Lithology and sandstone diagenesis types from petrophysical well logs -a tool for improved reservoir characterization in the Rotliegend formation, Permian basin, Northwestern Germany

Lithology and sandstone diagenesis types from petrophysical well logs - a tool for improved reservoircharacterization in the Rotliegend formation, Permian basin, Northwestern Germany

M. Hock1, T. Kraft1, F. Kloas2 and I. Stowe3

1BEB Erdgas und Erdoel GmbH, Hannover, Germany; 2Schlumberger Geophysikalische Service GmbH,Hannover, Germany; 3Schlumberger Log Services B.V., The Hague, Netherlands, now at GeoQuest, Houston,USA

First published in "First Break vol. 13, No 11, November 1995".

Published on the I2S, October 1997

Introduction

The success of Rotliegend gas exploration in North-Western Germany at depths between 4000 and 5000 m isdependent on the depositional environment but more significantly on the degree of diagenetic alterationencountered in the Rotliegend sandstones. Some sandstones are not productive due to the lack of primaryporosity caused by poor sorting and/or high clay content or the anhydrite cementation in sabkhas and ergdeposits. Nevertheless, even originally porous aeolian sandstones or shoreline sands are often unproductive, dueto the diagenetic cementation of the pore space by illite meshwork, kaolinite, carbonates (predominantly calcite)or quartz.

The core coverage in Rotliegend sequences is usally rather poor (on average about 10% of the total thickness)due to the enormous thickness of these sediments and the high cost of coring. Therefore, the obvious method toobtain facies information for uncored intervals and wells is the use of logs, since logs cover the entireRotliegend interval. Furthermore, logs are acquired on a routine basis for petrophysical evaluation and containmuch lithological information.

The value of openhole logs for facies analysis has been demonstrated in Zechstein carbonates of NorthernGermany (Stowe and Hock 1988). The following discussion outlines an equivalent methodololy for Rotliegendsequences.

Palaeogeography and Rotliegend stratigraphy

1. The Rotliegend sequence in the study area (Fig. 1) is subdivided into three major cycles:The Schneverdingen Cycle is deposited in graben areas and in the central parts of the Rotliegend Basinand consists predominantly of aeolian sandstones (Schneverdingen Sandstone), fanglometates andvolcanites.

2. The Slochteren (Dethlingen) Cycle overlies the Schneverdingen graben zones and covers large areas ofthe Rotliegend Basin; this cycle consists predominantly of waterlaid sandstones, basically the SlochterenHauptsandstein.

3. The Hannover Cycle or Hannover Formation as the youngest Rotliegend sequence in the area ischaracterized by the dominance of muddy lake sediments with intercalated reservoir sandstones(predominantly Wustrow Sandstone) and with intercalated salt layers towards the centre of the basin.

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Lithology and sandstone diagenesis types from petrophysical well logs http://i2s.houston.oilfield.slb.com/data/9706hock/9706hock.htm

Figure 1: Location map of study area and key wells.

These major cycles can be subdivided into distinct sedimentary cycles or depositional sequences. A generalizedRotliegend stratigraphy is shown in Fig. 2.

The main targets for gas exploration are the shoreline sands of the Wustrow Sandstone as a part of theHannover Cycle; subordinate targets are aeolian sands in the Slochteren Hauptsandstein and the SchneverdingenSandstone.

Figure 2: Rotliegend stratigraphy in the study area.

Integrated core/log evaluation

Key wells

Due to the great variability of facies and diagenesis in Rotliegend sandstones, a set of 15 key wells distributedover Northwestern Germany (Fig. 1) was selected to develop a petrophysical database for reservoircharacterization. The selection was based on the following criteria:

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1. Wells were representative, covering all reservoir types in the productive area as defined from regionalgeology and petrography.

2. A maximum core coverage between 0.9% and 39.6% with 14.8% on average was selected. The respectiveRotliegend thicknesses range between 140 m and 1900 m in the individual key wells (475 m on average).

