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Marine and Petroleum Geology 26 (2009) 1212–1227

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate /marpetgeo

Distribution of diagenetic alterations within depositional facies and sequencestratigraphic framework of fluvial sandstones: Evidence from the PetrohanTerrigenous Group, Lower Triassic, NW Bulgaria

Mohamed Ali Kalefa El-Ghali a,b,c,*, Sadoon Morad a,d, Howri Mansurbeg a,e, Miguel Angel Caja f,George Ajdanlijsky g, Neil Ogle h, Ihsan Al-Aasm i, Manhal Sirat d

a Department of Earth Sciences, Uppsala University, Villavagen 16, SE-752 36 Uppsala, Swedenb Department of Petroleum Geosciences, Universiti Brunei Darussalam, Tungku Link, Gadong BE1410, Bruneic Geology Department, Faculty of Science, Al-Fateh University, P.O. Box 13696, Tripoli, Libyad The Petroleum Institute, Geoscience Department, P.O. Box 2533, Abu Dhabi, United Arab Emiratese AGR Reservoir Evaluation Services AS, Karenslyst Alle 4, P.O. Box 444, NO - 0278 Oslo, Norwayf Departamento de Petrologıa y Geoquımica, Facultad de Geologıa, Universidad Complutense de Madrid, 28040 Madrid, Spaing University of Mining and Geology, St. Ivan Rilski, Sofia 1700, Bulgariah Environmental Engineering Research Centre, School of Civil Engineering, The Queen’s University of Belfast, Stranmillis Road, BT9 5AG, Irelandi Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada

a r t i c l e i n f o

Article history:Received 19 December 2007Received in revised form 29 July 2008Accepted 6 August 2008Available online 19 August 2008

Keywords:DiagenesisFluvial sandstonesSequence stratigraphyLower Triassic (NW Bulgaria)

* Corresponding author. Department of Petroleum G2461502.

E-mail addresses: melghali@fos.ubd.edu.bn, elghal

0264-8172/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.marpetgeo.2008.08.003

a b s t r a c t

Sequence stratigraphy of fluvial deposits is a controversial topic because changes in relative sea level willeventually have indirect impact on the spatial and temporal distribution of depositional facies. Changesin the relative sea level may influence the accommodation space in fluvial plains, and hence have impacton types of fluvial system, frequency of avulsion, and style of vertical and lateral accretion. This studyaims to investigate whether depositional facies and changes in the fluvial system of the Lower TriassicPetrohan Terrigenous Group sandstones (NW Bulgaria) in response to changes in the relative sea levelhave an impact on the spatial and temporal distribution of diagenetic alterations.Eogenetic alterations, which were encountered in the fluvial sandstones, include: (i) mechanicallyinfiltrated clays, particularly in channel and crevasse splay sandstones towards the top of the lowstandsystems tract (LST) and the base of the highstand systems tract (HST). (ii) Pseudomatrix, which resultedfrom mechanical compaction of mud intraclasts, occurs mainly in channel sandstones at the base of theLST and towards the top of the HST and thus led to porosity and permeability deterioration. (iii) Calcite(d18OVPDB¼�8.1& to �7.5& and d13CVPDB¼�7.8& to �6.3&) and dolomite (d18OVPDB¼�8.3& to�5.2& and d13CVPDB¼�8.3& to �7.1&), which are associated with palaeosol horizons developed on topof crevasse splay and channel sandstones of transgressive systems tract (TST) and LST. Such extensiveeogenetic calcite cements may act as potential layers for the formation of reservoir compartments forunderlying sandstones.Mesogenetic alterations include: (i) calcite (d18OVPDB¼�18.4& to �12.8& and d13CVPDB¼�8.6& to�6.8&) and dolomite (d18OVPDB¼�14.7& to �12.4& and d13CVPDB¼�8.0& to �7.0&), which wereformed in all depositional facies and systems tract sandstones, (ii) illite, which is the dominant diageneticclay mineral in all depositional facies and systems tracts, was associated with albitization of detritalK-feldspars, and (iii) quartz overgrowths, which are most abundant in TST rather than LST and HSTsandstones, because of the presence of suitable infiltrated clays and pseudomatrix in the latter sand-stones. Such cementation by calcite, dolomite, and quartz overgrowths and formation of illite led toporosity and permeability deterioration during mesodiagenesis.The results of this study revealed the importance of integration of diagenesis with depositional facies andsequence stratigraphy of fluvial sandstones in improving our ability to predict the spatial and temporaldistribution of eogenetic alterations and their subsequent impact on mesogenetic alterations, and thuson reservoir quality modifications.

� 2008 Elsevier Ltd. All rights reserved.

eosciences, Universiti Brunei Darussalam, Tungku Link, Gadong BE1410, Brunei. Tel.: þ673 2463001x1368; fax: þ673

i_geo@yahoo.co.uk (M.A.K. El-Ghali).

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M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–1227 1213

1. Introduction

Sequence stratigraphy techniques have been applied success-fully to predict the spatial and temporal distribution of paralic andshallow-marine depositional facies, and hence also the distributionand geometry of reservoir and source rocks in sedimentary basins(e.g. Posamentier et al., 1988a, b; Van Wagoner et al., 1988; Pos-amentier and Allen, 1993; Posamentier and Allen, 1999; Catuneanu,2002). These depositional settings are sensitive to changes inrelative sea level, which, induce, in turn, considerable changes inparameters that have profound impact on distribution of diageneticalterations in sandstones, such as pore water chemistry, detritalcomposition (e.g. intraclasts versus extraclasts), rate of sedimentsupply, organic matter, and bioturbation (Morad et al., 2000).

The application of sequence stratigraphy techniques to unraveland predict the response of fluvial style to changes in relative sealevel is less straightforward compared with paralic and shallow-marine environments. However, changes of the fluvial style frombraided to high sinuosity, meandering (i.e. architecture of fluvialdeposits) have been suggested by Wright and Marriott (1993),Shanley and McCabe (1994), and Posamentier and Allen (1999) tooccur as consequence of changes in the depositional base level,which are controlled by changes in the relative sea level. Changes inthe relative sea level, which also control the style of fluvial systems,have in conjunction with detrital sand composition, climatic condi-tions, and patterns of regional ground water flow, strong impact oneogenetic alterations in fluvial sediments (Morad et al., 2000).

This study aims to elucidate and discuss the distribution ofdiagenetic alterations and of their potential impact on reservoirquality evolution in fluvial sandstones of the Petrohan TerrigenousGroup (PTG), Lower Triassic, NW Bulgaria (Figs. 1 and 2) in thecontext of depositional facies and sequence stratigraphy. Diageneticregimes used in this study are: (i) eodiagenesis (0–2 km depth and<70 �C) that refers to alterations during which pore water chemistryis controlled by surface water, and (ii) mesodiagenesis (>2 km depth

Fig. 1. Location map of the study area in NW Bulgaria. Sampling of the Petrohan Terrigenoufrom Ajdanlijsky, 2002a).

and >70 �C) refers to diagenetic alterations that are mediated byevolved formation water (Morad et al., 2000).

In this study we adopted the widely applied sequence strati-graphic model for fluvial systems available in the literature, whichwas summarized by Wright and Marriott (1993), and Shanley andMcCabe (1994). According to these models, changes in relative sealevel control changes in depositional base level, and thus in thefluvial style. The lowstand systems tract (LST) forms when the baselevel falls (corresponds to fall in the relative sea level) and is,therefore, accompanied by low rate of accommodation creation. Asa result, channels migrate and accrete laterally, and form amal-gamated coarse-grained, braided fluvial sandstones with anerosional bounding surface at the base (i.e. sequence boundary, SB;Shanley and McCabe, 1994). During deposition of the late stage ofLST, accommodation starts to increase slowly owing to progressivebase level rise. As a result, floodplain deposits may develop, buttheir preservation is low owing to erosion by avulsing channels(Wright and Marriott, 1993; Shanley and McCabe, 1994).

