The influence of diagenesis on the reservoir quality of Cambrian and Carboniferous sandstones, southwest Sinai, Egypt

Joumd o/African Emlh sdmns. VOL 22, No. 3. p. 285400.19%

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The influence of diagenesis on the reservoir quality of Cambrian and Carboniferous sandstones, southwest Sinai, Egypt

EARLE F. MCBRIDE’, ANTAR ABDEL-WAHAB2 and ALAA M. K. SALEM*

‘Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, USA 2Department of Geology, Faculty of Education at Kafr El-She&h, Tanta University, Kafr EESheikh, Egypt

(Received 29 August 1995: revised version received 16 January 1996)

Abstract-The diagenetic influence on hydrocarbon reservoir quality was investigated for the Cambrian and Lower Carboniferous sandstones of southwestern Sinai. These quartzose and feldspathic Palaeozoic sandstones were not buried more than 1 to 1.5 km until Late Cretaceous and more recent times, when the most deeply buried rocks may have reached 25 km.

Porosity was reduced by compaction from an assumed original 45% to about 26%. In general, both Cambrian and Carbonihrmus sandstones lost more porosity by compaction (average of 19% for each) than by cementation (average of 17% and 13%, respectively). There is no significant difference in the degree of compaction shown by Cambrian (older, deeper buried) rather than Carboniferous sandstones.

Cementation by iron oxide, quartz, calcite and kaolinite reduced porosity to 12-15%, except in silcretes and some ferriaetes where porosity was reduced to ~5%. Significant secondary porosity was created (5.8 and 5.1% for Cambrian and Carboniferous sandstones, respectively ) chiefly by dissolution of feldspar. Kaolinite (maximum of 20%) is the most deleterious cement because it has high microporosity, which causes high residual water saturation, and occurs as tiny crystals that have the potential to break loose during rapid fluid flow and block the pore throats.

The presentday porosity in these sandstones averages 19% and ranges from 1.5 to 32%. Many sandstone samples (47% of a total of 178 samples) have permeability values higher than 1000 md. The plot of porosity versus the log of permeability has a good correlation indicating that microporosity, even though locally important, does not signifimntly influence reservoir quality. In spite of their age and the large volumes of groundwater that probably passed through them, these Palaeozoic sandstones retain sufficient porosity and permeability to possess excellent reservoir quality.

Resume - L’influence de la diagenese sur la qualite de reservoir a hydrocarbures a et6 &udi&e pour les gres du Cambrien et du Carbonifere inferieur du sud-ouest du SinaX Jusqu’au C&ace tardif, ces gres quartziques et feldspathiques du Paleozdique n’ont pas et6 enfouis sous une couverture excedant 1 a 1.5 km. Depuis lors, Yenfo wssement maximal n’a pas depa& 2.5 km.

Par compaction, la porosite estim&e initialement a 45%, est pass& h une valeur d’environ 26%. De facon g&&ale, tant les gr&s du Cambrien que ceux du Carbon&e ont perdu davantage de leur porosite par compaction (en moyenne 19% pour chacun d’eux) que par cementation (moyennes respectivement de 17% et 13%). 11 n’y a pas de difference significative dans le degn? de compaction apparaissant dans les g&s du Cambrien (plus anciens, enfouis a une plus grande profondeur) par rapport II celui observe dans ceux du Carbon&e.

Une cementation par des oxydes de fer, du quartz, de la calcite et de la kaolinite a reduit la porosite a 12-15%, sauf dans les &r&es et certaines ferricretes oh elle ne d&passe pas 5%. Une porosite secondaire significative est apparue (5.8 et 5.1% respectivement, pour les grb cambriens et carbon&es) principalement suite a la dissolution de feldspath. La kaolinite (maximum de 20%) constitue le ciment le plus nuisible h cause de sa microporosite elev&e entramant une saturation en eau msiduelle considerable. Par ailleurs, elle se presente en minuscules cristaux pouvant se detacher lors de la circulation rapide de fluides bouchant ainsi les interstices entre les pores.

Actuellement la porosite de ces gres, atteignant une moyenne de 19%, varie de 1.5 a 32%. Un grand nombre d’&hantillons de gr&s (47% sur un total de 178 6chantillons) atteint des valeurs de permeabilite d6passant 1000 md. Un diagramme de la porosite par rapport au logarithme de la permeabiliM montre une bonne correlation indiquant que la vti, m&me sielle peut localement etre importante, n’influence pas de facon signihcative la qualite du reservoir. Ces g&s pal6ozolques ont done garde une porosiM et une permeabilill suffisantes que pour presenter une qualite de r&en& excellente, ma&+ leur age et les quantiMs importantes en eau qui les ont probablement traverses.

INTRODUCI’ION

Most diagenetic studies and evaluations of hydrocarbon reservoir quality of sandstones are from hydrocarbon-bearing basins where rocks have been

buried 3 km or more. In these basins, temperature and pressure have strong influences on diagenetic processes. In contrast, this report describes the diagenetic processes affecting the reservoir quality of Palaeozoic sandstones that were deposited on the Arabian Shield

285

E. F. MCBRIDE ef nl. 286

29

29"40

28"ZO

N-+

P F

N-

Figure 1. Location map after Eyal et nl. (1980).

and which were buried less than 1.5 km for most of their history. The‘purpose of this study is to provide a case study of shallowly buried sandstones that can be compared with the far more common, deeper buried sandstones in most sedimentary basins. The sandstones are exposed in southwest Sinai, marginal to the Gulf of Suez (Fig. 1). Here, Palaeozoic and Precambrian rocks are exposed in an eastward- dipping fault block that brings these old rocks adjacent to Cretaceous and Eocene rocks along a fault with 2000 m of displacement (Shata, 1956). Rifting of the Gulf of Suez began in the early Miocene and continues to the present (Purser et al., 1993). The region also underwent minor folding during the Late Palaeozoic (Abdallah et al., 1992).