3. A modern logging suite was available consisting of density, neutron, photoelectric factor, sonic, naturalgamma ray spectroscopy, dual laterolog and micro-resistivity.

Core database

The cores from the key wells were macroscopically described in terms of lithology and sedimentary structures.These descriptions were presented in a schematic manner to avoid subjective core descriptions.

Thin sections were described with respect to their texture, mineralogical composition and diagenetic minerals.Semiquantitative mineralogical composition was obtained from point count analyses and from X-ray diffractionanalyses (XRD).

Microprobe analyses were used to determine the quantitative chemical composition of clay minerals andfeldspars.

Core plugs were measured in terms of porosity, grain density and permeability (routine petrophysical laboratorymeasurements).

Volumetric log analysis

The sequential steps of log and core evaluation to build a specific database for Rotliegend rocks areschematically sketched in Fig. 3. The various interpretation steps from log preprocessing to volumetric analysis,gas corrections, statistical cluster analysis, multiwell crossplot analysis to a tested multidimensional lithofaciesdatabase are outlined below.

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Figure 3: Schematic flow chart of integrated core/log evaluation.

The log evaluation needed careful data pretreatment such as log editing (including calibration validation,despiking, and precise depth matching), borehole corrections on all logs and invasion corrections on theresistivities. The correction algorithms are published by Dresser-Atlas (1985) and Schlumberger (1989).

The Schlumberger ELAN* Elemental Log Analysis program was used to compute mineral and fluid volumesfrom measured open hole logs. The method is published by Quirein et al. (1986). The program uses all availablecorrected logs simultaneously to compute mineral, water and hydrocarbon volumes of the formation in aniterative process by a least-squares solution of linear and non-linear tool response equations. Porosity andsaturation are computed from gas and water volumes in a subprocess. To determine the gas saturation fromresistivity and porosity logs the dual water model (Clavier et al. 1977) was used, reformulated in terms of fluidvolumes.

This precise volumetric analysis was a necessary step in order to calculate a correct gas saturation, which issubsequently used for the gas correction of the logs (see section gas-corrected logs). The ELAN resultsfurthermore provide the geologist with a mineralogical analysis of the formation.

From thin sections and XRD analyses the Rotliegend Formation in the study area is known to consistpredominantly of eight minerals (i.e. illite, chlorite, anydrite, plagioclase (albite), potassium-feldspar(orthoclase), haematite, calcite and quartz. Mostly subordinate and not always present are kaolinite, smectite(mixed layer clay mineral), dolomite, ankerite, siderite, barite and detritic heavy minerals like zircon andapatite. Ideally a standard petrophysical model for the Rotliegend should therefore include all eight predominantminerals. A matrix model with eight unknown volumes, however, requires eight respective response equations,hence eight logs to be measured. The eight logs used were density, neutron, photoelectric factor, velocity fromthe sonic log, gamma ray, and natural gamma ray spectroscopy (potassium, thorium, uranium). In wells withsignificantly different mineralogies adequately adjusted models were used.

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Since minerals such as illite, chlorite and plagioclase show variable log response parameters, depending on theirchemical composition and their diagenetic stage, results of microscopy (albite content of plagioclase) XRD data(illite crystallinity) and quantitative microprobe analyses have been used to calculate log response parametersfor these minerals from their respective chemical composition (method described in Ellis et al. 1988). Other logresponse parameters for the individual minerals used in ELAN were taken from Edmundson and Raymer(1979), Ellis et al. (1988), Serra (1990) and log interpretation charts (Dresser-Atlas 1985 & Schlumberger1989).

ELAN results were verified (Fig. 4) by comparing computed porosities with core porosities and computedmineralogical composition from XRD analyses, and from semiquantitative microscopic data (point countanalyses).

Figure 4: Results of ELAN (formation analysis) and LITHO (Litho-type) processing compared with core data(core analysis and diagenetic type). Vertical scale in meters indicated by ticks at the left margin of theLitho-Type column; legend for lithotype, formation and core analysis see Fig. 7; for description of diagenetic

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types refer to the section on single well cluster analysis.