During deposition of the transgressive systems tract (TST), baselevel rise increases as a consequence of sea level rise. As a result, theaccommodation space increases and leads to enhanced verticalrather than lateral accretion, and thus to an increase in the aggra-dations of floodplain deposits, with subordinate isolated channeland crevasse splay deposits. During deposition of the highstandsystems tract (HST), base level rise and the increase in accommo-dation space slow down leading to reduction of aggradation rate onfloodplains. Further decrease in the accommodation towards thetop of the HST leads to lateral rather than vertical accretion of thefluvial deposits, and thus increases sinuosity, which results inlateral accretion of fluvial channels (Wright and Marriott, 1993).

2. Geological setting

The Petrohan Terrigenous Group (PTG; z150 m thick) wasdeposited during the early Triassic (�Catalov, 1975; Tronkov and

s Group was focused on two locations, the Cerovo and the Oplinja outcrops (modified

Fig. 2. Schematic stratigraphic section of the Triassic fluvial succession in the studyarea in NW Bulgaria (modified from Ajdanlijsky, 2002b).

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–12271214

Ajdanlijsky, 1998; Ajdanlijsky, 2002a, b) in NW Bulgaria where thecomplete succession of the Triassic system is well exposed (Fig. 1).During deposition of the PTG, northwestern Bulgaria was part ofthe Eurasian platform, which was situated on the passive margin ofthe Eurasian craton and positioned between 30�and 40�N palae-olatitude (Philip et al., 1996). Palaeoclimatic conditions were arid tosemi-arid (Ajdanlijsky, 2002b). The PTG represents the initial stageof the Mesozoic transgression event, which is marked by clastic redbeds at its base (Tronkov, 1981). The PTG disconformably overliesPalaeozoic basement rocks and is in turn overlain by tide-domi-nated deltaic and tidal flat deposits of the Svidol Formation (Fig. 2).The PTG consists of sandstones, siltstones and mudstones, whichwere deposited in braided, anastomosing and high sinuosity,meandering fluvial systems (Ajdanlijsky, 2001; Ajdanlijsky, 2002b).

3. Depositional facies and sequence stratigraphy

The PTG in the study area consists of three third-order fluvialsequences (each ca. 35–45 m thick; Ajdanlijsky, 2002b), eachmarked at its base by a distinctive erosion surface incised intounderlying deposits, with amplitudes varying from several metersto over 30 m. These sequences, which are interpreted to begenerated during a base level fall (cf. Shanley and McCabe, 1994;

Fig. 3), comprise (i) amalgamated braided fluvial (lowstand systemstract, LST), (ii) mud-rich, anastomosing isolated fluvial channel(transgressive systems tract, TST), and (iii) high sinuositymeandering fluvial deposits (highstand systems tract, HST).

Fluvial LST deposits were formed during the time of base levelfall. During base level fall, the channels migrated laterally on pre-existing floodplains or channel deposits, and thus formeda multistory and multilateral amalgamated sandstone complexwithin incised valley with an erosional basal surface (cf. Shanleyand McCabe, 1994; Zhang et al., 1997; Fig. 3). These LST depositsare medium- to coarse-grained, stacked multistory, amalgamatedchannel sandstones with a small proportion of fine-grained,overbank deposits. The channel fill deposits are dominated bysandy bed-forms with a thickness that ranges between 3.6 m and4.9 m, and they exhibit high width/thickness ratios (Ajdanlijsky,2002a, b).

Fluvial TST deposits are interpreted to have been formed duringthe successive rapid rise of the base level, which resulted in a highrate of aggradation and in lowering of the fluvial gradient, and thusin anastomosing fluvial deposits with predominantly fine-grainedfloodplain sediments and crevasse splay sandstones developed (cf.Shanley and McCabe, 1994; Zhang et al., 1997; Fig. 3). The fluvial TSTdeposits comprise isolated channel sandstones, and crevasse splaysandstones interbedded with overbank mudstones (Ajdanlijsky,2002b). The channel deposits are composed of fine- to medium-grained, ribbon sand bodies, whereas the crevasse splay or leveedeposits are composed of fine-grained, tabular sand bodies. Theoverbank sediments are sheet-like in appearance, which are later-ally more extensive than the channel sandstones, and compriseboth massive and horizontally laminated interbedded siltstonesand mudstones (Ajdanlijsky, 2002b).

Fluvial HST was formed when the base level rise started toslow down, with reduction of aggradation rate, and this resultedin an increase in the lateral migration and the sinuosity of theriver system (cf. Shanley and McCabe, 1994; Zhang et al., 1997).The HST deposits are multistory and multilateral, channelcomplex sandstones and overbank mudstones of meanderingfluvial system. The overbank proportion of the HST is less and thechannel-fills greater compared with fluvial TST deposits (Ajdan-lijsky, 2002b; Fig. 3).

4. Samples and analytical procedure

One hundred and ten sandstone samples were collected fromtwo outcrop areas with well constrained depositional facies andsequence stratigraphic framework (Figs. 1 and 3). The samplescover the various depositional facies and systems tracts (Fig. 3).Detailed petrographic examination was performed on 110 thinsections, which were prepared subsequent to impregnation withblue epoxy resin under vacuum. Modal compositions wereobtained from the 110 sandstone samples by counting 300 points ineach thin-section (Table 1). A JEOL JSM-T330 scanning electronmicroscope (SEM) equipped with digital imaging system was usedto investigate the habits and textural relationships of diageneticminerals in 21 selected samples. The samples were coated witha thin layer of gold and examined under an acceleration voltage of20 kV and a beam current of 0.6 nA.

Carbon and oxygen stable isotope analyses (Table 2) were per-formed on 30 carbonate-cemented sandstone samples. Samplingusing microdrilling techniques was attempted in order to obtaincarbonate cements with different textures. However, the small sizeof the pores led to contamination among different generations ofcarbonate cements. Nevertheless, all analyses represent a certaindominant carbonate cement type in the samples. Sandstonesamples cemented by calcite and dolomite were subjected to thesequential chemical separation treatment described by Al-Aasm

Fig. 3. General sequence stratigraphic framework of the Petrohan Terrigenous Group fluvial sandstones (Lower Triassic, NW Bulgaria). LST refers to lowstand systems tract, TST totransgressive systems tract, and HST to highstand systems tract.

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–1227 1215

et al. (1990). These samples were analyzed using a Finnegan–MATDelta plus mass spectrometer. Calcite-cemented sandstone sampleswere reacted with 100% phosphoric acid at 25 �C for 4 h, anddolomite-cemented samples were reacted at 50 �C for 24 h. The gaswas analyzed using a SIRA-12 mass spectrometer. The phosphoricacid fractionation factors used were 1.01025 for calcite and 1.01060for dolomite. Precision of all analyses was better than �0.04&.Oxygen and carbon isotope data are presented in the d notationrelative to the Vienna PDB (PeeDee Belemnite) and SMOW (Stan-dard Mean Ocean Water) standards.

Chemical analysis (Table 2) of the carbonate cements was per-formed on 34 carbon-coated, polished thin sections using a CamecaCamebax BX50 microprobe (EMP), equipped with a backscatterelectron (BSE) detector, under accelerating voltage of 20 kV, beamcurrent of 10–15 nA, and a beam spot size of 1–5 mm. Analyticaltotals of carbonates (97–110%) were normalized to 100% forcomparison purposes.

5. Results

5.1. Sandstones: texture and composition

The Petrohan Terrigenous Group (PTG) fluvial sandstones aresubrounded to well rounded, poorly- to moderately-sorted, andfine- to coarse-grained quartzarenites and sublitharenites (av.Q95.8F1L3.2; Fig. 4 and Table 1). The dominant framework grain isquartz (range 21–78 vol.%; av. 61 vol.%), which is representedmainly by monocrystalline quartz (21–78%; av. 52%), and smalleramounts of polycrystalline quartz (1–50%; av. 11%). The feldspars(trace-3%; av. <1%) are K-feldspars and plagioclase (Table 1). Thelithic fragments (trace-6%; av. 1%) are mainly acid volcanic,plutonic, and trace amounts of sedimentary and metamorphicrocks (Table 1). Micas (trace-19%; av. 4%), which are more abundantin TST than in LST and HST sandstones (Table 1), include muscovite(trace-19%; av. 3%) and biotite (trace-11%; av. 0.5%, Table 1). Mudintraclasts (trace-12%; av. 2%), which are more abundant in LST andHST sandstones than in TST sandstones (Table 1), were squeezedbetween the rigid quartz grains and resulted in the formation ofpseudomatrix. Heavy minerals (trace-2%; av. <0.5%) include zircon,apatite and epidote.