The rocks studied are composed mainly of sandstones (90%) with thin layers of shales, sandy shales and siltstones. There is also a thin unit of limestone- dolomite (Urn Bogma Formation; not studied) and black to green shales intercalated with sandstones (Abu Durba Formation). The sandstones of interest to this study are exposed in a west-facing slope along a ridge of nearly continuous exposure for 100 km. Stratigraphic sections were measured and sampled at three localities: Urn Bogma (UB), Gebel Abu Durba (AD), and Gebel Naqus (N) (Fig. 1).

The age and stratigraphical subdivisions of the elastic formations in this area have been debated for decades (summary in Salem, 1995). For the AD and N localities the authors accept the stratigraphic nomenclature and age designations of Klitzsch

(1990), who considers the Araba and Naqus Formations to be entirely of Cambrian age and the Urn Bogma and Abu Thora to be of Early Carboniferous age (Fig. 2). For the Urn Bogma succession, the authors follow Weissbrod (1980), who divided the Lower Palaeozoic into four newly named formations and the Abu Thora into two unnamed members (Fig. 3). Correlation of Cambrian rocks between the UB and AD areas has not been established. Because the lower three formations at UB are much more feldspathic than the Araba and Naqus Formations, they may be older than the Araba or had a different provenance. For convenience, the Early Paleozoic rocks are all considered to be of Cambrian age, although Weissbrod (1980) and Abdallah et al. (1992) suggest that they may be as young as Ordovician.

PROCEDURES

For this study, three stratigraphic sections were measured and sampled (Fig. 3). Thin sections were examined from 120 samples, most of which were impregnated with blue dyed epoxy resin to aid in distinguishing the distribution of pores. Many samples were stained for either K-feldspar using sodium cobalinitrite or for calcite using alizarin Red- S. Four hundred counts per thin section were made of 100 samples to quantify the abundance of detrital grains, authigenic minerals and pore types. Grain size and sorting were determined by measuring the maximum diameter of 30 random grains under the microscope.

The porosity and permeability of 176 cylindrical plugs approximately 2.5 cm by 2.5 cm were measured to assess the hydrocarbon reservoir quality. A commercial porosimeter was used that employed mercury, air and Boyle’s law to determine the pore volume of each sample and the permeability of the plugs to air was measured using electronic flow meters as described by Sharp et al. (1994).

Detrital and authigenic minerals were identified using a combination of thin sections, X-ray diffraction, scanning electron microscopy, cathodoluminescence and electron microprobe analysis. The C, 0, and Sr isotopic signature of selected samples of authigenic calcite were determined to aid in interpreting the temperature and conditions of precipitation of this mineral phase and the 0 isotopic signature of authigenic quartz and kaolinite were determined to help discover the conditions of precipitation of these phases. Only a summary of the diagenetic history is presented here. A lengthier report is in Salem et al. (in prep.). This study builds on previous petrographic and diagenetic studies of the Cambrian sandstones by Weissbrod (1969), Abdel-Wahab et a2. (1987,1992), Allam (1989), Abdel-Wahab (1990) and Abdel-Wahab and McBride (1990, 1991).

The influence of diagenesis on the resevoir quality of Cambrian and Carboniferous sandstones

Lower Paleozoic

Silurian

Ordovician

Cambrian _

Naqus Fm. Araba Fm.

%fszf Abu Hamata

Sarabii El Khadim

Pra- Cambrian Basement

Pra- Cambrian Basement

Figure 2. Recent stratigraphical subdivisions of the Palaeozoic succession in Sinai. Conelation of the Cambrian formations of Weissbrod (1980) with the Araba and Naqus Formations have not been made. Cretaceous rocks unconformably overlie the Palaeozoic section at each locality. The total thickness of the Palaeozoic section at the UB lo&y is 330 m.

STRATIGRAPHY AND ROCK TYPES

Gebel Abu Durba - Gebel Naqus area

The Palaeozoic elastics are about 405 m thick at the AD locality and 335 m thick at the N locality. All the primary and secondary sedimentary structures recognized by Kholief et al. (1987) in the Palaeozoic sandstones from the Wadi Feiran-El Tor area are identified herein. A graphic representation of each succession and location of sampled beds is shown in Fig. 3. A synopsis of the stratigraphy is presented here (a detailed summary is given by Salem, 1995).

The Araba Formation overlies granite. The formation (95 m thick at AD and 75 m thick at N) consists mainly of medium-grained, well sorted, cross-bedded sandstones of multi-colours intercalated with laminated clayey siltstone (approximately 10%) and locally containing quartz pebbles, especially in the lower part. Trough cross-beds 20-30 cm thick are common. Skolithos tubes occur in about 50% of the beds. Colour moffles also outline irregular vertical burrows. Most of the Araba Formation is marine on the basis of trace fossils and local stromatolites and archeocyathids (Omara, 1972; lssawi and Jux, 1982; Allam, 1989; Klitzch, 1990; Magwood and Pemberton, 1990).

The Naqus Formation conformably overlies the Araba Formation and unconformably underlies the Abu Durba Formation. It consists of a sequence of poorly sorted, friable, permeable, cross-bedded, fine to coarse grained, locally pebbly sandstones 200 m thick in AD and 260 m thick in N. Naqus sandstones are mostly light to dark brown and do not show the multi-colours of the Araba. Trough and current-rippled cross-beds of a range of thicknesses and contorted and overturned cross-beds are common, although laminated beds are

also present. Deformed cross-beds of this type are widely recorded from fluvial sandstones (Robson, 1956; Jones, 1%2; Rust, 1968; Hendry and Stauffer, 1975,1977; Allen, 1985). Channel-fill deposits are also recognized where intraclasts of kaolin& pebbles occur in pebbly sandstones. About 10 m of aeolian dune deposits occur in the upper part of the Naqus at AD.

