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Marine and Petroleum Geology

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Research paper

Impact of diagenesis on reservoir quality evolution of the late CenomanianAbu Roash “G” Member in the Sitra Field, North Western Desert, Egypt

A. Abu Mostafaa,∗, A.M. Abu Khadrahb, A.A. Refaatb

a Badr Petroleum Company (BAPETCO), Heliopolis 5958, EgyptbGeology Department, Faculty of Science, Cairo University, Giza 12613, Egypt

A R T I C L E I N F O

Keywords:Abu Roash “G” MemberReservoir qualityDiagenesisSitra FieldNorth Western desertEgypt

A B S T R A C T

Reservoir quality evolution of the Abu Roash “G” Member was studied by petrographical analyses from wells inthe Sitra oilfield in the North Western Desert of Egypt. The sandstones of the Abu Roash “G” Member arepredominately subarkose to quartz arenite and exhibit a wide range of porosity (1.2%–23.8%), and permeability(0.008–126mD). Diagenetic features that influenced the reservoir-quality evolution include mechanical com-paction, pressure solution, cementation and dissolution of framework grains and cements. Compaction andcementation are generally the main factors that were responsible for the reduction of porosity and permeabilityin the upper Abu Roash “G” sandstones. Dissolution of detrital feldspars grains was the main drive for porosityenhancement of the upper Abu Roash “G” sandstones. Additionally, the corroded contacts between successivecarbonate phases and quartz grains followed by dissolution of the carbonate cements left behind secondaryporosity. On the other hand, the main porosity preservation in the upper Abu Roash “G” sandstones is due toscattered patches of carbonate cement, which prevented compactional collapse of the sandstone framework andto grain-coating chlorites that inhibited the precipitation of quartz overgrowths during burial. The Abu Roash“G” Member reservoir was subjected to a high diagenetic overprint resulting in marked reservoir heterogeneity.

1. Introduction

The main geologic factors that control reservoir quality in oil andgas reservoirs are porosity and permeability. These are controlled byprimary depositional texture and post-depositional processes of burialdiagenesis. Burial diagenesis controls the final geometry of the porestructure, grain orientation and packing, and the degree of cementationand clay filling of pore spaces (Slatt, 2006). Deep reservoir quality insandstones is the cumulative product of depositional, shallow diage-netic, and deep-burial diagenetic processes (Ajdukiewicz and Lander,2010; Bjørlykke and Jahren, 2012; Goldstein et al., 2012).

The Sitra field (Fig. 1) is located in the western part of the AbuGharadig Basin in the North Western Desert of Egypt. The explorationhistory of the area dates back to 1982 following hydrocarbon discoveryof Upper Cretaceous reservoirs in the Sitra 1-1, Sitra 3-1, and Sitra 5-1exploratory wells. The production started in 1990 from the Sitra 3/5Bahariya reservoir. This study aims to investigate the petrographiccharacteristics of the Abu Roash “G” Member sandstones in the Sitrafield, in order to elucidate the diagenetic impact on the reservoirquality evolution and to guide future exploration in the wider area andin similar settings.

2. Geologic setting

The subsurface of the Western Desert of Egypt (Fig. 1) is char-acterized by a complicated system of Mesozoic rift basins that cover anarea of approximately 200,000 km2 (Hantar, 1990; Sehim, 1993;Bosworth et al., 2008, 2015; Dolson et al., 2014). These basins form aseries of several discrete E-W to ENE-WSW and NE-SW oriented half-graben basins that were initiated in the Jurassic and continued tosubside through the Cretaceous, such as the Abu Gharadig, Alamein,Matruh, and Shoushan basins (Sultan and Halim, 1988; Emam et al.,1990; Taha, 1992; Moustafa, 2008; Bevan and Moustafa, 2012;Bosworth et al., 2015). The Abu Gharadig basin (Fig. 1) is an E-W or-iented asymmetric graben and represents one of the most importantproductive basins in the northern part of the Western Desert. It extendsfor about 300 km in length and 60 km in width. The sedimentary coverof this basin ranges in age from Late Jurassic to Miocene. The Sharib-Sheiba high constitutes the northern border of the basin; the Sitraplatform is its southern limit; the Kattaniya-Abu Roash high lies to itseast and Faghur-Siwa basin to its west (EGPC, 1992; Abdelmalek andZeidan, 1994). The Sitra field containes a series of right-stepping enechelon faults that strike WNW-ESE, defining a series of WNW-ESE

https://doi.org/10.1016/j.marpetgeo.2018.05.003Received 18 January 2018; Received in revised form 26 April 2018; Accepted 3 May 2018

∗ Corresponding author.E-mail address: a.mohamed@pg.cu.edu.eg (A. Abu Mostafa).