Gas-corrected logs

Gas corrections normalize the logs and are necessary to avoid detecting incorrect or false electrofacies due tohydrocarbon effects.

Gas effects on porosity logs are caused by residual gas saturations in the invaded zone (incomplete flushing bymud filtrate). The density, neutron, sonic and the photoelectric factor were corrected for these gas effects usingthe output from the volumetric analysis (ELAN) and replacing gas by water in each tool response equation. Forgas corrections on the sonic log the Wyllie equation was used.

Single well cluster analysis (Faciolog*) *Schlumberger trademark

The gas corrected logs were grouped using the Schlumberger Faciolog clustering process (Wolff andPelissier-Combescure 1982) to obtain log-derived facies intervals (electrofacies) for each individual key well.The logs were transformed into principal component (PC) space, which means a normalization of differentscales for comparison. Density responses with a dynamic range of 2.2-2.9 g cm-3 are not comparable to gammaray responses with a dymanic range of 0-300 API for a vectorial representation.

All scales were therefore normalized to a dynamic range of values between 0 and 1. After normalizationmultidimensional clusters or "modes" (groups of multidimensional vectors) were built of points having bothsimilar PC values and some vertical continuity, in this case 0.4572 m (1.5 ft) which is three sample increments.Since seven logs (density, neutron, photoelectric factor, sonic, sum gamma ray, thorium, potassium) were usedas an input to the Faciolog program, each layer (0.1524 m thick due to the sample increment) was representedby a 7-dimensional vector in 7-dimensional space. The uranium curve was not used since the uncertainty of thelog was considered to be too high but is used indirectly through the use of the total gamma ray which representsthe sum of thorium, potassium and uranium.

The number of clusters encountered was reduced by aggregation, those whose centre of gravity are closest beingcombined first, until a reasonable match between log-derived electrofacies, with facies zoning from the core,and microscopic data was found, without introducing conflicts. If required, final clustering of modes that belongto the same geological facies was performed manually.

By comparing the clusters with the core and with diagenetic features defined from thin sections it was foundthat the logs reflected mainly lithological subdivisions and various diagenetic features in the sandstones.Lithologies distinguished from logs were sandstones, mudstones (M), fanglomerates (G) and volcanites (V).

The diagenetic features found to create typical log responses in sandstones were quartz/calcite/anhydritecementation in sabkha environments (SB), illite coats (IC), haematite coats (H), chlorite overgrowth (C), illitemeshwork (IM) and kaolinite replacement (K) of feldspars accompanied by ankerite/siderite (A) and barite(BA) cements. A few diagenetic types showed no significant influence on the log response. The log response ofdolomite cement (D) as sometimes observed in sabkha environments (SB) was not significantly different fromcalcite cement and was therefore included in the Sabkha (SB) facies type. From observations on core material,bitumen impregnations (BI) showed too little vertical continuity to create significant, specific log responses.Feldspar leaching (FL) increases the porosity and was typically observed in diagenetic facies types with thehighest porosities such as chlorite types (C), haematite types (H) and illite meshwork type (IM). A detaileddescription of these diagenetic types is given in the section on the lithofacies database.

The multidimensional clusters from the key wells (Faciolog electrofacies) could not be simply combined into ageneral petrophysical database, as ambiguities caused by finer or coarser facies subdivisions in different keywells could not be excluded. To investigate for possible ambiguities, a multiwell crossplot analysis followed the

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single-well cluster analyses.

Multiwell crossplot analysis

In the next stage of facies analysis the gas-corrected log values for each electrofacies defined in Faciolog wereplotted in 2D crossplots resulting from the combination of all seven logs used. The 7-dimensional spaceinvestigated by Faciolog was therefore represented by 21 2D crossplots.