The Carboniferous Abu Durba Formation unconformably overlies the Naqus Formation and underlies the Malha Formation of Early Cretaceous age (Kora, 1989). It is composed of a sequence of interbedded sandstones (70% of the formation) and shales 80 m thick. The sandstones are cross-bedded, multi-coloured, friable and fine- to coarse-grained. There are several horizons of sandstones strongly cemented by goethite that warrant the term ferricrete. The shales in the formation display black, grey, yellow, green and red colours and excellent laminations. The Abu Durba Formation contains both marine and non- marine deposits.

Urn Bogma area

The Palaeozoic succession in the Urn Bogma area is about 330 m thick and has been subdivided into six rock units (Figs 2 and 3). The Cambrian formations form a conformable succession whose base rests on Precambrian granite.

The Sarabit El-Khadim Formation is composed mainly of coarse, pebbly sandstones intercalated with finer ones. Herringbone cross-bedding is common and points to a marine origin. The Abu Hamata Formation is composed mainly of cross- bedded sandstones intercalated with multi-coloured cross-laminated shaly siltstone (about 5%) in the middle part. The cross-bedded sandstones are

288 E. F. MCBRIDE ef al.

NAQUS SUPERGROUP (N) AEIU DURBA SUPERGROUP (AD) UMM BOGMA SUPERGROUP (UB)

Age Fm Spl. Lithology Qtz. Overgrowth ,. - r

Qtz. Overgrowth

Age Fm Spl. Lithology 0”” 10 20%

Qtz. Overgrowth

0”” 10 20% Age Fm Spl. Lithology

rl ,i’

/

Sample . . locatrons

.

20 m

[O

Dolostones

m

Shale Basement

vlfmcvc -

Cross bedding Sandstone

m 1.’

Figure 3. Schematic stratigraphical sequence of three localities. The dots show the sample sites. The lines show the amount of quartz cement in the samples; silcrete samples are shown with open circles.

tabular and locally contain some worm burrow tubes, which suggest a subtidal ‘shelf’ environment of deposition (cb Seilacher, 1967; Selley, 1978).

The Nasib Formation is composed mainly of fine- to medium-grained cross-bedded sandstones. It becomes more ferruginous upwards and is capped by cross-laminated shaly siltstone (approximately 5%). It displays channel-fill structures and overturned cross- beds in single sets 30-50 cm thick, both features typically of fluvial origin. The overturned cross-beds are similar to those in the Naqus Formation and are characteristic of fluvial sandstones. The lower three Cambrian formations are much more feldspathic and lithic rich than either the Araba and Naqus Formations and either had a different source or are not correlative with them.

The Adedia Formation is composed of fine- to medium-grained, multi-coloured (yellowish, brownish, reddish) sandstone beds, cross-bedded in parts, with iron oxide veinlets increasing upwards. Compositionally it may correlate with either the Araba or Naqus Formations to the south.

The Carboniferous Urn Bogma Formation unconformably overlies the Adedia Formation. It is composed mainly of hard, grey and pink dolomite and marly dolomite with thin layers of secondary gypsum and shales comprising lenses of ferruginous Mn ore at the base. Mart and Sass (1972) suggested a sedimentary origin for the Mn ore (chiefly psilomelane and pyrolusite) involving accumulation and differentiation stages.

The Abu Thora Formation conformably overlies the Urn Bogma Formation and is capped by a basaltic sill 20 m thick and then by Cretaceous rocks. The sill is of Late Triassic to Early Jurassic age (Weissbrod, 1969). Weissbrod (1969) divided the formation into two members: a lower sandy-clayey member 90 m thick and an upper sandy-quartzitic member 60 m thick. The lower member is composed of fine- to medium-grained, locally pebbly, cross-bedded sandstone, interbedded with carbonaceous shales and clays. The upper member is composed of whitish to milky, fine- to medium- grained, friable sandstones with a kaolinite-rich bed of about 1 m thick. The formation contains both marine

The influence of diagenesis on the resevoir quality of Cambrian and Carboniferous sandstones 289

Figure 4. Inferred burial and thermal history of rocks at the UB locality and the time of formation of major diagenetic events. The thickness of the stratigraphic units overlying the Carboniferous is from Kostandi (1959), Omara and Conil(1965), Weissbrod (1969,1980), Soliman and El-Fetouh (1970) and Kora (1984). The maximum thickness of each stratigraphic unit was plotted, a situation that might not apply to section UB. The tune of silcrete formation is unknown, but S shows the likely times of formation.

and non-marine beds (Omara and Shultz, 1965; Kora, 1984; Kora and Jux, 1985; Kora and Shultz, 1987).

BURIAL HISTORY

The burial history (total subsidence) of these rocks is inferred from the thickness (uncorrected for compaction) and age of the overlying rocks. Uncertainties exist about the thickness of rock removed by erosion at unconformites preceeding the Early Carboniferous, the Early Cretaceous and the present. Uncertainties in the burial history are shown in Fig. 4 by dashed lines. The base of the Cambrian at Urn Bogma was buried almost to 2.5 km by the end of the Eocene epoch (Fig. 4), but probably was not deeper than 1 or 1.5 km prior to Cretaceous time. Burial curves for the other localities are similar to that for the UB locality.

The thermal history is poorly known. The authors assume that during much of the time the surface temperature was 25°C and the geothermal gradient was 25°C km-‘. Thus, the base of the Cambrian probably did not reach temperatures in excess of 65°C and possibly not in excess of 5O”C, until after the Cretaceous (Fig. 4). A more detailed analysis of the possible thermal history is given by Salem (1995).

TEXTURE AND COMPOSITION: GENERAL COMMENTS

The Cambrian sandstones have an average composition of 65% framework grains (F), 17% cement

Wad1 Feiran-El Tor Sections

________~~ i________i i________, F L F L F L

Q Q cl

\, “\

7’ Restored

+ L_______ _‘I i________‘l : ____ ____\ ’ F L F L F L

Urn Bogma Bection

L-_______; L_______.! F L F L

0 Q

f’-_-___-+ i________,? J F L

Figure 5. Present and restored modal composition of the sandstones (see text for restoration procedure). The provenance fields are from Dickinson et al. (1983).