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Available online 04 May 20180264-8172/ © 2018 Elsevier Ltd. All rights reserved.

T

trending horst blocks, with the Sitra-8 horst representing the currentstudy area and the main production block of the Sitra field.

The stratigraphic section of the Sitra field follows the North WesternDesert regime (Fig. 2). The Abu Roash “G” Member which is the maintarget of this study is conformably overlain by the “F” Member of theAbu Roash Formation and is underlain by the Bahariya Formation. It iscomposed mainly of shale and sandstone with subordinate limestone.The Abu Roash “G” Member is divided into two main units; upper andmiddle/lower units separated by a transgressive Intra Abu Roash “G”carbonate marker which is a laterally very extensive layer in the AbuGharadig basin. This study focuses on the upper unit as it represents themain reservoirs of the Abu Roash “G” Member in the Sitra field.

3. Samples and methods

Samples were taken from conventional cores from the Abu Roash“G” Member, from two wells in the Sitra field. The samples are fromdepths of 3014–3122m (measured depth). Analytical techniques uti-lized in this study, include thin-section petrographyand scanning elec-tron microscope (SEM), undertaken at the facilities of the EREXServices Company, Egypt. 28 thin sections were prepared and stainedwith Alizarin Red-S and potassium ferricyanide for petrographic anddiagenetic analysis. Sandstone classification was petrographicallyclassified according to Folk (1980). Petrographic descriptions weresupplemented with studies of 14 samples by Scanning Electron Micro-scopy (SEM). The latter used a Philips XL 30-SEM equipped with back-scatter (BSE) and secondary electron detectors as well as an energydispersive spectrometer system (EDX) and an accelerating voltage of 30kv. SEM samples were first cleaned in cold xylene to remove hydro-carbon residues, then coated with gold using a sputter coater. HeliumPorosity and gas permeability on the studied core samples of the AbuRoash “G” sandstones were carried out at the Corex Services LimitedCompany, Egypt.

4. Results

4.1. Petrography of the sandstones

4.1.1. Sandstone composition and textureThe studied samples are composed of very fine to fine-grained

subarkose to quartz arenite sandstones (Fig. 4). The sand grains varyfrom angular to subrounded and are moderately sorted (Fig. 4a). Quartzis the predominant grains in the sandstones (up to 87%) and generallyoccurs as monocrystalline and rare polycrystalline phase. Feldsparsconstitute around (7%) of the total framework and comprise both po-tash and plagioclase feldspars. Few lithic fragments, mainly chert, arepresent and may reach about 0.5%. Rare bioclastic debris and phos-phatic fragments (Fig. 4a) were recorded as well as pelecypod shellfragments (Fig. 4b).Variable amounts of opaque and non-opaque (ru-tile, zircon, and tourmaline) heavy minerals (Fig. 4c), and occasionalmuscovite and biotite flakes (Fig. 4d) occur. Aggregates of framboidaland subcubic pyrite crystals (Fig. 4f), and sand-size, rounded glauco-nitic pellets are observed (Fig. 4b). The quartz: feldspars: lithic (QFL)ratios are displayed in Fig. 3.

4.1.2. Porosity and permeabilityThe pore types of the upper Abu Roash “G” reservoir sands are

dominated by secondary inter- and intra-particle pores (Fig. 4c ande),mainly formed through the partial to near complete dissolution ofdetrital grains (i.e., feldspars). Point counted porosity values rangebetween 1% and 10%. Average pore sizes are highly variable and rangefrom 10 to 140 μm. Pore interconnectivity ranges from poor to mod-erate-good. In the highly cemented samples, the point count porosity issignificantly reduced through the presence of calcite, chlorite andkaolinite cement. The helium core plug porosity ranges from 1.2% to23.8%. The core permeability of the sandstones shows a wide rangefrom 0.008 to 126mD.

