873-886

Milliken KL 1998 Carbonate diagenesis in non-marine foreland sandstones at the western edge of the Alleghanian overthrust belt southern Appalachians In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 87-105

Milliken KL 2003 Microscale distribution of kaolinite in Breathitt Formation sandstones (middle Pennsylvanian) implications for mass balance In Worden RH Morad S (Eds) Clay mineral cements in sandstones International Association of Sedimentologists Special Publications 34 343-360

Milliken ML 2005 Late diagenesis and mass transfer in sandstone-shale sequences In Mackenzie FT (Ed) Sediments diagenesis and sedimentary rocks Treatise on Geochemistry 7 ElsevierndashPergamon Oxford 159-190

Milliken KL Land LS 1993 The origin and fate of silt sized carbonate in subsurface Miocene-Oligocene mudstones south Texas Gulf Coast Sedimentology 40 107-124

Milliken KL McBride EF Cavazza W Cibin U Fontana D Picard MD Zuffa GG 1998 Geochemical history of calcite precipitation in Tertiary sandstones northern Apennines Italy In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 213-239

Milovanov P Petrov I Zhelev V Sinyovski D Marinova A Klimov I Valev V Ichev M Ilieva E 2009 Geological map of the Republic of Bulgaria 150 000 Map sheet Delchevo and Simitli Apis 50 Sofia

Mondol NH Bjoslashrlykke K Jahren J K Hoslasheg K 2007 Experimental mechanical compaction of clay mineral aggregates ndash changes in physical properties of mudstones during burial Marine and Petroleum Geology 24 289-311

Morad S 1998 Carbonate cementation in sandstones distribution patterns and geochemical evolution In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentlogists Special Publications 26 1-26

Morad S De Ros LF Nystuen JP Bergan M 1998 Carbonate diagenesis and porosity evolution in sheet-flood sandstones Evidence from the Middle and Lower Lunde Members (Triassic) in the Snorre Field Norwegian North Sea In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 53-85

Morad S Al-Aasm IS Sirat M Sattar MM 2010a Vein calcite in Cretaceous carbonate reservoirs of Abu Dhabi Record of origin of fluids and diagenetic conditions Journal of Geochemical Exploration 106 156-170

Morad S Al-Ramadan K Ketzer JM de Ros LF 2010b The impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy American Association of Petroleum Geologists Bulletin 94 1267-1309

Morad S Aldahan AA 1987a A SEM study of diagenetic kaolitization and illitization of detrital feldspars in sandstones Clay Minerals 22 237-243

Morad S Aldahan AA 1987b Diagenetic chloritization of feldspars in sandstones Sedimentary Geology 51 155-164

Morad S De Ros LF 1994 Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group Murchison Field North Viking Graben (northern North Sea ndash comment) Sedimentary Geology 93 135-141

Morad S Ketzer JK De Ros LF 2000 Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks implications for mass transfer in sedimentary basins Sedimentology 47 (Suppl 1) 95-120

Morad S Ketzer JK De Ros LF 2012 Linking diagenesis to sequence stratigraphy An integrated tool for understanding and predicting reservoir quality distribution In Morad S Ketzer JK De Ros LF (Eds) Linking diagenesis to sequence stratigraphy International Association of Sedimentlogists Special Publications 45 1-36

Muchez P Viaene W Dusar M 1992 Diagenetic control on secondary porosity in flood plain deposits an example of the Lower Triassic of northeastern Belgium Sedimentary Geology 78 285-298

Nedkvitne T Bjoslashrlykke K 1992 Secondary porosity in the Brent Group (Middle Jurassic) Huldra Field North sea implications for predicting lateral continuity of sandstones Journal of Sedimentary Petrology 62 23-34

Nilsen TH Sylvester AG 1995 Strike-slip basins In Busby CJ Ingersoll RV (Eds) Tectonics of sedimentary Basins Blackwell Cambridge 425-457