(C) and 18% porosity (P); Carboniferous sandstones average F,C,,P,,. The detrital matrix is essentially absent from the sandstones, although infiltered clay is present in some samples. All sandstones in the Carboniferous, Naqus, Araba (except two samples) and Adedia Formations are quartz arenites; samples from the lower three formations at the UB locality are subarkoses to arkoses. Rock fragments, mica and heavy minerals are minor components in the samples. The present modal compositions are shown in Fig. 5. Point count data are available from the authors.

The framework composition of the sandstones has been modified during diagenesis primarily through feldspar dissolution and kaolinization. The authors determined the amount of feldspar lost by these processes by assuming that all oversize pores (pores originally inferred to have been occupied by feldspar grains) and all grains pseudomorphed by kaolinite and calcite were originally feldspar (cf McBride, 1987). This

290 E. F. MCBRIDE et al.

Compaction - - - ----

clay coats -- - -

Fe-oxides ----

cluartz cement

-feldspar cement -

Calctte cement - - --

Dissolution ___----

Kaolinite cement

Barite cement -

Gypsum cement -

Halite cement -

Figure 6. The paragenetic sequence of authigenic minerals. The bold lines identify the main episodes of each event. K-feldspar and halite are present only in the Cambrian samples. Calcite, kaolinite and bar& occur both as cement and replaced grains. Dissolution affected the feldspar, rock fragments and calcite.

amount of feldspar was added to the present amount of feldspar to get the original feldspar content. Restored modal compositions are plotted in Fig. 5. In spite of the difference in their age, both the Cambrian and Carboniferous sandstones are remarkably similar in composition and diagenetic character except for feldspar (average of 9.3% in Cambrian sandstones and 0.2% in Carboniferous sandstones) and kaolinite (average of 6.0% in Cambrian sandstones and 0.2% in Carboniferous sandstones).

The mean framework grain size in both Cambrian and Carboniferous sandstones is chiefly fine to medium, but is locally coarse. Most samples are unimodal, but some have a bimodal size distribution of the fine and coarse modes. This kind of bimodality, common in Early Palaeozoic rocks (Blatt, 1992, p. 67) is the product of an aeolian history (Folk, 1968; Johansen, 1988). Another type of bimodality formed when windblown silt-sized grains (loess) infiltered through the intergranular pore spaces among coarser quartz grains. The bimodality of aeolian origin can survive marine transgression (Wilson, 1962; Johansen, 1988).

Cement minerals, in order of decreasing abundance, are iron oxides (hematite, goethite, maghemite), kaolinite, quartz, calcite, illite, smectite, mixed-layer illite-smectite, halite, gypsum and barite. Their influence on reservoir quality is described below. The porosity of Cambrian sandstone ranges from 2 to 32% and averages 19%; the porosity of Carboniferous sandstones ranges from 1 to 34% and averages 19%.

FRAMEWORK GRAINS

Quartz is the major detrital component. It makes up 78 to 99% of the framework fraction of Cambrian sandstones and 99 to 100% of Carboniferous sandstones. The average values for the two ages of the samples are

87 and 99.6%, respectively. Feldspar does not exceed 2% of the framework grains, except in all 17 samples of the lower three Cambrian units at the UB locality and in 2 samples of the lower part of the Araba Formation. It reaches 37% in one sample of the Serabit El-Khadim Formation. However, dissolution by groundwater has removed an average of 7.5% feldspar from the Cambrian and 5.0% from Carboniferous samples based on the abundance of oversize pores (see below). The pre-diagenetic framework composition has been calculated by assuming that oversize pores, including those filled by kaolinite, were originally feldspar. Thus, most samples originally were subarkoses (Fig. 5). Feldspar is more abundant in finer grained samples. Rock fragments and mica average less than 1% of the framework grains. In decreasing abundance they are metamorphic (schist), sedimentary (chert, shale rip-up clasts and hematitic sandstone), volcanic (silicified and devitrified rhyolitic rocks) and plutonic (granite). Muscovite and biotite are rare. Detrital heavy minerals are minor components; they consist of hematite, goethite, magnetite, zircon, tourmaline and ruble.

MATRIX

Detrital clay is essentially absent in the samples except as rare claystone rip-up clasts. However, thin clay coatings (cutans), a few micrometres thick, of kaolinite and lesser illite occur on detrital framework grains in a few samples and makes up 3.8% of one sample. The clay is generally pigmented by iron oxides, has particles orientated parallel with the detrital grain surfaces or around the margin of pores and in places forms meniscus-shaped pore bridges. These are characteristics of infiltered clay: clay particles introduced in suspension when muddy water percolates into dry sand in non-marine environments (cF Crone, 1975; Walker et al., 1978; Matlack ef al., 1989; Abdel-Wahab and Turner, 1991). The infiltered clay did not significantly reduce the porosity. However, it may have inhibited the precipitation of quartz cement in a few samples in the manner described by Heald and Larese (1974).

CEMENT AND OTHER AUTHIGENIC MINERALS

General comments

Authigenic minerals include those that simply filled pores (cement) in the sandstones plus those that partly or completely replaced detrital grains or other authigenic phases. The paragenetic sequence of authigenic minerals, as determined by textural relations in thin sections and scanning electron microscopy, is shown in Fig. 6. A summary of the abundance of each authigenic phase is shown in Table 1. The inferred depth and temperature of cementation of major authigenic


Table 1. Summary of abundance of the main authigenic minerals

YMineral ‘Range (%) Average (% ) Influence on reservoir quality Iron oxide 3 to 40 5.4 modest Quartz (non-silcrete) Tr-8 2.6 slight Quartz (silcrete) 10 to 18 13.8 severe K-feldspar 0 to 3 0.3 insignificant Calcite 3 to 20 1.6 significant only locally Ka oto20 4.3 gnlfi olinite si * ‘cant locally

Abundances are per cent of total rock. Values for calcite and kaolinite include both pore-filling cement and grain replacements.

phases is shown in Fig. 4. Iron oxide (hematite, goethite and rare maghemite),

quartz and kaolinite are the main authigenic minerals that influence reservoir quality in Cambrian sandstones and iron oxide and quartz are the main minerals that influence reservoir quality in Carboniferous sandstones. Quartz has a fairly uniform distribution in sandstones of both ages, except for three silcrete beds where it fills most pores (McBride et al., in prep.). Other cements have an uneven distribution so that their influence on reservoir quality varies from bed to bed. Histograms of the abundance of authigenic minerals are shown in Fig. 7. Authigenic calcite is important only in the few beds where it exceeds 2% (Fig. 7). Authigenic illite, smectite, mixed-layer illite/smectite, barite, gypsum and halite are present in such small amounts that they have no important effect on reservoir quality and are not discussed further here. Furthermore, halite and gypsum are probably restricted to outcrops and are probably not present in the same rocks in the subsurface.