Figure 5 shows a multiwell crossplot (density-neutron) with log data from all key wells for haematite typesandstones and sabkha sandstones as identified from clustering and after comparison with cores. The twoplotted facies types form homogenous clusters of data in the 2D density-neutron space. However, distinguishingall Rotliegend facies types from just one single crossplot (e.g. density-neutron) is not possible.

Figure 5: Multiwell crossplot with gas-corrected log data of two major lithofacies (haematite type and sabkhasandstones); the various symbols represent samples from different key wells.

Description of lithofacies database (LITHO)

The Schlumberger LITHO-Analysis process (Delfiner et al. 1984) was used to define a local detailed databasefor the electrofacies determined. The clusters from the multiwell crossplot analysis were used to definemulti-dimensional ellipsoids by constructing confidence ellipses around each cluster in the 2D crossplots, thuscreating the final lithofacies. Figure 6 shows an example of confidence ellipses for some major electrofacies inthe density-neutron crossplot. Lithology and diagenetic types used for lithofacies descriptions are based on theclassifications defined by Gaupp (1989a, b).

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Figure 6: Density-neutron crossplot with probability ellipses for major lithofacies; shaly sandstones,fanglomerates and volcanites are not plotted due to overlap with other facies in this crossplot.

All lithofacies differentiation from log responses are based on differences in mineralogical composition, porevolume and porosity types. These features are largely the result of diagenetic processes, at least in thesandstones. Diagenetic features, however, are not independent of sedimentary environments. Therefore, thelithofacies obtained from logs do not directly describe the geological facies, but an interpretation of diageneticfeatures often allows conclusions regarding the depositional environment predating the diagenesis, i.e.haematite-type sandstones indicate aeolian deposits, anhydrite cementation indicates sabkha facies, chlorite istypical for shoreline deposits and illite coats for red bed conditions. A legend for all the Rotliegend lithofaciesdefined is given in Fig. 7. These lithofacies are described below.

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Figure 7: Legend of lithofacies for LITHO-plots and minerals and fluids for formation analysis (ELAN) andcore analyses (see Fig. 4).

Sandstones

Chlorite type (C) Chlorite-type sandstones are the best reservoir facies and have the highest porosity andpermeability. C-type sandstones have high intergranular pore volume (7-24%, on average 15%) withpermeabilities of 1-30 md (measurements under surface conditions). C-type sandstones are fine-grained andmoderately to well sorted. They are characterized by early diagenetic Mg-chlorite overgrowths on the sandgrains, i.e. on quartz and feldspar grains. C-type sandstones usually have only minor feldspar content, probablydue to the observed feldspar leaching. Spotted anhydrite and bitumen impregnations are common. The chloritetype sandstones are considered to represent a belt of shore-line sands subparallel to a former E-W orientation ofthe Rotliegend lake. They occur predominantly in the Wustrow Cycle and are rare in the uppermost SlochterenCycle.

Haematite type (H) Haematite-type sandstones, the second best reservoir facies have porosities between 10 and20%. H-type sandstones are mostly massive and often without visible bedding, medium- to coarse-grained andmoderately- to well- sorted. Haematite coatings around detrital grains are characteristic. H-type sandstonesusually have a high feldspar content, predominantly potassium feldspar (orthoclase). Feldspar leaching iscommon, but seems to be selective on plagioclase (albite). Haematite-type sandstones are well developed inthick aeolian sands, predominantly in the Schneverdingen Cycle and in the Slochteren Cycle.

Illite coats (IC) Illite-coated type sandstones are poor reservoir sandstones, the sections with the relativelyhighest porosities are producible after hydraulic fracture treatment. IC-type sandstones have reducedintergranular pore volume (porosity 2-10%, average permeability 0.01-1 md) due to enhanced pressure solution(tight grain packing) and anhydrite, carbonate and quartz cements are common. IC-type sandstones are fine- tomedium- grained and medium- to poorly-sorted. Early diagenetic tangential illite grain coats are either detrital,pedogenetic or by infiltration. The illite coats are a catalyst for pressure solution and can act as nuclei for laterillite growth (illite meshwork). Typical also are high contents of both feldspars, i.e. albite and orthoclase(partially as single fledspar grains but often in detrital grains of volcanic material). Illite coated sandstones occurin red beds and are most common in the Slochteren Cycle.