Ochoa M Arribas J 2005 Diagenetic paths in the margin of a Triassic Basin NW zone of the Iberian Chain Spain International Journal of Earth Sciences 94 256-266

Osborne M Haszeldine RS Fallick AE 1994 Variation in kaolin morphology with growth temperature in isotopically mixed pore-fluids Brent Group UK North Sea Clay Minerals 29 591-608

Parcerisa D Goacutemez-Gras D Traveacute A 2005 A model of early calcite cementation in alluvial fans evidence from the Burdigalian sandstones and limestones of the Vallegraves-Penedegraves half-graben (NE Spain) Sedimentary Geology 178 197-217

Paxton ST Szabo JO Ajdukiewicz JM Klimentidis RE 2002 Construction of an intergranular volume compaction curve for evaluating and predicting compaction and porosity loss in rigid-grain sandstone reservoirs American Association of Petroleum Geologists Bulletin 86 2047-2067

Pittman ED Larese RE 1991 Compaction of lithic sands experimental results and applications American Association of Petroleum Geologists Bulletin 75 1279-1299

Platt JD 1993 Controls on the clay mineral distribution and chemistry in the early Permian Rotliegend of Germany Clay Minerals 28292-416

Playagrave E Traveacute A Caja MA Salas R 2010 Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well Lower Cretaceous Valencia Trough Spain) Geofluids 10 314-333

Pollastro RM 1993 Considerations and applications of the illitesmectite geothermometer in hydrocarbon-bearing rocks of Miocene to Mississippian age Clays and Clay Minerals 41119-133

Putnis A 2009 Mineral replacement reactions Reviews in Mineralogy and Geochemistry 70 87-124

24

Renard F Ortoleva P Gratier JP 1997 Pressure solution in sandstones Influence of clays and dependence on temperature and stress Tectonophysics 280 257-266

Richter DK 1985 Die Dolomite der Evaporit- und der Dolcrete-Playasequenz im mittleren Keuper bei Coburg (NE-Bayern) Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 170 87-128

Ridgway KD DeCelles PG 1993 Petrology of mid-Cenozoic strike-slip basins in an accretionary orogen St Elias Mountains Yukon Territory Canada In Johnsson MJ Basu A (Eds) Processes controlling the composition of clastic sediments Geological Society of America Special Papers 284 67-89

Rossi C Alaminos A 2014 Evaluating the mechanical compaction of quartzarenites The importance of sorting (Llanos foreland basin Colombia) Marine and Petroleum Geology 56 222-238

Sachsenhofer RF Rantitsch G Hasenhuumlttl C Russegger B Jelen B 1998 Smectite to illite diagenesis in Early Miocene sediments from the hyperthermal western Pannonian Basin Clay Minerals 33 523-537

Saigal GC Bjoslashrlykke K 1987 Carbonate cements in clastic reservoir rocks from offshore Norway ˗ relationships between isotopic composition textural development and burial depth In Marshall JD (Ed) Diagenesis of sedimentary sequences Geological Society London Special Publications 36 313-324

Sarkisyan SG 1972 Origin of authigenic clay minerals and their significance in petroleum geology Sedimentary Geology 7 1-22

Schmid S Worden RH Fisher QJ 2006 Sedimentary facies and the context of dolocrete in the Lower Triassic Sherwood Sandstone Group Corrib Field west of Ireland Sedimentary Geology 187 205-227

Schmidt V McDonald DA 1979 Texture and recognition of secondary porosity in sandstones In Scholle PA Schluger PR (Eds) Aspects of diagenesis Society of Economic Paleontologists and Mineralogists Special Publications 26 209-225

Schoumlner R Gaupp R 2005 Contrasting red bed diagenesis the southern and northern margin of the Central European Basin International Journal of Earth Sciences 94 897-916

Scotchman IC Jones LH Miller RS 1989 Clay diagenesis and oil migration in Brent Group sandstones of northwestern Hutton Field UK North Sea Clay Minerals 24 339-374

Shalaby MR Hakimi MH Abdullah WH 2014 Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin northern Western Desert Egypt Geological Journal 49 239-255