POROSITY

General comments

Thin section porosity was divided into three categories:

i) intergranular pores (pores between sand grains); ii) oversized pores (pores larger than normal

intergranular pores and comparable to the size of sand grains); and

iii) intragranular pores (pores within grains). Of the 18% average thin-section pores, 69% are intergranular pores, 26% are oversized pores and 5% are intragranular pores. In this study, the term ‘secondary porosity’ refers to the total of intragranular pores plus oversized pores, whereas intergranular porosity refers to all porosity between grains, whether it is primary or has resulted from the dissolution of pore- filhng cement. Essentially all intergranular pores are thought to be primary in these sandstones for reasons given below. Macropores are intergranular pores and intragranular pores large enough to be identified and point counted in thin section, whereas micropores are

tiny pores (those with pore throats CO.5 pm; Pittman, 1977) that occur between detrital and authigenic clays and inside rock fragments and some feldspars. Micropores are too small to analyze in thin section, but their abundance is assessed below.

Secondary porosity

Secondary porosity formed in the Sinai sandstones by the dissolution of framework grains (FGD) and to a lesser and unknown extent by the dissolution of calcite cement. Criteria for the recognition of secondary porosity in sandstones are given by Schmidt and McDonald (1979) and Shanmuggan (1984). Partially dissolved feldspars in the Sinai sandstones suggest that oversized pores formed chiefly by the complete dissolution of feldspar grains. It is possible that a few oversized pores formed where rock fragments and unstable heavy minerals dissolved also. Porosity formed by FGD is thus the sum of intragranular pores and oversized pores. It averages 5.8% for Cambrian sandstones and 5.1% for Carboniferous ones. Porosity generated by FGD makes up 32% of the total thin section porosity in the Cambrian sandstones and 27% in the Carboniferous ones; thus, it contributes significantly to the present total porosity.

Secondary porosity which results from the dissolution of mineral cements, whether they are replacive or purely pore-filling, is termed cement- dissolution porosity (Burley and Kantorowicz, 1986). Ragged, corroded edges (regular v-shaped or spiky patterns) of calcite cement crystals in the Sinai sandstones indicates that calcite was being dissolved in outcrop. Furthermore, micritic calcite cement occurs in some samples as a few widely scattered grains per thin section and some quartz grains in samples uncemented by calcite have notches and embayments on their margins. Both of the latter features are possible clues to the presence of former calcite cement that has been dissolved (Shanmuggan, 1984; Burley and Kantorowicz, 1986). Cement dissolution porosity can be created by the dissolution of calcite cement in outcrop or in the subsurface by pore fluid (meteoric or marine- derived formation water) that is undersaturated with respect to carbonates (Bjrarlykke, 1984). Although large

292 E. F. MCBRIDE ef al.

Table 2. Porosity and permeability values of Cambrian and Carboniferous sandstones

1) Araba Sp.No 3NQ 4 5 6 7 9 11 12 15 16 18 19 21 22 24 26 4D 7 9 10 11 14 15 16 18 19 20 21 22 24 25 26 27 28 29 30 32 33 36 37 40 Average 3) Abu D Sp.No. ‘95D 96 99 100 101 ~105 107 110 111 112 113 114 116 117 120

Nrmation ‘m0(%) Ts0% Micro0 K (md)

32.3 24.4 1233 22.4 15

24.2 24.8 15.8

11.9 7.9 10.5 971

142 1284 11.09 1171

22.7 19.3

1.5 5.5

9.6 13.2 15

17.6 23.7 15.2

4.8 10.2 70 6

101 1519

56

17.4 21.9 13.2 18.4 14.1 19.2 14

18.9 19.4 20.5 20.4 13.4 21.8 17.4 10.9 14.5 18.1 19.4 7.8 18.5 19.8 24.9 19.3 22.2 20.5

11.6 6.7 20.7 10.4 27.1

10.7 1.2 2.8 -8.7

17 7 10

1.5 4

16.5 4 19.8 0.6

21.5 0.3 15.5 1.9

23.5 -9 15.2 2.9

17 -9.2

21.9 -2.1 7 17.9

1095 1407 56

382 34

244 97

620 269 97

397 50

818 800 66 504

1556 777 887

1195 1597 494 248

1002 11%

23.6 18.9 26.9

1318 26.6 0.3 1610

20.8 15.9 4.9 338 ba Formation Pm 0% Ts0% Micro 0 K (md)

2.9 6 -3.1 17 -

24.2 - - 24.2 26.7 -2.5 17.8 - - 25.7 22.5 3.2 25.3 24.7 0.6 20.9 - 22.3 - - 22.9 - - 22.7 19 3.7 33.8 - - 21.4 - 24.2 28.7 -4.5