5.2. Diagenetic alterations: petrology, geochemistry, anddistribution patterns

5.2.1. Silicates5.2.1.1. Clay minerals. Clay minerals (trace-35%; av. 15%) includegrain-coating and/or pore-bridging clays (trace-7%; av. 2%), illite (trace-35%; av. 15%) and chlorite (trace-9%; av. 0.5%). Grain-coating and/orpore-bridging clays occur as thin (z2–15 mm thick) platelets that aretangentially arranged on detrital grain surfaces or bridges betweendetrital grains (Fig. 5A and B, respectively). SEM examinations revealedthat grain-coating clays display honeycomb-like texture (Fig. 5C), beingmost common in channel and crevasse splay sandstones towards thetop of LST and the base of HST successions (trace-7%; av. 3% and trace-5.1%; av. 2%, respectively; Table 1), compared with TST successionswhere they occur as trace amounts (Table 1). Grain-coating clays arethick (z10–15 mm) and continuous in LST and HST sandstones but arerelatively thin (z2–4 mm) and discontinuous in TST sandstones. Insome cases, clays occur as discontinuous coatings being thickest in thegrain embayments (Fig. 5D).

However, illite occurs mainly as pore-lining and replacement ofgrain-coating clays and detrital grains such as micas and pseudo-matrix. Pore-lining illite displays fibrous and hair-like crystals(20 mm long) oriented perpendicular to the detrital grain surfaces,as well as flake- and honeycomb-like crystals with spiny termina-tions. In some cases, illite extends and bridges the pores. The micasare replaced by fibrous, filamentous, and booklet-like illite thatinflates into the adjacent pores (Fig. 6A and B). Illite is engulfed by,and thus pre-dates, quartz overgrowths. Illite is common in alldepositional facies and systems tracts (Table 1).

Chlorite occurs as scattered platelets within framework grains(Fig. 6C) such as lithic fragments, micas and, rarely, pseudomatrix,and as grain-coating clay (Fig. 6D). Chlorite is engulfed by, and thuspre-dates, quartz overgrowths. Chlorite occurs in small amounts inall depositional facies and systems tracts (Table 1).

5.2.1.2. Quartz. Quartz cement (trace-25%; av. 6%) occurs as euhe-dral, syntaxial overgrowths around detrital quartz grains (100–150 mm thick) and, very rarely, as outgrowths. Quartz overgrowthsare common near sites of intergranular contacts. The boundariesbetween the quartz overgrowths and the detrital quartz grains are

Table 1Modal composition (maximum, minimum, average and standard deviation) of 110 samples from the Petrohan Terrigenous Group fluvial sandstones, Lower Triassic, NW Bulgaria

Depositional facies and sequence stratigraphy CS HST (n¼ 6) CH HST (n¼ 22) CS TST (n¼ 14) CH TST (n¼ 6) CS LST (n¼ 10) CH LST (n¼ 52)

Min. Max. Mean S.D. Min. Max. Mean S.D. Min. Max. Mean S.D. Min. Max. Mean S.D. Min. Max. Mean S.D. Min. Max. Mean S.D.

Detrital grainsMonocrystalline quartz 54.0 77.5 61.2 5.3 33.3 75.1 50.5 7.7 44.7 58.3 49.1 5.8 18.3 61.0 39.6 20.0 48.7 67.7 55.8 5.9 20.3 59.7 48.1 8.1Polycrystalline quartz 4.0 13.3 8.8 3.4 4.0 30.0 14.5 8.5 3.3 18.0 9.9 4.4 2.3 8.3 4.2 2.2 0.7 11.3 6.2 3.9 1.0 50.0 12.5 9.6Potassium feldspars 0.7 1.7 1.1 0.3 0.0 1.7 0.5 0.6 0.3 1.3 0.9 0.4 0.0 1.0 0.5 0.4 0.0 1.7 0.5 0.5 0.0 1.7 0.4 0.6Plagioclase feldspars 0.0 0.3 0.1 0.1 0.0 1.0 0.1 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.2 0.3 0.0 1.0 0.1 0.2Volcanic lithic fragments 0.0 2.3 0.7 0.9 0.0 4.7 1.3 1.3 0.0 3.3 0.8 1.0 0.0 2.3 1.4 0.9 0.0 3.7 1.3 1.1 0.0 4.3 1.2 1.3Plutonic lithic fragments 0.0 0.3 0.1 0.1 0.0 5.0 0.5 1.1 0.0 2.3 1.0 0.9 0.0 3.0 0.5 1.2 0.0 1.7 0.2 0.5 0.0 5.7 1.0 1.5Sedimentary lithic fragments 0.0 0.7 0.1 0.3 0.0 0.0 0.0 0.0 0.0 0.7 0.1 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.7 0.3 0.8Metamorphic lithic fragments 0.0 0.0 0.0 0.0 0.0 0.7 0.1 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Muscovite 0.0 8.7 2.4 3.1 0.0 15.7 2.9 4.4 0.0 7.3 4.2 2.3 0.7 9.7 3.6 3.4 0.3 7.0 3.6 2.7 0.0 10.3 2.3 2.8Biotite 0.0 0.0 0.0 0.0 0.0 1.0 0.2 0.3 0.0 0.0 0.0 0.0 0.0 1.3 0.2 0.5 0.0 3.3 0.9 1.3 0.0 1.7 0.1 0.4Chert 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Heavy minerals 0.0 0.3 0.1 0.2 0.0 2.0 0.4 0.6 0.0 1.3 0.7 0.5 0.0 0.7 0.1 0.3 0.0 1.7 0.6 0.7 0.0 2.0 0.4 0.5Mud intraclasts 0.0 3.3 1.8 1.5 0.0 12.1 1.1 1.5 0.0 2.3 0.5 0.8 0.0 3.7 1.1 1.7 0.0 3.7 0.6 1.3 0.0 10.6 1.3 1.9

Diagenetic alterationsInfiltrated clays 0.0 3.2 1.4 0.0 0.0 5.1 1.7 0.7 Trace Trace Trace Trace Trace Trace Trace Trace 0.0 3.2 2.2 0.4 0.0 6.7 3.9 1.7Quartz overgrowths 0.0 9.4 3.3 5.9 2.0 8.7 3.6 5.3 1.0 16.3 6.8 6.1 0.0 21.0 10.7 8.5 0.0 5.0 1.9 1.9 0.0 8.3 4.2 5.4Pseudomatrix 0.0 6.7 2.2 2.5 0.0 12.3 2.2 3.9 0.0 1.7 0.6 0.7 0.0 0.0 0.0 0.0 0.0 13.3 1.5 4.2 0.0 15.7 1.6 2.9Illite 7.3 26.0 9.4 6.6 4.0 23.7 11.3 5.9 11.7 24.7 12.4 4.4 0.0 23.3 7.4 9.0 5.0 22.0 10.1 5.2 5.0 35.3 10.5 6.3Chlorite 0.0 0.0 0.0 0.0 0.0 3.3 0.4 0.9 0.0 0.0 0.0 0.0 0.0 0.7 0.1 0.3 0.0 0.7 0.1 0.2 0.0 9.3 1.1 2.6Albite in feldspars 1.3 10.3 3.1 3.7 0.0 6.3 1.5 1.9 1.0 4.7 2.7 1.3 0.0 1.0 0.3 0.4 0.0 8.0 1.9 2.6 0.0 7.7 1.4 2.2Calcite replaces detrital grains 0.0 6.3 1.3 2.5 0.0 8.3 1.8 2.5 0.0 13.3 2.0 4.3 0.7 9.0 4.6 3.0 0.0 2.0 0.5 0.7 0.0 15.0 2.2 3.6Microcrystalline calcite 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0 6.0 4.5 2.6 0.0 7.3 2.4 0.8 0.0 4.7 1.6 0.7Blocky calcite 0.0 4.3 1.1 1.7 0.0 15.3 2.7 4.2 0.0 7.0 2.5 2.7 2.3 20.7 8.5 7.4 0.0 13.7 3.5 5.4 0.0 16.3 2.9 4.4Poikilotopic calcite 0.0 1.0 0.2 0.4 0.0 6.3 0.4 1.4 0.0 1.3 0.3 0.6 0.0 8.7 5.1 3.6 0.0 12.0 2.9 3.8 0.0 13.7 0.8 2.3Dolomite 0.0 2.0 0.5 0.8 0.0 6.7 0.6 1.6 0.0 2.7 1.0 1.2 0.0 18.0 6.5 7.7 0.0 11.0 2.1 3.3 0.0 10.3 1.0 2.0Pyrite 0.0 1.0 0.3 0.4 0.0 1.7 0.4 0.6 0.0 2.3 0.7 0.9 0.0 3.3 1.2 1.6 0.0 1.3 0.4 0.5 0.0 3.7 0.5 0.8Fe-oxide 0.0 2.7 0.8 1.0 0.0 2.0 0.6 0.7 0.0 5.7 2.1 2.0 0.0 0.0 0.0 0.0 0.0 4.6 0.6 0.4 0.0 4.7 0.5 0.7