Fig. 1. Mesozoic and Cenozoic basins in the Western Desert and Sinai (modified from Dolson et al., 2001; Bevan and Moustafa, 2012; El Gazzar et al., 2016).Reproduced with permission from Elsevier.

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5. Discussion

5.1. Diagenetic processes

In the light of the above-mentioned description, the effects of di-agenesis alteration and paragenesis on reservoir quality are elucidatedas follows:

5.1.1. CompactionTextural features in the studied samples indicate the occurrence of

various degrees of both mechanical and chemical types of compaction,depending on the extent to which the rocks have been buried. Themajor grain contact types are pointing to long and concavo-convextypes dominating and minor sutured suggesting a moderate degree ofcompaction (Fig. 4a and e). Bending of flexible muscovite grains

Fig. 2. Generalized litho-stratigraphic column of the North Western Desert of Egypt (modified from Schlumberger, 1995, 1984).

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(Fig. 4d) due to mechanical compaction is noted. Chemical compactionpresent in the form of pressure solution was recognized by the presenceof microstylolites (Fig. 4d).

5.1.2. CementationThe most common diagenetic minerals which are identified in the

upper Abu Roash “G” sandstones are authigenic clay minerals, carbo-nate, and iron oxide cements. Minor amounts of quartz overgrowths,pyrite and siderite cement were also identified. Carbonate cement(ferroan and non-ferroan) are represented by corrosion of detritalquartz grains at the contact with the calcite cement, resulting in apoikilitic texture (Fig. 4a), whereas patchy pore-filling ferroan calcitecement (Fig. 5e) occurs in some samples. Authigenic clay mineral ce-ment includes kaolinite and chlorite. In SEM photomicrographs(Fig. 5d), kaolinite and chlorite are easily recognized as the mostabundant clay minerals acting as pore-filling cement and coating det-rital grains. Kaolinite is the dominant authigenic clay mineral and oc-curs as pore-filling cement within secondary pores and pseudomorphicreplacement of feldspars (Worden and Morad, 2003). Kaolinite booklets(Fig. 5d) are moderately crystallized and locally showing partly cor-roded pseudo-hexagonal basal sections. Chlorite needles were identifiedin most of the samples and occur as poorly to moderately crystallizedpore-filling and minor grain-coating chlorite plates (Fig. 5d). Ironoxides are also observed in most of the samples and constitute ce-menting material and/or patches filling pore spaces, micro-concretions,veinlets and stylolites (Fig. 4f). Ferrugination of the matrix by ironoxides was recorded in very fine-grained sandstone samples (Fig. 4f).

Other authigenic phases in the upper Abu Roash “G” sandstonesinclude secondary quartz, pyrite, and siderite cements. Poorly to well-developed euhedral, smooth-faced and pyramidal quartz overgrowths(Figs. 4e and 5d) were recorded in most of the samples. Siderite mi-crocrystals locally replaces detrital clays occur in minor amounts insome samples (Fig. 5a).

5.1.3. Dissolution processesThe dissolution of authigenic minerals or grains can enhance por-

osity of a secondary type (Schmdit and Mcdonald, 1979; Ehrenberg,1990; Shalaby et al., 2014). Secondary porosity occurs locally in the

upper Abu Roash “G” sandstones and results from partial to completedissolution of detrital framework grains(quartz, feldspars and glauco-nite pellets), and to the dissolution of calcite cement. Evidence of dis-solution includes corroded contacts between successive carbonatephases and quartz grains (Fig. 4a), followed by dissolution of carbonatecement leaving behind a secondary intra-particle porosity (Fig. 5c), andassociated with authigenic cement with kaolinite replacement of feld-spar (Fig. 5f). Some of the glauconite pellets display a reddish-browncolor due to oxidation to iron oxides (Fig. 5b).