305 1069 1192 2039 1142 1911 2565 1791 2024 2163 1801 2503 2048 751

12.8 0.9 11.9 2

9NQ cl

4

5 B 1 3 1 7 ? 1 2 5 !3 9 2 1 5 s 0 2 3 1 5 6 7 B

9

D

2 3 2D 3 5 B 0 2 4 B D 2 7 2 3 9 0 3 6 7 9 3 4 iverage

ormation Pm 0 % Ts 0% Micro 0 K (md)_

19.7 1221

24.2 21.2

18.3 21.4 24.2 17.7 22.2 19.9 22

21.8 20.9 21.7 18.4 25

24.9 17.2 21.5

20.7 18.1 20.2 19.8 19.8 21.8 10.5 1.5 4.2 19.3 11.1 16.6 13.7 22.4

19.7 22

24.2 24.2 25.4 21.5

21.6 19.9 19

18.8 19.4 17.5 19.7

12.6 15

11.5 14

9.7 198 330

26 -4.6

18.7 3.5 11.2 8.7

22.8 -1.9

9.7 15.3 21.7 3.2

16.6 23.3

4.9

33 1202 1473 1020 1152 115

1412 287 1975 2035 1905 1532 138

1743 1329

20.5 1.3 4.3 6.2 1.3 0.2 2.8 1.4

6.5 10.1

231 2122 2019 1831 2020 287 68 3 2

1274 170 15

136 1448

22.3

23.5 0.7

1558 1570 1603 1177 2106 1115

17.8

18.2 21.8

15

7.1

1.7 -2.8

4.4

12.6

1534 1297 1550 1826 1534 1748 2121

22.4 2603 18.2 12.6 5.6 618


Table 2. continued

4Um 6 7 9 12 14 15 16 18 21 22 22 23 Average 2)Abu-H 26Um 27 29 31 33 36 39 Average 3)Nasib I 41Um 43 44 48 51 52 54

‘128 1 21.9 - - 2157 Average 1 20.4 14.1 6.3 922 B) Urn Bogy ma Area 1) Sarabit E LKhadim Sp. No. Pm0 % Ts0% Micro0 K(md)

1 FOI

PmQ%

Ts0%

12.3 - 97 18.8 - - 357 23.2 25.7 -2.5 211 12.1 - - 575 17.1 5.8 11.3 28O 17.6 6.3 11.3 148 16.5 15 1.5 251 15.2 25 -9.8 211 15.8 14.5 1.3 392 16.5 - 937

17.3 - - 18.4 17.3 1.1 1364 13.2 - - 757 17.1 11.8 5.3 351

xata Formation 20.3 - 20.2 22 13.4 - 16.7 - 21.6 21.4 21.5 20.7 21.4 20.3 21.5 20.8

rmation 20.1 - 16.5 15 16.2 - 21.2 15.5 26.2 25.8 24.2 22.2 16.9 -

107 -1.8 355

- 1119 - 28

0.2 413 0.8 837 1.1 589 0.7 304

- 1317 1.5 430 - 64

5.7 360 0.4 439 2 601

328 55 1 22.2 21.9 0.3 301 Average 1 22.1 20.1 2 369 4)Adedia Formation 59Um 21.3 19.5 1.8 589

13.7 12 1.7 115 18.3 17.7 1.5 190

= Total porosity measured by a porosimeter.

= Thin-section porosity.

9.8 - - 15 12.6 - -

21.9 - - 31 23.4 22.7 0.7 1189 15.4 - - 77 21.1 18.5 2.6 728 19.6 - - 358 16.9 16 0.9 431

j)Abu-Thora Formation 1 Pm0% Ts0% Micro0 K (md) jp.No.

XJUm 32 33 35 37 w LO2 LO4 105 LO7 LO9 111 114 115 116 118 120 121 123 125 127 128 129 130 133 136 138 139 141 144 145 148 150 153 154 156 157 159 161 162 163 164 165 167 170 Average

22.5

15.2 21

17.9 21.2 20.9 16.6 17.2 15

28.1 21.8

17 22.5 4.5 22 1.1 17.5 21.9 22.1 10.3 21.6 19.1 5.3 21.4 20.5 18.9 20.6 23.5 18.4 10.9 17.4 20

20.3 22.1

9.9

21.6 24.9 17.5

20.1

20.4 16.3

2.1

20.2 -3.6 21.5 -4.3

24 21.4 16.9

4.1 0.4

22 0.5

25 -3

22.5 -5

8.8 1.5

25.7 -6.6

30.5 -11.6

22.7 0.8

22.3

19

12.3

10.6 26.1 24

-4.9

1.3

-4.5 0.9

17.4

19

2034 850 298 222 733

1494 1600 1459 1456 727

1202 1047

1 96 141 955 984 547 20

1041 643

2 1418 1716 1142 1543 1528 1693

59 14% 1973 1722 2307

66

1395 1378 881

1220 5.2 4.8 0.4 1 19.9 18.6 1.3 426

Micro 0% = Total0 - Macroporosity.

K (md) = P-ability in millidarhs; average Permeability is the geometric mean.


30 35

Cambrian 25

s Mean = 5.3%

30 Mean .4.1%

E20

I?=46 25 "=52

d 20 z 15 g 10 15

<2 4 6 6 10 12 14 16 z-16 <2 4 6 6 10 12 14 16 z-16

Iron Oxide Cement (%) Iron Oxide Cement (%)

Cambrian

In20 Mean =2.6%

_m n=46

:I5 rn

6

$10

25

;2 h io 1'2 i4 i6 >;6

Quartz Cement (%)

35 Cambrian

230 Mean=14%

f n=46

CYJ 25

_,20

$ 15 E 3 10 z

a 10 12 14 16 z-16

Authigenic Calcite (%)

. ”

35

30

25

20

15

10

5

0

Carbonifsmus

Mean = 2.1%

" =52

<2 k 6 10 12 14 16 >16

Quartz Cement (%)

40,

;2 ; i a 10 12 14 16 x.16

Authigenic Calcite (%.)