Shanmugam G 1984 Secondary porosity in sandstones basic contributions of Chepikov and Savkevich American Association of Petroleum Geologists Bulletin 68 106-107

Sheldon HAWheeler JWorden RH Cheadle MJ 2003 An analysis of the roles of stress temperature and pH in chemical compaction of sandstones Journal of Sedimentary Research 73 64-71

Shishkov G Valcheva S Minchev D Salabasheva V Stefanova Y 1989 Petrology of the coal basins and deposits in Bulgaria Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 78 livre 1 ndash Geacuteologie II 122-137 (in Bulgarian with English and Russian abstracts)

Sliaupa S Cyziene J Molenaar N Musteikyte D 2008 Ferroan dolomite cement in Cambrian sandstones burial

history and hydrocarbon generation of the Baltic sedimentary basin Acta Geologica Polonica 58 27-41

Smith JT Ehrenberg SN 1989 Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium Marine and Petroleum Geology 6 129-135

Smosna R 1989 Compaction law for Cretaceous sandstones of Alaskarsquos North slope Journal of Sedimentary Petrology 59 572-584

Smosna R Bruner K 1997 Depositional controls over po-rosity development in lithic sandstones of the Appalachian Basin reducing exploration risk In Kupecz JA Gluyas J Bloch S (Eds) Reservoir quality prediction in sandstones and carbonates American Association of Petroleum Geologists Memoirs 69 249-265

Souza RS Silva CM 1998 Origin and timing of carbonate cementation of the Namorado Sandstone (Cretaceous) Albacora Field Brazil implications for oil recovery In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 309-325

Spoumltl C Wright VP 1992 Groundwater dolocretes from the Upper Triassic of the Paris Basin France a case study of an arid continental diagenetic facies Sedimentology 39 1119-1136

Stefanov Y 2011 Sedimentology of Paleogene rocks of the Padesh Basin Southwestern Bulgaria PhD thesis Sofia University ldquoSt Kliment Ohridskirdquo Sofia 183 pp (in Bulgarian)

Stefanov Y Chatalov A 2007 Formation of high-Mg calcite pisoids and ooids in brackish-water environment an example from the Priabonian sequence in the Padesh basin Southwestern Bulgaria Comptes rendus de lrsquoAcadeacutemie bulgare des Sciences 60 291-297

Stefanov Y Chatalov A 2008 Mineral composition of the clay fraction in the Paleogene shales from the Padesh Basin southwestern Bulgaria In Proceedings ldquo60 years Geology in the Sofia University ldquoSt Kliment Ohridskirdquo Sofia University Publishing House 145-150 (in Bulgarian)

Stefanov Y Chatalov A Yaneva M 2007 Petrographic composition and provenance analysis of the Paleogene clastic rocks from the Padesh Basin I Petrographic composition of the clastic rocks Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 100 livre 1 ndash Geacuteologie 283-335 (in Bulgarian with English abstract)

Stefanov Y Chatalov A Yaneva M 2008 Petrographic composition and provenance analysis of the Paleogene clastic rocks from the Padesh Basin II Provenance analysis Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 101 livre 1 ndash Geacuteologie 57-77 (in Bulgarian with English abstract)

Surdam RC Boese SW Crossey LJ 1984 The chemistry of secondary porosity In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 129-149

Surdam RC Crossey LJ Hagen ES Heasler HP 1989 Organic-inorganic interactions and sandstone diagenesis American Association of Petroleum Geologists Bulletin 73 1-23

Taylor TR Giles MR Hathon LA Diggs TN Braunsdorf NR Birbiglia GV Kittridge MG Macaulay CI Espejo IS 2010 Sandstone diagenesis and reservoir quality prediction models myths and reality American Association of Petroleum Geologists

25

Renard F Ortoleva P Gratier JP 1997 Pressure solution in sandstones Influence of clays and dependence on temperature and stress Tectonophysics 280 257-266