5.2. Paragenesis

Burial history, temperature and pore pressure evolution of the AbuRoash “G” Member (Fig. 6) was modelled in one dimension usingSchlumberger's PetroMod (V. 2011) basin modelling software. The AbuRoash “G” Member experienced a relatively long shallow burial phase(∼70 million years) followed by a phase of rapid burial starting be-tween 60 and 40 Ma to their present maximum burial depth. The phaseof rapid burial was accompanied by significant temperature and porepressure increases. The burial history models show a burial rate in-crease from ∼60 Ma onwards, leading to a present-day maximumburial depth of ∼3000m below seafloor (Abu Roash “G” Member top)and temperature (approximately 117 °C). Pore pressures are at theirmaxima at the present day, around 35MPa for the Abu Roash “G”Member.

The relative timing of the major diagenetic minerals in the analyzedsandstone samples of the upper Abu Roash “G” reservoirs (Table 1)were inferred from their textural relations as observed in thin sectionand SEM analysis. Aprecise knowledge of the timing and duration ofeach of the diagenetic events was difficult to achieve due to the un-availability of K/Ar dating, oxygen and carbon isotope measurementsand fluid-inclusion analysis that provide information on the timing andtemperature of formation of authigenic mineral phases and on thenature and origin of the diagenetic fluids. The diagenetic events aredivided into two regimes: Eogenesis (< 2 km) and Mesogenesis(> 2 km; Morad et al., 2000).

Fig. 3. The studied analyzed samples plotted on the QFL(Quartz, Feldspar, Lithic ratios) triangular diagram of Folk (1980).

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5.2.1. Eogenesis stageDiagenetic modifications that occur during near-surface diagenesis

are strongly governed by parameters that are related to changes in therelative sea level, such as chemical composition of surface waters(meteoric, marine and brackish), detrital composition of the sand, de-position rates, organic matter content and degree of bioturbation(Morad et al., 2000). The main eogenetic modifications in the upperAbu Roash “G” reservoirs sandstones include the following:

1 Alteration of fecal pellets to glauconite which occurs soon after burial.The characteristics of glaucony grains (sand-size, rounded pellets)present in the upper Abu Roash “G” reservoirs are most probably ofparautochthonous type, i.e., reworked and intraformational (e.g.Amorosi, 1995). This process could be attributed to very early di-agenetic stages.

2 Mechanical infiltration of clays into sandy sediments may also occurduring the early stages of diagenesis (e.g. Dunn, 1992).

3 Formation of pyrite: Aggregates of framboidal and subcubic pyritecrystals (Fig. 4f) indicate an eogenetic form (Worden and Burley,2003) which results from the microbial reduction of detrital ferriciron and typically the presence of seawater sulphate during earliestburial (Love, 1967).

3 Formation of siderite: Siderite-dominated eogenetic carbonates occur

owing to the local enrichment of iron in the system, resulting fromthe alteration of ferric detrital clays (Browne and Kingston, 1993;Baker et al., 1996).

4 Mechanical compaction: The initial phase of deformation of grainframeworks typically forms by mechanical compaction immediatelysubsequent to deposition (Worden et al., 2000). With increasingdepth, the sandstones of the upper Abu Roash “G” reservoirs suf-fered a moderate degree of compaction, as revealed from bending offlexible muscovite grains (Fig. 4d), and changing of grain contactsfrom point to long, concavo-convex and minor sutured types.

5 Kaolinization of detrital plagioclase (Fig. 5f), K-feldspar and micasthat may be attributed to various extents of mixing between me-teoric and seawater in the coastal environment (Worden and Morad,2000).

5.2.2. Mesogenesis stageAt burial depths greater than 2 km, diagenetic modifications are

strongly controlled by temperature and chemically evolved formationwaters, and by the extent and pattern of shallow-burial diageneticmodifications induced in the sandstones (Salem et al., 2005). The me-sogenetic alterations in the studied samples of the upper Abu Roash “G”Member include the following:

Fig. 4. - Thin-section photomicrographs of sand-stones in the upper Abu Roash “G” Member.(a).Quartz arenite consists of subrounded and an-gular quartz grains (Q) with calcite cement (C) in theform of poikilotopic crystals enclosing and corrodingseveral detrital quartz grains (see arrows) and asso-ciated with a minor phosphatic grain (Ph), graincontact are of long and concavo-convex type. PPL (b)Probably recrystallized pelecypod fragment (Pf),sand-size glauconite pellets (G) and detrital quartz(Q) floating in an association of a clay matrix andcarbonate cement. XPL (c) Opaque (Op) and non-opaque rutile (R) and zircon (Z) heavy minerals oc-cupy inter-particle pores. PPL (d) Quartz grains (Q)with mica flakes (M) arranged longitudinally alongthe bitumen-filled (B) microstylolite fracture, showbending. XPL (e) Quartz grains (Q) with roundedovergrowths (O) and corroded contacts, grain contactare of concavo-convex type. XPL; (f) Aggregate offramboidal and subcubic pyrite crystals (Py) andfracture filled with a ferruginated clayey material (F).Reflected light.

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1 Chlorite Coating: Authigenic chlorite forms coatings of pseudohex-agonal crystals arranged perpendicular to the detrital grain surfaceforming a coating (Fig. 5d). Grain-coating chlorite derives from thediagenetic transformation of syn-depositional Fe-rich clays (pri-marily berthierine) at burial depths greater than about 3 km andtemperatures greater than 90–100 °C (Ehrenberg, 1993; Aagaardet al., 2000; Ryan and Hillier, 2002; Beaufort et al., 2015). In thestudied samples, chlorite coatings are absent from compacted con-tacts between sutured grains, suggesting the later developed aftermechanical compaction and local hindrance of development ofquartz overgrowths, and thus prior to precipitation of well devel-oped quartz overgrowths (Fig. 5d).

2 Quartz overgrowth: Moderately developed euhedral and smooth-faced quartz overgrowths (Figs. 4e and 5d) around detrital quartzgrains occur in the studied samples. The presence of quartz cementas a syntaxial overgrowth near the sites of intergranular dissolutionand around tightly packed detrital quartz grains (Fig. 4e) indicates amesogenetic origin, as postulated by Worden and Morad (2000).The possible source of quartz cement is predominantly pressuredissolution and stylolite formation within sandstones (Brown et al.,1989). The best evidence of pressure dissolution in the studied

samples are the presence of sutured quartz grains (microstylolites)(Fig. 4d) and corroded contacts between successive quartz grains(Fig. 4e).

3 Ferroan Calcite cement: Patchy, pore-filling, blue-stained ferroancalcite cement occurs in some of the studied samples (Fig. 5e). Inorder to incorporate ferrous iron into the calcite lattice to produce aferroan calcite, reducing conditions should exist. If the pore-watersare oxidizing, any ferrous iron present is rapidly oxidized to ferriciron and precipitated as iron hydroxide. Reducing conditions aremore likely to occur at depth than at near-surface (Adams et al.,1984).

The patchy distribution of ferroan carbonate cement may reflectinitially patchy carbonate precipitation, or it may be due to the sub-sequent partial removal of more evenly distributed cement owing todissolution during burial (Boggs, 2009).

4 Formation of iron oxides: Iron oxide cement exhibits different forms,namely coating around detrital quartz and feldspar grains as well asisolated patches and pervasive pore and microstylolites fillings. Thiniron coating on detrital grains is possibly inherited from source

Fig. 5. - Thin-section and SEM photomicrographs ofsandstones in the upper Abu Roash “G” Member (a)Part of sideritic concretion (darker colored mass oc-cupying the left side of the photo) encompasses veryfine quartz grains. XPL; (b) Fresh glauconite pellet(G) and degraded glauconite pellet (Dg) set in aclayey matrix. PPL; (c) Intergranular porosity withmoderate connectivity (blue areas), with the devel-opment of secondary intra-particle porosity (arrows)a result of the partial dissolution of the detrital pla-gioclase (P) and quartz (Q) grains, associated withtourmaline grain (T). PPL; (d) SEM photomicrographshows moderately crystallized pore-filling authigenickaolinite booklets (K), associated with poorly tomoderately crystallized pore-filling and grain-coatingchlorite plates (Ch). Minor, moderately to well de-veloped euhedral, smooth-faced and pyramidalquartz overgrowths (O)are noted. PPL; (e) Patchypore-filling, blue-stained ferroan calcite cement(FC).PPL (f) SEM photomicrograph shows partialdissolution of kaolinized detrital plagioclase withinterconnected pores (Φ). (For interpretation of thereferences to color in this figure legend, the reader isreferred to the Web version of this article.)