PorosityIntergranular porosity 0.0 0.0 0.0 0.0 0.0 2.0 0.2 0.6 0.0 2.3 0.3 0.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.7 0.2 0.9Intragranular porosity 0.0 0.3 0.1 0.1 0.0 2.0 0.2 0.6 0.0 2.0 0.2 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Moldic porosity 0.0 0.0 0.0 0.0 0.0 5.0 0.5 1.5 0.0 1.7 0.2 0.6 0.0 0.0 0.0 0.0 0.0 1.0 0.1 0.3 0.0 0.3 0.0 0.0Fracture porosity 0.0 0.0 0.0 0.0 0.0 1.3 0.1 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 0.0 0.3

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Table 2Elemental chemical composition, carbon and oxygen isotopic analyses of calcite and dolomite cements of the Petrohan Terrigenous Group fluvial sandstones Lower Triassic,NW Bulgaria

Sample number Systems tract Facies Cement MgCO3 CaCO3 FeCO3 MnCO3 d13CvPDB& d18OvPDB&

CalcitePTG/OV-02/13 p1 HST Channel Calcite replace quartz 1.4 97.1 0.1 1.4 �7.6 �18.1PTG/OV-02/04 p1 HST Channel Coarse-crystalline calcite 0.1 99.8 n.d. 0.1 �7.1 �18.4PTG/OV-02/03 p2 HST Channel Coarse-crystalline calcite 0.4 97.2 n.d. 2.4 �8.6 �16.5PTG/OV-02/03 p3 HST Channel Coarse-crystalline calcite 0.3 99.7 n.d. n.d.PTG/OV-02/02 p1 HST Channel Coarse-crystalline calcite 0.0 98.1 n.d. 1.9 �7.7 �17.0PTG/OV-02/02 p2 HST Channel Coarse-crystalline calcite 1.6 95.9 n.d. 2.5PTG/OV-02/45 p1 TST Crevasse splay Coarse-crystalline calcite 0.6 99.4 n.d. n.d. �7.1 �18.0PTG/CV-02/54 p1 TST Crevasse splay Coarse-crystalline calcite 1.1 96.3 n.d. 2.6 �7.0 �12.8PTG/CV-02/54 p2 TST Crevasse splay Coarse-crystalline calcite 0.3 97.9 0.1 1.7PTG/CV-02/55 p3 TST Crevasse splay Poikilotopic calcite 0.9 97.7 n.d. 1.3 �6.8 �15.3PTG/CV-02/53 p1 TST Crevasse splay Micritic and microcrystalline calcite 0.1 98.0 n.d. 2.0 �7.1 �7.8PTG/CV-02/53 p2 TST Crevasse splay Micritic and microcrystalline calcite 0.5 97.8 n.d. 1.6 �6.3 �7.5PTG/CV-02/53 p3 TST Crevasse splay Micritic and microcrystalline calcite 0.5 99.4 n.d. 0.1 �7.8 �8.1PTG/CV-02/53 p5 TST Crevasse splay Micritic and microcrystalline calcite 0.1 99.9 n.d. n.d. �6.9 �7.7PTG/CV-02/16 p1 TST Crevasse splay Poikilotopic calcite 0.7 97.9 n.d. 1.4 �7.9 �14.2PTG/CV-02/17 p2 TST Crevasse splay Coarse-crystalline calcite n.d. 100.0 n.d. n.d.PTG/OV-02/52 p4 LST Channel Poikilotopic calcite 0.5 98.7 0.2 0.6 �8.1 �17.2PTG/OV-02/52 p1 LST Channel Poikilotopic calcite 0.5 97.8 n.d. 1.6PTG/OV-02/33 p1 LST Channel Poikilotopic calcite 0.2 97.7 1.2 0.9 �7.7 �17.2PTG/CV-02/45 p4 LST Channel Poikilotopic calcite 0.1 99.9 0.0 0.0 �7.1 �17.1PTG/CV-02/45 p6 LST Channel Poikilotopic calcite 1.0 99.0 n.d. n.d.PTG/CV-02/43 p1 LST Channel Poikilotopic calcite 1.1 98.2 n.d. 0.7PTG/CV-02/40 p1 LST Channel Poikilotopic calcite 0.4 94.7 2.1 2.8 �8.1 �12.8PTG/CV-02/40 p2 LST Channel Poikilotopic calcite 0.3 99.7 n.d. n.d.PTG/CV-02/41 p3 LST Channel Poikilotopic calcite 0.1 99.8 0.1 n.d. �7.4 �13.8PTG/CV-02/41 p4 LST Channel Poikilotopic calcite n.d. 99.2 0.2 0.5PTG/CV-02/14 p3 LST Channel Poikilotopic calcite 0.5 95.0 1.8 2.7PTG/OV-02/52 p1a LST Channel Coarse-crystalline calcite 0.8 97.8 1.1 0.2 �7.2 �15.0PTG/OV-02/52 p1b LST Channel Coarse-crystalline calcite 0.5 99.4 n.d. n.d.PTG/OV-02/51 p2a LST Channel Coarse-crystalline calcite 0.1 98.5 0.4 1.0 �8.2 �17.2PTG/OV-02/51 p2b LST Channel Coarse-crystalline calcite 0.2 98.9 0.1 0.8PTG/OV-02/31 p1 LST Channel Coarse-crystalline calcite n.d. 98.5 0.3 1.2 �7.0 �17.3PTG/CV-02/45 p5 LST Channel Coarse-crystalline calcite 0.6 99.2 n.d. n.d. �6.9 �12.9PTG/CV-02/43 p2 LST Channel Coarse-crystalline calcite 0.1 99.1 0.4 0.4 �7.6 �17.1PTG/CV-02/43 p3 LST Channel Coarse-crystalline calcite 1.2 98.8 0.0 0.0PTG/CV-02/49 p5 LST Channel Coarse-crystalline calcite 1.7 97.5 0.4 0.3 �7.5 �18.3PTG/CV-02/14 p4 LST Channel Coarse-crystalline calcite 3.3 96.0 0.4 0.3 �7.3 �15.6PTG/CV-02/07 p4 LST Channel Coarse-crystalline calcite n.d. 96.3 0.9 2.7 �8.1 �17.5PTG/CV-02/08 p5 LST Channel Coarse-crystalline calcite 0.2 98.9 0.1 0.8 �8.2 �17.5

DolomitePTG/CV-02/53 p4 TST Crevasse splay Rhombic dolomite 44.8 54.3 0.3 0.5 �7.1 �5.2PTG/CV-02/53 p6 TST Crevasse splay Rhombic dolomite 43.9 54.5 0.2 1.4 �7.4 �5.6PTG/CV-02/53 p7 TST Crevasse splay Rhombic dolomite 44.7 54.4 0.0 0.8PTG/CV-02/53 p8 TST Crevasse splay Rhombic dolomite 43.7 55.2 0.3 0.8 �8.3 �6.1PTG/CV-02/53 p9 TST Crevasse splay Rhombic dolomite 44.1 54.6 n.d. 1.2PTG/CV-02/53 p10 TST Crevasse splay Rhombic dolomite 45.1 53.6 0.2 1.2 �7.2 �8.3PTG/CV-02/48 p4 LST Channel Rhombic dolomite 42.7 53.6 1.9 1.8 �7.0 �14.7PTG/CV-02/14 p2 LST Channel Rhombic dolomite 41.6 54.7 1.7 2.0 �8.0 �12.4

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well defined by fluid inclusions, clay coatings and/or iron oxides.Quartz overgrowths engulf, and thus post-date, illite, but areengulfed by, and thus pre-date, coarse-crystalline calcite anddolomite II. Quartz overgrowths are most common in channel andcrevasse splay TST sandstones (trace-21%; av. 8%) compared withLST (trace-11%; av. 3%) and HST (trace-9%; av. 2%) sandstones.