Richter DK 1985 Die Dolomite der Evaporit- und der Dolcrete-Playasequenz im mittleren Keuper bei Coburg (NE-Bayern) Neues Jahrbuch fuumlr Geologie und Palaumlontologie Abhandlungen 170 87-128

Ridgway KD DeCelles PG 1993 Petrology of mid-Cenozoic strike-slip basins in an accretionary orogen St Elias Mountains Yukon Territory Canada In Johnsson MJ Basu A (Eds) Processes controlling the composition of clastic sediments Geological Society of America Special Papers 284 67-89

Rossi C Alaminos A 2014 Evaluating the mechanical compaction of quartzarenites The importance of sorting (Llanos foreland basin Colombia) Marine and Petroleum Geology 56 222-238

Sachsenhofer RF Rantitsch G Hasenhuumlttl C Russegger B Jelen B 1998 Smectite to illite diagenesis in Early Miocene sediments from the hyperthermal western Pannonian Basin Clay Minerals 33 523-537

Saigal GC Bjoslashrlykke K 1987 Carbonate cements in clastic reservoir rocks from offshore Norway ˗ relationships between isotopic composition textural development and burial depth In Marshall JD (Ed) Diagenesis of sedimentary sequences Geological Society London Special Publications 36 313-324

Sarkisyan SG 1972 Origin of authigenic clay minerals and their significance in petroleum geology Sedimentary Geology 7 1-22

Schmid S Worden RH Fisher QJ 2006 Sedimentary facies and the context of dolocrete in the Lower Triassic Sherwood Sandstone Group Corrib Field west of Ireland Sedimentary Geology 187 205-227

Schmidt V McDonald DA 1979 Texture and recognition of secondary porosity in sandstones In Scholle PA Schluger PR (Eds) Aspects of diagenesis Society of Economic Paleontologists and Mineralogists Special Publications 26 209-225

Schoumlner R Gaupp R 2005 Contrasting red bed diagenesis the southern and northern margin of the Central European Basin International Journal of Earth Sciences 94 897-916

Scotchman IC Jones LH Miller RS 1989 Clay diagenesis and oil migration in Brent Group sandstones of northwestern Hutton Field UK North Sea Clay Minerals 24 339-374

Shalaby MR Hakimi MH Abdullah WH 2014 Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin northern Western Desert Egypt Geological Journal 49 239-255

Shanmugam G 1984 Secondary porosity in sandstones basic contributions of Chepikov and Savkevich American Association of Petroleum Geologists Bulletin 68 106-107

Sheldon HAWheeler JWorden RH Cheadle MJ 2003 An analysis of the roles of stress temperature and pH in chemical compaction of sandstones Journal of Sedimentary Research 73 64-71

Shishkov G Valcheva S Minchev D Salabasheva V Stefanova Y 1989 Petrology of the coal basins and deposits in Bulgaria Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 78 livre 1 ndash Geacuteologie II 122-137 (in Bulgarian with English and Russian abstracts)

Sliaupa S Cyziene J Molenaar N Musteikyte D 2008 Ferroan dolomite cement in Cambrian sandstones burial

history and hydrocarbon generation of the Baltic sedimentary basin Acta Geologica Polonica 58 27-41

Smith JT Ehrenberg SN 1989 Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium Marine and Petroleum Geology 6 129-135

Smosna R 1989 Compaction law for Cretaceous sandstones of Alaskarsquos North slope Journal of Sedimentary Petrology 59 572-584

Smosna R Bruner K 1997 Depositional controls over po-rosity development in lithic sandstones of the Appalachian Basin reducing exploration risk In Kupecz JA Gluyas J Bloch S (Eds) Reservoir quality prediction in sandstones and carbonates American Association of Petroleum Geologists Memoirs 69 249-265

Souza RS Silva CM 1998 Origin and timing of carbonate cementation of the Namorado Sandstone (Cretaceous) Albacora Field Brazil implications for oil recovery In Morad S (Ed) Carbonate cementation in sandstones International Association of Sedimentologists Special Publications 26 309-325