Illite meshwork (IM) Illite meshwork sandstones are poor or non-reservoir sandstones. The sections with therelatively highest porosities are sometimes producible or candidates for hydraulic fracture treatment. They arecharacterized by relatively high porosities (on average 9-13%), but usually very low permeabilities (0.006-0.8md). Characteristic for IM-type sandstones are radial overgrowths on detrital grains of platy to fibrous illite("Maschenillit", "Faserillit") which have a severe negative influence on permeability. The occurrence ofillite-meshwork is independent from the depositional environment and seems to be formed by three differentprocesses.

1. IM-type sandstones with low porosities are formed IC-type sandstones in the Slochteren Cycle, where theillite coats of the IC-types acted as nuclei of illite growth.

2. IM-type sandstones with higher porosities seem to be formed by illite growth in H-type aeoliansandstones. They often occur sandwiched between fanglometate layers in the Schneverdingen and in theLower Hauptsandstein. Leaching of potassium feldspars from detrital volcanite grains might be the sourceof the potassium meeded for the illite growth.

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3. Porous IM-types occur in former C-type sandstones of the Wustrow Cycle. This extensively developedmeshwork illite is considered to be caused by acidic fluid influx from the Carboniferous and seems tooccur in an aureole of about 1-2 km around late normal faults cutting through Rotliegeng andCarboniferous.

Kaolinite type (K) Kaolinite type sandstones are usually non-reservoir sandstones, and at best candidates forhydraulic fracture treatment. Porosities are well below 10%. Characteristics are permeability reducing kaoliniteaggregates, which replace partly or totally leached feldspars. The kaolinitization is often associated withbitumen impregnation, ankerite/siderite and barite cementation. The occurrence of kaolinite-type sandstones isindependent from the depositional environment. Kaolinite replacement of feldspars occurs as an aureole ofseveral 100 m around late normal faults, when Rotliegend is in juxtaposition to Carboniferous. Thekaolinitization is considered to be induced by acidic solutions derived from Carboniferous coal measures in thelateral or vertical vicinity of the altered sandstones.

Sabkha (SB) Characteristic for non-reservoir Sabkha sandstones (SB-type) is an intergrowth ofquartz-anhydrite-calcite filling the intergranular pore volumes (resulting porosities usually well below 5%). Thepredominant texture is a wavy, but irregular, diffuse bedding. SB-types occur predominantly in the vicinity ofsabkha shales (preferentially as intercalated sandstone layers in the Hannover Cycle mudstones). They also formoccasionally thick sequences (10-15 m) in the middle Slochteren sandstone, which can be interpreted as ergmargin deposits.

Shaly sandstones (Sd) Shaly sandstones are dense (non-reservoir) with anhydrite and/or quartz and carbonatecements like SB-type sandstones but with substantial clay content. The clay content is usually concentrated onwavy or diffuse laminations.

Mudstones (M) Mudstones are either massive, laminated, rippled or diffuse lacustrine sediments with transitionsto the sabkha environment. Mudstones in the study area have total porosites of max. 5% (pore space occupiedby clay-bound water, no effective porosity). This lithofacies represents several geological facies groupedtogether, since they are generally non-reservoirs for natural gas. They vary from silt-rich mudstones containinglower proportions of clays to mudstones with high clay volumes and lower silt (grain) content.

Fanglometates (G) Fanglometates range between grain to matrix supported and contain predominantly volcanicdebris and few mud clasts. The porosity of fanglometates lies well below 2% (non-reservoir).