5.2.1.3. Feldspars. Feldspars (trace-10%; av. 2%) occur as albitizeddetrital K-feldspars and trace amounts of albitized plagioclase and,rarely, as feldspar overgrowths around detrital feldspar grains(z50 mm thick). Albite that has replaced the detrital feldsparsoccurs as small prismatic crystals arranged along the twinningplanes of the detrital feldspars. The albitized feldspars, which aresimilar petrographically to those described by Morad (1988) andMorad et al. (1990), occur in all depositional facies and systemstracts (Table 1).

5.2.2. Carbonates5.2.2.1. Calcite. Calcite (trace-29%; av. 4%) occurs as micritic tomicrocrystalline (�50 mm) and coarse-crystalline (up to 200 mm),

pore-filling cement, which replaces partly and/or totally detritalgrains. Micritic and microcrystalline calcite are commonly associatedwith palaeosol horizons that developed on crevasse splay andchannel TST and HST sandstones. Micritic and microcrystallinecalcite occur as grain-coatings around detrital grains (Fig. 7A), asdense texture within which scattered detrital grains are embedded(Fig. 7A), and as local patches (200–800 mm). Rarely, microcrystallinecalcite displaces mica and occurs as rhizocretions (Fig. 7B).Conversely, coarse-crystalline calcite is dominated by blocky (Fig. 7C)and poikilotopic crystals (Fig. 7D) that tend to fill small pores intightly packed framework grains (intergranular volume 5–15%)and/or replace framework grains (Fig. 7E). Coarse-crystalline calcite,which engulfs, and thus post-dates illite and quartz overgrowths(Fig. 7F), is common in all depositional facies and systems tracts.

All types of calcite cements are nearly pure CaCO3 end-member(94.7–100%; av. 98.2%; Table 2) with small amounts of MgCO3

(trace-3.3%; av. 0.6%), FeCO3 (trace-2.1%; av. 0.5%) and MnCO3

(trace-2.8%; av. 1.2%). Bulk isotope composition of micritic andmicrocrystalline calcite (Table 2) reveals a narrow range of d18OVPDB

values (�8.1& to �7.5&) and d13CVPDB (�7.8& to �6.3&), whereas

5

Quartzarenite

Sublitharenite

Lithicsub-arkose

Suba

rkos

e

Quartz

Feldspar Lithic fragments

5

2525

LithareniteArkose

highstand systems tracttransgressive systems tractlowstand systems tract

Fig. 4. Modal composition of 110 sandstone samples from the Petrohan TerrigenousGroup fluvial sandstones Lower Triassic, NW Bulgaria, plotted on McBride (1963)classification.

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–12271218

coarse-crystalline calcite (Table 2) reveals a fairly wide range ofd18OVPDB values (�18.4& to �12.8&) and a narrow range ofd13CVPDB values (�8.6& to �6.8&). The cross plot of d18O and d13Cof the micritic and microcrystalline calcite and coarse-crystallinecalcite displays relatively weak correlation (r¼þ0.4; Fig. 8).

5.2.2.2. Dolomite. Dolomite (trace-18%; av. 1%) occurs as smallrhombic crystals (z50–150 mm). Two generations of dolomite havebeen distinguished, which are hereafter referred to as dolomite I

Fig. 5. (A) Photomicrograph (crossed polarizers) showing mechanically infiltrated clays (a(arrow) of clays between detrital grains. (C) SEM image showing close-up view of grain-inherited clay coatings (black solid arrow) engulfed by quartz overgrowths (white solid arr

and dolomite II. Dolomite I replaces the host sediments (Fig. 9A),forms local patches and/or tends to fill large intergranular pores(z200–800 mm) in loosely packed framework grains (intergran-ular volume 30–48%). Dolomite I is most common in palaeosolhorizons that developed on crevasse splay and channel sandstonesof TST and HST successions. Conversely, dolomite II occurs asscattered rhombic crystals that tend to fill small pores (Fig. 9B) intightly packed framework grains (intergranular volume 5–15%).Dolomite II engulfs, and thus post-dates, illite and quartz over-growths. Dolomite II is common in all depositional facies andsystems tracts.

Dolomites I and II are somewhat Ca-rich (53.6–55.2%; av. 54.4mole %; Table 2) and contain small amounts of FeCO3 (trace-1.9%;av. 0.7 mole %) and MnCO3 (0.8–2%; av. 1.2 mole %). Bulk isotopeanalysis of dolomite I (Table 2) reveals a narrow range of d18OVPDB

values (�8.3& to �5.2&) and d13CVPDB values (�8.3& to �7.1&),and the same applies to dolomite II (Table 2), which revealsa narrow range of d18OVPDB values (�14.7& to �12.4&) andd13CVPDB values (�8.0& to �7.0&). The cross plot of d18O and d13Cof dolomite I and dolomite II displays no correlation (r¼�0.1;Fig. 8).

5.2.3. PseudomatrixPseudomatrix (trace-16; av. 2%) occurs as deformed and squeezed

mud intraclasts in between rigid grains, which extends into adjacentpores (Fig.10A). In some cases, the original shape of the mud intraclastis still recognizable. The volume of pseudomatrix, which is a function ofamounts of mud intraclasts and degree of compaction, is higher inchannel and crevasse splay sandstones, being more abundant towardsthe base of LST (trace-16%; av. 2%) and the top of HST (trace-12%; av. 2%)successions, compared with channel and crevasse splay TST sand-stones, in which it occurs as traces (Table 1).

5.2.4. Other diagenetic alterationsPyrite (trace-4%; av. 0.5%) occurs as small, scattered euhedral crys-

tals (z20 mm) that fill intergranular pores or occurs within

rrow) coating detrital grains. (B) SEM image showing grain-coating clays and bridgescoating clays, typical for smectite. (D) Photomicrograph (crossed polarizers) showingow).

Fig. 6. (A) Photomicrograph (crossed polarizers) showing illitized mica that expanded into the adjacent pores. (B) SEM image showing fibrous and hair-like crystals of illitizedmicas. (C) Photomicrograph (crossed polarizers) showing platelets of chlorite (chl) within framework grains. (D) SEM image showing platelets of chlorite (chl) coating detrital grain.

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–1227 1219

pseudomatrix as well as in the vicinity of mica flakes. Pyrite is engulfedby, and thus pre-dates, calcite and quartz overgrowths. Pyrite is morecommon in crevasse splay TSTsandstones (trace-3%; av. z1.5%) than increvasse splay LST and HST sandstones (trace-2%; av. z0.5% and trace-4%; av. z0.5%, respectively). Fe-oxide (trace-5%; av. 0.5%) occurs asintergranular pore-filling and detrital grain-coatings, being closelyassociated with micas. Fe-oxide is abundant in crevasse splay TST(trace-5%; av. 2%) and less common in crevasse splay LST and HSTsandstones, where it occurs in trace amounts (Table 1).

5.3. Compaction and sandstone porosity

The sandstones of the Petrohan Terrigenous Group displayvariable degrees of mechanical and chemical compaction, particu-larly when the early cements were lacking. Mechanical compactionis evidenced by bending of micas (Fig. 10B) and pseudoplasticdeformation of mud intraclasts into pseudomatrix (Fig. 10A).Chemical compaction, which occurs along detrital quartz grains,has resulted in the development of concave–convex and suturedcontacts. Chemical compaction is most extensive when the quartzgrains are coated with thin illite or when the mica occurs at theinterface along quartz grain contacts (Fig. 10B).