Spoumltl C Wright VP 1992 Groundwater dolocretes from the Upper Triassic of the Paris Basin France a case study of an arid continental diagenetic facies Sedimentology 39 1119-1136

Stefanov Y 2011 Sedimentology of Paleogene rocks of the Padesh Basin Southwestern Bulgaria PhD thesis Sofia University ldquoSt Kliment Ohridskirdquo Sofia 183 pp (in Bulgarian)

Stefanov Y Chatalov A 2007 Formation of high-Mg calcite pisoids and ooids in brackish-water environment an example from the Priabonian sequence in the Padesh basin Southwestern Bulgaria Comptes rendus de lrsquoAcadeacutemie bulgare des Sciences 60 291-297

Stefanov Y Chatalov A 2008 Mineral composition of the clay fraction in the Paleogene shales from the Padesh Basin southwestern Bulgaria In Proceedings ldquo60 years Geology in the Sofia University ldquoSt Kliment Ohridskirdquo Sofia University Publishing House 145-150 (in Bulgarian)

Stefanov Y Chatalov A Yaneva M 2007 Petrographic composition and provenance analysis of the Paleogene clastic rocks from the Padesh Basin I Petrographic composition of the clastic rocks Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 100 livre 1 ndash Geacuteologie 283-335 (in Bulgarian with English abstract)

Stefanov Y Chatalov A Yaneva M 2008 Petrographic composition and provenance analysis of the Paleogene clastic rocks from the Padesh Basin II Provenance analysis Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 101 livre 1 ndash Geacuteologie 57-77 (in Bulgarian with English abstract)

Surdam RC Boese SW Crossey LJ 1984 The chemistry of secondary porosity In McDonald DA Surdam RC (Eds) Clastic diagenesis American Association of Petroleum Geologists Memoirs 37 129-149

Surdam RC Crossey LJ Hagen ES Heasler HP 1989 Organic-inorganic interactions and sandstone diagenesis American Association of Petroleum Geologists Bulletin 73 1-23

Taylor TR Giles MR Hathon LA Diggs TN Braunsdorf NR Birbiglia GV Kittridge MG Macaulay CI Espejo IS 2010 Sandstone diagenesis and reservoir quality prediction models myths and reality American Association of Petroleum Geologists

25

Bulletin 94 1093-1132 Thyne G 2001 A model for mass transfer between adjacent

sandstone and shale Marine and Petroleum Geology 18 743-755

Traveacute A Calvet F 2001 Syn-rift geofluids in fractures related to the earlyndashmiddle Miocene evolution of the Valleacutes-Penedeacutes half-graben (NE Spain) Tectonophysics 336 101-120

Tucker ME Wright VP 1990 Carbonate Sedimentology Blackwell Oxford 482 pp

Ufnar D Gonzalez L Ludvigson G 2004 Diagenetic overprinting of the sphaerosiderite palaeoclimate proxy are records of pedogenic groundwater δ18O values preserved Sedimentology 51 127-144

Van den Bril K Swennen R 2008 Sedimentological control on carbonate cementation in the Luxembourg Sandstone Formation Geologica Belgica 12 3-23

Vangelova V Vangelov D 2013 Evolution of strike-slip basin systems and the related hydrothermal activity Padesh Basin SW Bulgaria Annuaire de lrsquoUniversiteacute de Sofia ldquoSt Kliment Ohridskirdquo Faculteacute de Geacuteologie et Geacuteographie 103 livre 1 ndash Geacuteologie 37-55 (in Bulgarian with English abstract)

Vatsev М Kamenov B Juranov S 2003 Depo-sitional stages and correlation of the Paleogene from the graben basins in Southwest Bulgaria Annual of the University of Mining and Geology ldquoSt Ivan Rilskirdquo 46 Part I Geology and Geophysics 39-44

Vatsev M Juranov S Seferinov S 2011 Contribu-tion to stratigraphy of the Eocene sediments of Pa-desh basin (South-Western Bulgaria) Comptes rendus de lrsquoAcadeacutemie bulgare des Sciences 64 81-90