Volcanites (V) Volcanites in the Rotliegend are rhyolite, rhyodacite, andesite or basalt. Volcanites are usuallydense; porous volcanites (around 5%, max. 10%) due to vesicules are rare. Since this porosity is notinterconnected the permeabilities are negligible. Gas production from naturally fractured Rotliegend volcanitesis an exception.

Verification and application of the lithofacies database

The established lithofacies database was finally tested on two independent test wells. Results of theLITHO-analysis were verified by comparison with cored intervals in the test wells.

This lithofacies database has now been successfully applied to numerous Rotliegend wells as a routineprocedure. The results are regularly controlled by petrophysical core measurements and petrographicdescriptions (core and thin sections).

The lithofacies derived from logs can be used for reservoir description and geological facies interpretation. Aschematic profile illustrates the relationships between sedimentary facies, sandstone diagenesis,syn-sedimentary and late tectonics based on log-derived lithofacies information (Fig. 8).

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Figure 8: Schematic E-W profile of lithofacies in a Rotliegend graben showing non-reservoir illite coat typesandstones above a Carboniferous paleohigh, chlorite type sandstones (gas reservoir) in the shoreline belt,meshwork illite reducing permeabilities close to a late normal fault and non reservoir kaolinite type sandstonein the deepest graben, where Rotliegend is in juxtaposition to Carboniferous; for legend for lithotypes see Fig.7.

Conclusions

Reasonable gas reservoir quality is associated with porous, permeable sandstones such as the chlorite-type andthe haematite-type sandstones. Illite coated sandstones and even kaolinite-type and illite-meshwork types can beproductive after a hydraulic fracture treatment. The volcanites, which can be sub-economic gas producersbecause of intensive natural fracturing, are rare exceptions.

Log analysis can be used to identify these reservoir facies. The corresponding net lithofacies sections are netreservoir thicknesses, which cannot be determined from porosity cut-offs due to overlap in porosity rangesbetween productive and non-productive facies types.

The lack of petrographic core information can be partially compensated by petrophysical openhole logs, andthus allows a reduction in coring programs in Rotliegend wells.

It must, however, be emphasized that the effort in terms of manpower, time and expertise considerably exceedspetrophysical routine log analysis.

To obtain an adequate log database as shown for the Rotliegend, the following crietria have to be met:

Sufficient key wells with good core coverage must be chosen to define all facies encountered in the studyarea. Relatively complete sets of high-quality logs must be available especially for the creation of the database. Close co-operation between log analysts, petrographers, geochemists and geologists is required. With subsequent routine processing, petrographic verification must still be carried out, but core coverage,

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and hence costs, can be reduced. The electrofacies reflect the in-situ rock composition and texture (influence of diagenesis is verysignificant). Interpretation within the context of the regional framework is needed to translate thelog-derived lithofacies into geological facies and hence a depositional model.

Acknowledgements

The authors wish to thank BEB Erdgas und Erdoel GmbH and Schlumberger for permission to publish thispaper. Special thanks to Prof. Dr. Reinhard Gaupp (University of Mainz, formerly BEB Erdgas und ErdoelGmbH) for providing the basics and systematic classifications for the petrographic descriptions; i.e. diagenenisand lithology, upon which this study is based. Dr. Franz Brauckmann (BEB Erdgas und Erdoel GmbH)supervised the petrographic descriptions and organized the hundreds of XRD analyses and point count analyses.The petrographic core descriptions have been carried out by Dr J?rgen Lewandowski (University Bochum),Dipl.-Geol. Helmut Michel (University GieBen) and Cand.-Geol. Michael Pasternak (Technical UniversityClausthal). Descriptions of diagenetic minerals from thin sections have been performed by Dipl.-Geol. StefanSchpoint count analyses of thin sections have been provided by Dipl.-Geol. Andreas MUniversity Berlin).

MS received September 1994, accepted June 1995.

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Lithology and sandstone diagenesis types from petrophysical well logs -a tool for improved reservoir characterization in the Rotliegend formation, Permian basin, Northwestern Germany