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rocks and the presence of corroded quartz grains suggests the pre-sence of earlier carbonate cement which was replaced by iron oxidethrough solution of iron carbonates with subsequent re-precipitationof iron oxides. The patchy distribution of iron oxides patches sug-gests either aborted cementation. Authigenic iron oxides (hematiteand goethite) were derived by oxidation of iron-bearing minerals(such as biotite) in oxic pore waters (cf. Boggs, 2009). Iron oxides insediments may have formed just after deposition at the sedimentwater interface but were regenerated during burial (Walker, 1974).

5 Chloritization: As evidenced by the occurrence of thin chlorite plateson the alteration surfaces of feldspar (Fig. 5d), it is possible thatkaolinite may have been altered to chlorite during progressiveburial. Chlorite may form diagenetically from smectite and fromkaolinite when a source of Fe and Mg is present (Bjørlykke, 1998).Kaolinite can be replaced by chlorite, especially in systems that havelittle available potassium (thus preventing the occurrence of illitegrowth at the expense of kaolinite). Boles and Franks (1979) re-corded that the chloritization of kaolinite occurs at burial depthsbetween about 3.5 and 4.5 km (T= 165–200 °C). In contrast,chloritization of kaolinite has been reported at depths of about2.5 km (T=100 °C; Ehrenberg, 1993).

5.3. Implications for reservoir quality

5.3.1. Compositional controls on reservoir qualityThe upper Abu Roash “G” Member is composed of moderately to

well sorted, very fine to fine-grained sandstone. Fine-grained sand-stones commonly have a lower initial permeability than those con-sisting of coarse grains shortly after deposition (Shepherd, 1989). Thisimplies that the upper Abu Roash “G” sandstones had a relatively lowinitial permeability during deposition. In addition, the mineral com-position of rocks has an important influence on subsequent diageneticprocesses, such as compaction (Ramm, 2000; Bjørlykke, 2014), whichin turn influences the reservoir quality. The Abu Roash “G” sandstonescontain an average content of approximately 7% potash and plagioclasefeldspars, which were altered to authigenic clay mineral cements thatmay negatively affect the porosity and permeability of the reservoirs.Biotite mica flakes will lead to relatively rapid porosity lost throughductile deformation, due to compaction during progressive burial.Therefore, the textural and mineralogical compositions of the AbuRoash “G” sandstones are susceptible to compaction and dissolutionwhich affect the reservoir quality.

5.3.2. Diagenetic controls on reservoir qualityThe main diagenetic processes that might be controlled the evolu-

tion of porosity and permeability in the upper Abu Roash “G” sand-stones (Table 2) are compaction, calcite and iron oxide cementation,and kaolinite and chlorite precipitation. Compaction and cementationare generally the main factors that were responsible for the reduction ofporosity and permeability in the upper Abu Roash “G” sandstones.Calcite is the most common cement in the Abu Roash “G” sandstonesand fills pores to varying degrees. The inverse correlations between thecalcites and porosity and permeability (Fig. 7a and b) suggest that thecalcite cements occlude both pores and pore throats in the Abu Roash“G” sandstones. However, the patchy carbonate cements could haveplayed an important role in the reservoir quality by slowing downmechanical compaction and creation of a secondary porosity by dis-solution. Iron oxides cements exhibit a negative impact on the porosityand permeability of the upper Abu Roash “G” sandstones owing to theiroccurrence as a cementing material and/or patches filling pore spaces,micro-concretions, veinlets, and stylolites. Quartz overgrowths in theupper Abu Roash “G” sandstones played a subordinate role in reducingthe porosity and permeability by occluding pore throats; this could be

Fig. 6. Burial and thermal histories of the Abu Roash “G” sandstones for well Sitra 8-3 in the Sitra Field.

Table 1Paragenetic sequence of the upper Abu Roash “G” sandstones as established inthis study.