Total thin-section porosity of the sandstones, which includesboth primary and secondary pores, reveals a narrow range fromtrace to 6% (Table 1). The primary intergranular porosity revealsa range from trace to 6% (Table 1), whereas the secondary intra-granular and moldic porosity, which was derived from partial tocomplete dissolution of feldspars, reveals a range from trace to 5%(Table 1). Microporosity was difficult to quantify under the petro-graphic microscope but it does exist mainly as intragranularmicropores within albitized feldspars and between clay crystals. Aplot of total intergranular volume versus total intergranular cementand pseudomatrix (Fig. 11) indicates that the loss of depositionalporosity was greater due to compaction than to cementation. Thedistribution of porosity within different depositional facies andsystems tracts of fluvial sandstones does not show any significantvariation, owing to its small amounts.

6. Discussion

Linking diagenesis to sequence stratigraphic framework offluvial deposits is fraught with difficulties and uncertainties owingto the merely indirect impact of changes in the relative sea level onthe architecture of such deposits. However, such linking would inprinciple reflect changes in the rate of accommodation, which, inturn, influence the fluvial depositional system (Shanley andMcCabe, 1994). Another difficulty arises from the complex spatialand temporal distribution of eo- and mesogenetic alterations in theTriassic fluvial deposits. Nevertheless, eogenetic grain-coatingclays, calcite and dolomite and pseudomatrix, as well as meso-genetic quartz overgrowths and illite, show fairly systematic spatialand temporal distribution patterns with depositional facies andsystems tracts. Conversely, mesogenetic albite, calcite, and dolo-mite, display no such distribution patterns. Although it is notpossible to determine the precise timing of diagenetic alterationsdue to the lack of a burial history curve, the textural relationshipsbetween diagenetic alterations combined with the isotopiccomposition enabled us to determine the relative timing of anoverall paragenetic sequence (Fig. 12).

6.1. Distribution of diagenetic silicates in contexts of depositionalfacies and sequence stratigraphy

6.1.1. Clay mineralsGrain-coating clays, which occur in the Triassic fluvial sand-

stones as platelets that are tangentially arranged around thedetrital grain surfaces, are typically formed by mechanical infil-tration (cf. Matlack et al., 1989; Moraes and De Ros, 1990, 1992). Thehoneycomb-like texture of these grain-coating clays is possiblymixed layer illite/smectite. Smectitic, infiltrated grain-coating clayspreferentially form during weathering under arid to semi-aridclimatic conditions (Keller, 1970; Moraes and De Ros, 1992; De Roset al., 1994), such as those prevailed during deposition of theTriassic sandstones (Ajdanlijsky, 2002a, b). Grain-coating clays arecommon in fluvial deposits (Morad et al., 2000; Ketzer et al., 2003;

Fig. 7. Photomicrographs (crossed polarizers) showing: (A) microcrystalline calcite coats (solid arrow), micritic concretionary calcite, and microcrystalline calcite replacing the hostsediments (dashed arrow), (B) microcrystalline calcite occurs as rhizocretions, (C) blocky coarse-crystalline calcite (arrow) filling pore between tightly packed framework grains, and(D) poikilotopic coarse-crystalline calcite (arrow) filling pore between tightly packed framework grains. (E) BSE image showing blocky coarse-crystalline calcite filling small pores(solid arrow) and replacing detrital grains (dashed arrow). (F) Photomicrograph (crossed polarizers) showing blocky coarse-crystalline calcite (dashed arrow) engulfing quartzovergrowths (solid arrow).

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Worden and Morad, 2003; El-Ghali et al., 2006). Conversely,discontinuous grain-coating clays, which are restricted to grainembayments in the fluvial sandstones, are likely inherited clays(Wilson and Pittman, 1977; Wilson, 1994).

The distribution of infiltrated clays within channel and crevassesplay sandstones, particularly in LST, HST and, rarely, in TSTsuccessions, was most likely related to percolation of mud-richsurface waters (cf. Moraes and De Ros, 1992; Ketzer et al., 2003).However, clays infiltration in channels and crevasse splay depositstowards the top of LST sandstones most likely occurred during latestages of LST and possibly subsequent TST. During deposition of latestage LST and subsequent TST, accommodation space wasprogressively created as a result of base level rise, allowing depo-sition of considerable amounts of floodplain muds (Wright andMarriott, 1993; Shanley and McCabe, 1994). Deposition of thesemuds produced ideal conditions for mud-rich surface waters,which resulted in the formation of infiltrated clays (Moraes and DeRos, 1992). The abundant and thick infiltrated clays in LST sand-stones is probably attributed to their high depositional perme-ability (cf. Moraes and De Ros, 1992).

Infiltrated clays in the fluvial TST sandstones most likelyoccurred by crevassing (Kirschbaum and McCabe, 1992) during

aggradations of floodplains owing to increasing of accommodationspace, which occurs as a result of base level rise (Wright andMarriott, 1993). Crevassing during aggradations of floodplainpromotes the development of mud-rich surface waters and thusideal conditions for clay infiltration into crevasse splays andchannels sands. The presence of relatively small amounts and thethin nature of infiltrated clays in TST sandstones are attributed tothe rapid sealing of the underlying crevasse splay and channel sandby floodplain mud (cf. Ketzer et al., 2003) and/or due to the lowdepositional permeability in crevasse splay sands.

Infiltrated clays in channels and crevasse splays sand at the baseof HST are interpreted to be formed during early stage of the HST(El-Ghali et al., 2006). Progressive decrease in accommodationcreation as a consequence of slow rise in the base level (i.e. slowrise in the relative sea level) in early stage of the HST resulted indeposition of floodplain muds, but less copiously compared to theTST (cf. Wright and Marriott, 1993; Miall, 1997). Mud-rich surfacewaters, which prevailed during deposition of these floodplainmuds, would percolate into underlying channel and crevasse splaysandstones and thus produced ideal conditions for infiltration ofclays into the sandstones towards the base of the HST (Ketzer et al.,2003; El-Ghali et al., 2006).

keymicritic and microcrystalline calcite

dolomite I

-20 -18 -16 -14 -12 -10 -8 -6 -4-9.0

-8.5

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

coarse-crystalline calcite

dolomite II

Fig. 8. d13CVPDB& versus d18OVPDB& plot of bulk calcite and dolomite cementsshowing: (i) very weak positive correlation (r¼þ0.4) for calcite cements, which isattributed partly to slightly increasing input in 12C from thermal alteration of organicmatter during progressive burial and increasing temperature, and (ii) no correlation(r¼�0.1) for dolomites, which is attributed by multiple sources of dissolved carbon,such as decay of C3 plants and thermal alteration of organic matter.

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–1227 1221

The fibrous, hair-like, flaky and honeycomb-like and with spinyterminations and booklet-like habits of illite indicate diageneticorigin (Morad et al., 2000; Lemon and Cubitt, 2003). Illite, whichtypically forms during progressive burial (i.e. mesodiagenesis)under high temperature (90–130 �C; Morad et al., 2000), requiredhigh aK

þ/aHþ ratio in the pore waters (Ehrenberg et al., 1993; Morad

et al., 1994). The high aKþ/aHþ ratio required to achieve the illitization

process in the fluvial sandstones is attributed partially to thesimultaneous albitization of detrital K-feldspars, which contributedthe required Kþ ions to the pore waters (cf. Morad, 1988). Thepresence of grain-coating, honeycomb-like illite crystals with spinyterminations, suggests transformation of infiltrated clays, whichwere originally smectite (Moraes and De Ros, 1992), into illite viamixed layer illite/smectite (Keller et al., 1986; Morad et al., 2000).Conversely, the booklet-like illite crystals with fibrous and spinytermination in the micas, which have inflated into adjacent pores,are interpreted to be formed by illitization of kaolinitized micas.Preservation of typical booklet-like stacking crystal habits ofkaolinite within the illitized micas has been reported from theSilurian–Devonian Furnas Formation of the Parana Basin in Brazil(De Ros, 1998).

The distribution of illite (excluding illitized micas) is controlledby the spatial and temporal distribution of eogenetic infiltrated

Fig. 9. (A) BSE image showing small rhombic dolomite crystals (arrow) in palaeosol horizonfilling small pores in tightly packed framework grains.

clays, mud intraclasts, and pseudomatrix, which are more abun-dant in LST and HST compared with TST sandstones. The dominanceof illite over chlorite in all depositional facies and systems tracts ofthe fluvial sandstones is attributed partially to: (i) albitization ofdetrital K-feldspars, which leads to increase in aK+ /aH+ ratio, andhence stabilization of illite (ii) scarcity of Fe-rich minerals such asbiotite, and (iii) lack of Fe-rich clay minerals.