Vaacutezquez M Nieto F Morata D Droguett B Carrillo-Rosua FJ Morales S 2014 Evolution of clay mineral assemblages in the Tinguiririca geothermal field Andean Cordillera of central Chile an XRD and HRTEM-AEM study Journal of Volcanology and Geothermal Research 282 43-59

Welch SA Ullman WJ 1993 The effect of оrganic acids on plagioclase dissolution rates and stoichiometry Geochimica et Cosmochimica Acta 57 2725-2736

Welton JE Link MH 1982 Diagenesis of sandstones from Miocene-Pliocene Ridge basin southern California In Crowell JC Link MH (Eds) Geologic history of Ridge Basin Southern California Society of Economic Paleontologists and Mineralogists Pacific Section 181-190

Wilkinson M Haszeldine RS Fallick AE 2004 Hydrocarbon filling history from diagenetic evidence Brent Group UK North Sea Marine and Petroleum Geology 21 443-455

Wilkinson M Haszeldine RS Fallick AE 2006 Jurassic and Cretaceous clays of the northern and central North Sea hydrocarbon reservoirs reviewed Clay Minerals 41 151-186

Wilkinson M Milliken KL Haszeldine RS 2001 Systematic destruction of K-feldspar in deeply buried rift and passive margin sandstones Journal of the Geological Society 158 675-683

Wilson MD Pittman ED 1977 Authigenic clays in sandstones recognition and influence on reservoir properties and paleoenvironmental analysis Journal of Sedimentary Petrology 57 3-31

Wintsch RP Kvale CM 1994 Differential mobility of elements in burial diagenesis of siliciclastic rocks Journal of Sedimentary Research A64 349-361

Worden RH Morad S 2003 Clay minerals in sandstones controls on formation distribution and evolution In Worden RH Morad S (Eds) Clay mineral cements in sandstones International Association of Sedimentologists Special Publications 34 3-42

Worden RH Myall M Evans IJ 2000 The effect of ductile-lithic sand grains and quartz cement on porosity and permeability in Oligocene and Miocene clastics south China Sea prediction of reservoir quality American Association of Petroleum Geologists Bulletin 84 345-359

Wysocka A Swierczewska A 2003 Alluvial deposits from the strike-slip fault Lo River Basin (OligoceneMiocene) Red River Fault Zone north-western Vietnam Journal of Asian Earth Sciences 21 1097-1112

Xia F Brugger J Chen G Ngothai Y OrsquoNeill B Putnis A Pring A 2009 Mechanism and kinetics of pseudo-morphic mineral replacement reactions A case study of the replacement of pentlandite by violarite Geochimica et Cosmochimica Acta 73 1945-1969

Yang L Xu T Wei M Feng G Wang F Wang K 2015 Dissolution of arkose in dilute acetic acid solution under conditions relevant to burial diagenesis Applied Geochemistry doi101016japgeochem201501007

Yuan G Cao Y Jia Z Gluyas J Yang T Wang Y Xi K 2015 Selective dissolution of feldspars in the presence of carbonates the way to generate secondary pores in buried sandstones by organic CO2 Marine and Petroleum Geology 60 105-119

Zagorchev I 2001 Introduction to the geology of SW Bulgaria Geologica Balcanica 31 3-52

Zagorchev I Popov N 1968 Geology of the Padesh Paleogene Basin In Tsankov V (Ed) Anniversary Geological Collection Bulgarian Academy of Sciences Sofia 23-35 (in Bulgarian with English abstract)

Zagorchev I Popov N Ruseva M 1989 Paleogene stratigraphy in part of Southwestern Bulgaria Geologica Balcanica 19 41-69 (in Russian with English abstract)

Zamanzadeh SM Amini A Rahimpour-Bonab H 2009 Eogenetic dolomite cementation in Lower Permian reservoir sandstones southern Zagros Iran Geological Journal 44 501-525

Printed 2015

26