Digenetic Event Eogenesis Mesogenesis

Alteration of fecal pellets toglauconite

—————————

Mechanical infiltration of clays ——————————Pyrite ——————————Siderite ——————————Mechanical compaction ——————————Kaolinitization —————————Chlorite coating ————————Quartz overgrowth ——————————Ferroan calcite ——————————Iron Oxides —————————— ——————————Chloritization ——————————

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attributed to the chlorite coating around detrital quartz grains, whichinhibits precipitation of quartz overgrowths.

The type of clay minerals present is important for the reservoirpotential of sandstone (Tucker, 2001). Authigenic clay mineral cementsin the upper Abu Roash “G” sandstones include kaolinite and chlorite.There is an inverse relationship between the porosity and permeabilityand kaolinite for the Abu Roash “G” sandstones (Fig. 7c and d). Porositydecreases relatively with increasing chlorite content (Fig. 7e). Perme-ability is positive regarding chlorite content (Fig. 7f), although it can

also fill in pore spaces. However, chlorite coatings can effectively pre-vent quartz overgrowth (Ehrenberg, 1993; Ajdukiewicz and Larese,2012; Saïag et al., 2016), so chlorite has a more positive than negativeimpact on permeability in the Abu Roash “G” sandstones. Dissolution offeldspar produces microporosity as the dissolved grains are replaced byclay minerals (kaolinite and chlorite), which may result in the furtherheterogeneity of the reservoirs (Wang et al., 2017). This microporositydoes not contribute much to the reservoir quality of the Abu Roash “G”sandstones, as it is an oil-bearing reservoir. Oil emplacement in the Abu

Table 2Diagenetic effects on porosity evolution of the upper Abu Roash “G” sandstones, as established in this study.

Diagenesis Diagenetic alteration Effect on porosity

Compaction Clay mineral and mica bedding orientation NegativeCementation Quartz Quartz overgrowth Minor

Carbonate Calcite and ferroan calcite and siderite precipitation in pore space MinorSulphide Aggregates of framboidal and subcubic pyrite NegativeIron Oxide Cementing material and/or patches filling pore spaces and stylolites Negative

Authigenic Clay Minerals Formation Moderately crystallized pore filling authigenic kaolinite booklets NegativePore-filling chlorite NegativeGrain-coating chlorite Positive

Dissolution Partial to complete dissolution of detrital feldspar and quartz grains, PositiveCarbonate cement dissolution Positive

Hydrocarbon emplacement Formation of hydrocarbon Positive

Fig. 7. Cross plots showing the relationship between content of common cements and helium porosity and permeability in the Abu Roash “G” sandstones.

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Roash “G” reservoir during early mesogenesis may have protected thereservoir from further cementation and compaction.

6. Conclusions

The Abu Roash “G” Member is composed of very fine to fine-grained, moderately sorted, subarkose to quartz arenite sandstones witha wide range of porosity and permeability. The main diagenetic pro-cesses affecting the reservoir quality are compaction, carbonate ce-mentation, clay minerals cementation and dissolution. Mechanicalcompaction and cementation reduced large amounts of primary por-osity. The presence of carbonate and clay mineral cements played amajor role in the permeability loss of the Abu Roash “G” sandstones.However, the patchy carbonate cements could have played an im-portant role in the reservoir quality of the Abu Roash “G” sandstones byslowing down mechanical compaction and creation of a secondaryporosity by dissolution. Permeability is positive regarding chloritecontent, as chlorite coatings can effectively prevent quartz overgrowth,so chlorite has a predominately positive impact on the permeability ofthe Abu Roash “G” sandstones. Dissolution of feldspar grains and calcitecement is a major contributor to secondary pores in the Abu Roash “G”sandstones.

This study helps to understand the variations in the pathways ofdiagenetic evolution and their impacts on the reservoir quality andheterogeneity of the Abu Roash “G” sandstones in the Sitra field, toguide future hydrocarbon exploration and to provide clues for similarreservoirs.

Acknowledgements

The authors extend grateful thanks to the Egyptian GeneralPetroleum Corporation (EGPC) and to the Badr Petroleum Company(BAPETCO) for data release and permission to publish this article.Thanks are, also, due Prof. Dr. Aleya Hafez, Geology Department,Faculty of Science, Cairo University for her critically reviewing themanuscript.

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