6.1.2. QuartzThe presence of quartz cement as syntaxial overgrowths near

sites of intergranular dissolution and around closely packed detritalquartz grains indicates mesogenetic origin (McBride, 1989; Wordenand Morad, 2000). The distribution of quartz overgrowths wasmainly controlled by the spatial and temporal distribution of grain-coating, infiltrated clays and pseudomatrix. Silica required forquartz cements was partly sourced internally from pressuredissolution of quartz grains, which was enhanced by the presenceof thin and discontinuous clay coatings and the occurrence of micasat the interface along quartz grain contacts (Giles et al., 1992;Gluyas et al., 1993). The TST sandstones were extensively cementedby quartz overgrowths during mesodiagenesis due to the presenceof thin and discontinuous nature of grain-coating clays and to thesmall volumes of pseudomatrix. Conversely, LST and HST sand-stones, in which the grains are pervasively coated with infiltratedclays and contain more pseudomatrix, were less cemented byquartz overgrowths during mesodiagenesis (Moraes and De Ros,1992). The thick nature of the grain-coating clays and occurrence ofpseudomatrix prevented chemical compaction and renderednucleation sites of quartz overgrowths scarce.

6.2. Distribution of pseudomatrix in contexts of depositional faciesand sequence stratigraphy

The increase in the amounts of pseudomatrix, which resultedfrom the mechanical compaction of mud intraclasts during burial,towards the base of the LST and the top of the HST sandstones,suggests incorporation of these mud intraclasts into the sandstonesduring early LST and late HST events. During deposition of early LST,base level fall was associated with decreasing accommodationspace (Shanley and McCabe, 1994), and resulted in lateral migrationof channels and erosion of floodplain deposits. As a result of lateralchannel migration, considerable amounts of mud intraclasts wereincorporated into channel sandstones (Ketzer et al., 2003), whichwere transformed into pseudomatrix during burial. Likewise,deposition of the late HST, which was concomitant with base levelfall, there was decrease of the accommodation space towards thetop of HST (cf. Shanley and McCabe, 1994; Miall, 1997). As accom-modation space decreases, channels will migrate laterally, resulting

. (B) Photomicrograph (crossed polarizers) showing rhombic dolomite crystals (arrow)

Fig. 10. Photomicrographs (crossed polarizers) showing: (A) pseudomatrix (arrow) formed by squeezing of mud intraclasts between detrital quartz grains by mechanicalcompaction, (B) chemical compaction along intergranular contacts between detrital quartz grains, enhanced by occurrence of micas (arrows).

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–12271222

in erosion of adjacent floodplain fines and thus incorporation ofmud intraclasts into channel sandstones towards the top of HST.

6.3. Distribution of diagenetic carbonates in contexts ofdepositional facies and sequence stratigraphy

6.3.1. CalciteThe micritic, microcrystalline, and coarse-crystalline textures of

calcite cement, and its paragenetic relations with other diageneticminerals, suggest precipitation in various diagenetic regimes.Micritic and microcrystalline calcite occur in palaeosol horizons,which developed on crevasse splay and channel sandstones of LSTand TST. Such a distribution pattern of micritic and microcrystallinecalcite related to spatial and temporal distribution of palaeosols isknown elsewhere (Esteban and Klappa, 1983; Wright and Tucker,1991; Mack et al., 1993; Beckner and Mozley, 1998; Garcia et al.,1998; Hall et al., 2004). Micritic and microcrystalline calcite inpalaeosol have presumably formed in the vadose zone (Hall et al.,2004) during near-surface diagenesis, as evidenced by densemicritic texture (Mora et al., 1993; Beckner and Mozley, 1998) and

Fig. 11. Plot of intergranular volume (IGV) versus volume of cement (Houseknecht,1988; modified by Ehrenberg, 1989) for 110 fluvial sandstone samples. LST, TST and HSTrefer to lowstand, transgressive and highstand systems tracts, respectively. Porositywas destroyed more by mechanical compaction than by cementation, except for thesamples associated with palaeosol horizons where the porosity was destroyed by earlycarbonate cementation.

rhizocretionary structure (Retallack, 1988; Monger et al., 1991; Hallet al., 2004). Using the d18OVPDB values of micritic and microcrys-talline calcite (�8.1& to �7.5&), the fractionation equation ofFriedman and O’Neil (1977), and assuming pore water with d18OV-

SMOW values (�7& to �5&) which are equivalent to those ofmeteoric waters of the basin during the Lower Triassic (Craig andGordon, 1965), precipitation would have occurred at temperaturesof 19 �C and 32 �C (Fig. 13), which supports the inferred near-surface, vadose diagenetic origin. The d13CVPDB values of micriticand microcrystalline calcite (�7.8& to �6.3&) indicate that dis-solved carbon was derived mainly from the decay of C3 plants andfrom atmospheric CO2 (cf. Cerling, 1984; Garcia et al., 1998; Morad,1998).

Coarse-crystalline calcite, which fills small pores in tightlypacked framework grains (intergranular volume 5–15%) andengulfs quartz overgrowths, is interpreted to have precipitatedfrom evolved formation water during deep burial diagenesis. Usingthe d18OVPDB values of calcite II (�18.4& to �12.8&), the fraction-ation equation of Friedman and O’Neil (1977), and assuming thed18OV-SMOW values for the pore waters (�2& to 0&), which arecommon for evolved formation water relative to the contemporaryLower Triassic meteoric water (�7& to �5&, Lundegard and Land,1986), coarse-crystalline calcite would have precipitated attemperatures between 75 �C and 140 �C (Fig. 13). These tempera-tures agree well with the inferred deep burial origin for precipita-tion of coarse-crystalline calcite, based on the petrographicexamination, and with the post-quartz overgrowths parageneticsequence; quartz overgrowths are typically of mesogenetic origin(80–130 �C; McBride, 1989; Worden and Morad, 2000). Thed13CVPDB values of coarse-crystalline calcite (�8.6& to �6.9&) aresimilar to the d13CVPDB values of the micritic and microcrystallinecalcite, which may suggest derivation of carbon from the dissolu-tion of these eogenetic calcites. Slight input of 12C duringprogressive burial and increasing temperature from the maturationof organic matter is indicated by the weak correlation between d18Oand d13C (Fig. 8; Irwin et al., 1977; Surdam et al., 1984; Morad, 1998).

6.3.2. DolomiteBased on the textural characteristics, presence of dolomite I in

palaeosol horizons, and occurrence as local patches with micro-crystalline textures and as small rhombs that fill large poresbetween loosely packed framework grains (intergranular volume30–48%), precipitation is inferred to have occurred at near-surfaceduring eodiagenesis. Dolomite precipitation is attributed to anincrease of the Mg2þ/Ca2þ ratio in pore waters due to evaporativeionic concentration (Made et al., 1994; Garcia et al., 1998) underarid to semi-arid climatic condition, which prevailed duringdeposition of the fluvial sandstones (Ajdanlijsky, 2002a, b). Usingthe d18OVPDB values of dolomite I (�8.3& to �5.2&), the fraction-ation equation of Land (1983), and assuming the d18OV-SMOW values

Fig. 12. Diagram displaying the present-day spatial and temporal occurrence of diagenetic minerals in the Triassic fluvial sandstones. The boundary between eodiagenesis andmesodiagenesis is sensu Morad et al. (2000).

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–1227 1223

(�7& to�5&), which are equivalent to meteoric waters during theLower Triassic (Craig and Gordon, 1965), dolomite I would haveprecipitated at temperatures of 20 �C and 50 �C (Fig. 13). Thesetemperatures are compatible with the inferred near-surface toshallow burial diagenetic origin and based on the petrographicresults. The d13CVPDB values of dolomite I (�8.3& to�7.1&) indicatethat dissolved carbon was derived mainly from C3 plants (Cerling,1984; Garcia et al., 1998; Morad, 1998).