20

16

g 16

El4

jl2 s 10

#:

24

35

30 Carboniierous

Mean = 0.2%

25

20

15 10

2 5

0 0

<2 4 6 6 10 12 14 16 z-16 <2 4 6 6 10 12 14 16 >18

Authigenic Kaolinite (%) Authigenic Kaolinite (%)

Figure 7. Histograms showing the abundance of quartz, calcite, iron oxide and kaolinite cement in individual samples.

volumes of meteoric water passed through the sandstones when silica for quartz cement was introduced, calcite cement had not yet precipitated. Thus, the meteoric water that precipitated the quartz cement could not have dissolved the calcite cement. In the final analysis, evidence indicates that some calcite cement has been dissolved from the sandstones recently, but the exact amount is uncertain. Because there is no major source of carbonate (i.e. marine shells, carbonate rock fragments, calcrete) in the sandstones and the

sandstones adjacent to the Urn Bogma Formation (manganiferous dolomite) are not extensively cemented by calcite, the authors take the conservative view that the sandstones probably did not contain much more calcite cement originally than it has now and that cement dissolution produced only small amounts of secondary porosity.

Of the total present porosity in all samples, primary porosity averages 69% (43% total rock porosity) and secondary porosity averages 31% (=6% total rock porosity).


30-

‘i; @ 0 25

f20- B 3 $j 15-

1 .c lo- S

r = 0.61 n = 92

0 0 ,

0 I I I I I I

1

0 5 10 15 20 25 30 35 Porosimster Porosity (%)

Figure 8. Plot of porosity measured by a porosimeter versus the porosity measured by point counts on thin sections.

Figure 9. Scanning electron micrograph of authigenic kaolin& showing the micropores between kaolinite crystals.

Porosimeter porosity

The average porosimeter porosity of all measured samples (n=178) is 18.9% with values ranging from 1.5 to 34% (Table 2). A plot of thin section porosity versus porosimeter porosity for 92 samples is shown in Fig. 8 (r=0.61). The p orosimeter measures both macropores and micropores, whereas only macropores can be tabulated in thin section. Two factors may explain the relatively low correlation coefficient:

i) for some samples the thin section was made from rock 1 cm from the place where the porosity was measured by the porosimeter; and

ii) some samples, espcecially those with abundant kaolinite, have a significant amount of microporosity

(Fig. 9). In most samples, thin section porosity is less than porosimeter porosity. The difference between the two values is considered to be the amount of microporosity in the sample. The unusually high total porosity and microporosity values for some samples are probably spurious values and likely result from the presence of fractures within the samples. Negative microporosity values result from the calculation procedure (subtracting thin section porosity from porosimeter porosity) and are impossible values. The limitations of the Kobe-type porosimeter explains these values.

Micropores range from absent to 1556, but generally are less than 5%. They are, predictably, most abundant in samples with kaolin& cement. Authigenic kaolin& typically has from 25 to 50% micropores between individual crystals (Fig. 9; Hurst and Nadeau, 1995). Thus, micropores are rare in Carboniferous samples.

PERMEABILITY AND POROSITY

The permeability of the sandstones to air was measured at surface pressure conditions (760 psi). In the Wadi Feiran-El Tor area, the permeability averages (geometric mean) 936 md for Cambrian sandstones and 1480 md for Carboniferous ones. Permeability in the Urn Bogma area averages 458 md and 978 md for the Cambrian and Carboniferous sandstones, respectively (Tables 2 and 3). About 47% of the samples analyzed have a permeability greater than 1000 md. Sandstones of this permeability magnitude make an excellent aquifer and a very good hydrocarbon reservoir.

The samples in this study show a moderate correlation between porosity and permeability (r=0.7, Fig. 10) commensurate with samples dominated by macropores. However, the relationship is not a simple one. For any given porosity there is a wide range of permeability. This is due, in part, to textural effects (grain size, sorting, packing, etc.), but is mainly due to different amounts of kaolinite (micropores) in the samples. In addition, anomalously large permeability values are measured if the samples have fractures. Carboniferous sandstones, because of their low content of kaolinite cement, have higher permeability and less micropores than the Cambrian sandstones. For example, the Abu Durba Formation (Carboniferous) has 2.4% microporosity and 1480 md permeability, whereas the underlying Naqus Formation (Cambrian) has 4.7% microporosity and 1175 md permeability. The former contains only 0.2% kaolinite cement versus 5.5% for the latter.

Many people consider authigenic kaolinite (maximum of 20% including micropores) to be a potentially deleterious cement because of its high microporosity, which causes high residual water saturation, and because its small crystals can break loose during hydrocarbon production and block pore throats (Almon and Davies, 1978; Neasham, 1977; Pittman,

296 E. F. MCBRIDE ef a/.

Table 3. Average percentage of intergranular porosity, pore-filling cement, intergranular volume and porosity

loss due to compaction in both Cambrian and Carboniferous sandstone.

Formation A) Cambrian Naqus Araba Adidia Nasib Abu-Hamata Sarabit El-khadim Average B) Carboniferous Abu Durba Abu-Thora Average

n

18 20 5 5 4 7

7 17

INTP PFC IGV COPL Pm0 TS0 Micro 0 K (md)

10 12.6 22.7 16.2 18.2 12.6 5.6 618

12.4 12.2 24.6 17.9 20.8 15.9 4.9 338 11.8 17.7 29.5 23 19.3 17.7 1.5 190

13 11.2 24.3 17.7 22.1 20.1 2 369

14.1 15.1 29.2 22.7 21.5 20.8 0.7 304

11 14.5 25.5 18.8 17.1 11.8 5.3 351

12.1 13.9 26 19.4 19.7 15.4 4.3 413

13.9 11.5 25.5 18.8 20.4 14.1 6.3 922

16.2 10.2 26.4 19.7 22.4 20.9 1.3 426

15.1 10.9 26 19.3 20.1 17.5 2.6 571

Porosity values are in percent; permeabilities are in millidarcies.

n=Number of samples; INTF’=Intergranular porosity; PFC=Pore-fiUing cement; IGV=Intergranular volume (INTP +PFC);

COPL=Compaction-porosity loss.

Pm 0=Mean porosity measured by porosimeter; TS 0=Mean thin section porosity; Micro e)=Porosimeter 0 - Thin section 0;

K (md)=Permeability; average permeability is geometric mean

1989). Howard (1992), however, believes that kaohnite has little tendency to migrate through pores.

POROSITY LOSS BY COMPACTION

At the time of deposition, clean well sorted sandstones have an initial porosity of approximately 45% (Pryor, 1973; Atkins and McBride, 1992). This value was assumed for both the Cambrian and Carboniferous sandstones. After deposition, that porosity was reduced by compaction processes and cementation. The total amount of porosity lost by compaction (COPL) of a sandstone can be expressed by the following equation (Ehrenberg, 1989):

,,,,~,,_(l00~1~V)-(0PxlCrV) (lOO-IGV) ’

where OP=the original porosity (45%) and IGV=the intergranular volume (intergranular porosity + pore filling cement).

The intergranular volume, which is synonymous with minus-cement porosity (also called pre-cement porosity), is easily quantified by point counting as the sum of intergranular porosity and pore-filling cement (but not grain-replacement cement). Using this variable in Ehrenberg’s equation, the average amount of porosity loss due to compaction is approximately 19% for both the Cambrian and Carboniferous sandstones (range is 16 to 2396, Table 3). That is, the intergranular volume was reduced to an average of 26% by compaction alone.

Compaction in sandstones takes place chiefly by the combination of grain rearrangement (grain slippage and

repacking), ductile grain deformation and intergranular pressure solution (Ftichtbauer, 1967). This relationship can be expressed by the following formula (McBride et al., 1991):

COPL=0 lost by grain rearrangement + 0 lost by ductile grain deformation + 0 lost by intergranular pressure solution.

In the Sinai sandstones there are almost no ductile grains and cathodoluminescence shows that pressure solution at the grain contact is insignificant. Thus, the average 19.4% porosity loss in these sandstones (18.7% for Cambrian and 20.5% for Carboniferous rocks) was by grain rearrangement only. The degree of compaction shown by the Sinai Palaeozoic sandstones is common for quartzose sandstones that undergo maximum physical compaction but not pressure solution (Fiichtbauer, 1967; Houseknecht, 1987).

POROSITY LOSS BY COMPACTION VERSUS CEMENTATION

When assessing the diagenetic modification of primary porosity, it is useful to separate the effects of compactional processes from the effects of cementation. The relative importance of the two processes can be visualized by the use of Houseknecht’s (1987) diagram or its modification by Ehrenberg (1989). The diagram (Fig. 11) illustrates how the intergranular porosity of the sandstones is dependent upon the intergranular volume and the proportion of the intergranular volume that is occluded by cement. The clusters of data show no significant differences between Cambrian and


r = 0.7 n= 175

ll,,,ct,,/,, ,,,, ,,,, ,,,, ,,,, ,,,, 0 5 10 15 20 25 30 35

Porosimeter Porosity (%)

Figure 10. Plot of porosimeter porosity versus permeability.

Carboniferous samples. However, Carboniferous samples are less cemented and have more intergranular porosity than Cambrian ones. The diagram (Fig. 11) also shows that for 3/4 of the formations studied, more porosity was lost by compaction than by cementation. The world-wide importance of porosity loss in sandstones by compaction was stressed by Lundegard (1992).

SUMMARY AND CONCLUSIONS

The quartzose and feldspathic Palaeozoic sandstones that were studied were probably not buried more than 1 to 1.5 km until Late Cretaceous times and later, when

the deepest rocks reached 2.5 km. In spite of their age and the large volumes of groundwater that probably passed through them, they retain, with a few exceptions, sufficient porosity and permeability to possess excellent reservoir quality. In fact, the groundwater dissolved enough feldspar grains from the sandstones to contribute 25 to 30% of the total pore space.

ordinary quartz cement were separate events in the Cambrian versus Carboniferous rocks. Some secondary porosity was created (5.8 and 5.1% for Cambrian and Carboniferous sandstones, respectively) by the dissolution of feldspar and probably some rock fragments and calcite cement. Regeneration of porosity to more than 25% occurred in some samples, but in others, porosity was later reduced by kaolinite cementation. Authigenic kaolinite (maximum of 20% including micropores) is potentially the most deleterious cement because it has high microporosity, which causes high residual water saturation, and because its small crystals have the potential to break loose during hydrocarbon production and block pore throats.

Porosity was reduced by compaction from an estimated original 45% to about 26%. Grain rearrangement was the main mechanism of compaction; intergranular pressure solution and ductile grain deformation are insignificant. There is no significant difference in the degree of compaction shown by Cambrian (older, deeper buried) as opposed to Carboniferous sandstones.

The present-day porosity in these sandstones averages 19% and ranges from 1.5 to 32%. Many sandstone samples (47% of a total of 178 samples) have permeability values higher than 1000 md. The plot of porosity versus the log of permeability has a good correlation indicating that microporosity, even though locally important, does not control reservoir quality. In general, the Palaeozoic sandstones of this study have excellent aquifer and hydrocarbon reservoir quality.

Acknowledgements

Following and during compaction, cementation Financial support was provided by the Egyptian by iron oxide, quartz and calcite further reduced the Educational Cultural Bureau and the J. Nalle porosity to 12-15%, except in silcretes and some Gregory Chair in Sedimentary Geology of the ferricretes where the porosity was reduced to ~5%. Department of Geological Sciences of the University Cementation by calcite and kaolinite in both the of Texas at Austin. Constructive comments on the Cambrian and Carboniferous may have occurred at manuscript were made by reviewers Mamdouh the same time, but cementation by iron oxide and Shebl and Abhijit Basu.

Cement (%) 5 10 15 20 25 30 35 40

0 100 Original porosity destmyd by cementation (%)

Figure 11. Diagram showing the relative importance of compaction versus cementation to porosity development in the sandstones. The dots are the averages for each formation.


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Geological map

Geological map of Sinai; 1: 500 000.1980. Compiled by Eyal, M., Bartov, Y., Shimron, A. E. and Bentor, Y. K. Geological Society of Israel, Jerusalem.

Documents

The influence of diagenesis on the reservoir quality of Cambrian and Carboniferous sandstones, southwest Sinai, Egypt