Dolomite II, which fills small pores in tightly packed frameworkgrains (IGV¼ 5–15%) and engulfs quartz overgrowths, is inter-preted to have precipitated after significant burial and compaction.Using the d18OVPDB values of dolomite II (�14.4& to �12.4&),fractionation equation of Land (1983), and assuming the d18OV-

SMOW values for the pore waters (�2& to 0&), which is equivalentto the evolved formation water relative to the contemporary LowerTriassic meteoric water (�7& to �5&, Lundegard and Land, 1986),dolomite II would have precipitated at temperatures of 100 �C and145 �C (Fig. 13). These calculated temperatures are typical for

dolomite precipitation during deep burial diagenesis and are in linewith the petrographic examination of later quartz overgrowths,which are typically of mesogenetic origin (80–130 �C; McBride,1989; Worden and Morad, 2000). The d13CVPDB values of dolomite II(�8.0& to �7.0&) suggest that dissolved carbon was derived frommultiple sources, such as thermal alterations of organic matter andfrom the dissolution of eogenetic carbonate cements, which issupported by the lack of correlation between d18O and d13C (Fig. 8;Irwin et al., 1977; Surdam et al., 1984; Morad, 1998).

7. Summary model for the diagenetic alterations andreservoir quality evolution of fluvial sandstones withina sequence stratigraphic framework

A general model for the distribution of diagenetic alterations inthe fluvial sandstones within a sequence stratigraphic framework isgiven in Fig. 14. The sandstones have undergone various eo- andmesogenetic alterations that can be linked to variable extents to

Fig. 13. Diagrams showing the range of temperatures calculated from the assumedoxygen isotopic composition of the pore water and the d18O values of: (A) eogenetic(micritic and microcrystalline) and mesogenetic (coarse-crystalline) calcite cementsusing the fractionation equation of Friedman and O’Neil (1977), and (B) eogeneticdolomite I and mesogenetic dolomite II using the fractionation equation of Land(1983).

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depositional facies and systems tracts. Prediction of these diage-netic alterations in relation to depositional facies and systems tractshas led to better understanding of the spatial and temporal distri-bution of reservoir quality evolution. Mechanically infiltrated clays,which are more abundant in sandstones toward the top of LST andthe base of HST than in TST, were developed by percolation of mud-rich surface waters as a result of increasing accommodation spaceand deposition of floodplain muds. Infiltrated grain-coating clayshave resulted in the reduction of intergranular porosity andpermeability by blocking pore throats (Fig. 5A and B), and increasedthe volume of ineffective micropores. Porosity and permeability inTST sandstones were less affected compared with sandstones of LSTand HST owing to the occurrence of small amounts of mechanicallyinfiltrated clays.

Pseudomatrix, which is more abundant in sandstones towardthe base of LST and the top of HST than in TST, has resulted frommechanical compaction of mud intraclasts. These intraclasts wereincorporated mainly into channel and crevasse splay sandstones byerosion of underlying and/or adjacent floodplain muds, duringlateral migration of channels as a consequence of base level fall.Accordingly, the occurrence of abundant pseudomatrix in sand-stones toward the base of LST and the top of HST sandstones hasstrongly reduced original intergranular macroporosity by filling theadjacent pores, and lowered the permeability by blocking the porethroats (cf. Howard, 1992; Bloch, 1994; Smosna and Brune, 1997).Conversely, the TST sandstones, which contain small amounts ofpseudomatrix, were less affected, and thus retained more inter-granular macropores. Despite the presence of mechanically infil-trated clays and pseudomatrix, precipitation of extensive eogeneticcalcite and dolomite was associated with palaeosol horizons thatdeveloped on top of crevasse splay and channel sandstones of LSTand TST. Occurrence of such extensively cemented horizons withvery low porosity is significant for reservoir quality evaluation,because such horizons act as barriers for fluid flow, and thus formpotential reservoir compartments in underlying sandstones.

Distribution of mesogenetic quartz overgrowths wascontrolled by the distribution of mechanically infiltrated clays and

pseudomatrix. The presence of limited amounts of quartz over-growths in the LST and HST sandstones is attributed to thepresence of abundant grain-coating mechanically infiltrated claysand pseudomatrix. These diagenetic clays have presumably pre-vented chemical compaction and rendered nucleation sites ofquartz overgrowths unavailable. In contrast, the formation ofporosity–permeability deteriorating, quartz overgrowths in theTST sandstones is attributed to the discontinuous and thin natureof the grain-coating, infiltrated clays. The dominance of illite overchlorite in all depositional facies and systems tracts is attributedto the dioctahedral nature of the infiltrated clays (Moraes and DeRos, 1992) and the presence of eogenetic kaolinite (Morad et al.,2000). These clay minerals are susceptible to illitization duringmesodiagenesis (Morad et al., 2000) in the presence of Kþ sour-ces. Potential source of Kþ in the Triassic fluvial sandstonesincludes albitization of detrital K-feldspars. Sandstones in alldepositional facies and systems tracts were, subsequently,pervasively cemented by calcite and dolomite, which resulted indeterioration of the remaining porosity. Thus, the Triassic fluvialsandstones serve as analogs for tight gas reservoirs in which mostof the intergranular porosity (and hence permeability) has beeneliminated by extensive mechanical and chemical compaction aswell as by cementation.

8. Conclusions

Linking diagenesis to sequence stratigraphy of meandering andbraided fluvial deposits should be done cautiously owing to theindirect control of changes in the relative sea level on the archi-tecture of fluvial systems. This study revealed, however, that thespatial and temporal distribution of diagenetic alterations and oftheir impact on reservoir quality evolution in the Lower Triassic(Bulgaria) can be linked to depositional facies and sequencestratigraphic framework. Eogenetic infiltrated clays, calcite, dolo-mite and pseudomatrix as well as mesogenetic quartz overgrowths,and illite, show relatively systematic distribution patterns withindepositional facies and sequence stratigraphic units. Conversely,mesogenetic albite, dolomite, and calcite, do not show suchdistribution patterns.

Thick, extensive mechanically infiltrated clays, which deterio-rated reservoir quality, are more common in sandstones towardsthe top of LST and the base of HST sandstones. These mechanicallyinfiltrated clays were formed by increase of the accommodationspace and deposition of floodplain muds, which allowed mud-richsurface waters to percolate into these sandstones. Precipitation ofextensive eogenetic calcite and dolomite cements on top ofcrevasse splay and channel sandstones of LST and TST successionshas the potential to induce reservoir compartmentalization in suchfluvial successions for underlying sandstones. Pseudomatrix ismore common towards the base of LST and the top of HST sand-stones owing to the incorporation of mud intraclast into thesesandstones by erosion of floodplain muds as a result of channelsavulsion during base level fall.

Quartz overgrowths are more common in the TST sandstonesthan in LST and HST sandstones owing to the presence of thinner,incomplete grain-coating mechanically infiltrated clays in theformer sandstones. The dominance of illite over chlorite within alldepositional facies and systems tracts is attributed to the presenceof precursor dioctahedral smectite and kaolinite in the fluvialsandstones. Potassium needed for the illitization reactions waspresumably derived partly from the albitization of detrital K-feld-spars. The remaining pores in sandstones of all depositional faciesand systems tracts were filled totally by calcite and dolomitecements during deep mesodiagenesis. Thus, the Triassic fluvialsandstones are suggested to serve as analogs for deep, tight

Fig. 14. Schematic diagenetic model displaying the evolution pathways and spatial and temporal distribution of diagenetic minerals in the Triassic fluvial sandstones withina sequence stratigraphic framework. 1 Refers to the lower part fluvial channels in the LST and HST, 2 refers to the upper part of fluvial channels in the LST and HST, 3 refers to theupper pat of fluvial channels in the LST and TST which are close to TS and MFS, and 4 refers to the fluvial channels in the TST.

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reservoirs in which the intergranular porosity was nearlycompletely eliminated by compaction and cementation.

This case study demonstrates that a predictable conceptualmodel for distribution of diagenetic alterations and reservoirquality evolution in fluvial sandstones can be constructed bycombining the knowledge of diagenesis into depositional facies andsequence stratigraphy. Additionally, this study can serve as ananalog for fluvial tight gas reservoir rocks.

Acknowledgments

The authors thank Hans Harryson for aiding with microprobeanalysis. Ihsan Al-Aasm would like to acknowledge the continuoussupport from the Natural Science and Engineering Research Councilof Canada (NSERC). The authors thank the anonymous reviewers andthe Editor-in-Chief of the Journal of Marine and Petroleum Geologyfor their critical and constructive comments on the manuscript.

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