Distribution of diagenetic alterations within depositional facies and sequence stratigraphic framework of fluvial sandstones: Evidence from the Petrohan Terrigenous Group, Lower Triassic, NW Bulgaria

Marine and Petroleum Geology 26 (2009) 1212–1227

Contents lists available at ScienceDirect

Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Distribution of diagenetic alterations within depositional facies and sequence stratigraphic framework of fluvial sandstones: Evidence from the Petrohan Terrigenous 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

¨ gen 16, SE-752 36 Uppsala, Sweden Department of Earth Sciences, Uppsala University, Villava Department of Petroleum Geosciences, Universiti Brunei Darussalam, Tungku Link, Gadong BE1410, Brunei Geology Department, Faculty of Science, Al-Fateh University, P.O. Box 13696, Tripoli, Libya d The Petroleum Institute, Geoscience Department, P.O. Box 2533, Abu Dhabi, United Arab Emirates e AGR Reservoir Evaluation Services AS, Karenslyst Alle´ 4, P.O. Box 444, NO - 0278 Oslo, Norway f Departamento de Petrologı´a y Geoquı´mica, Facultad de Geologı´a, Universidad Complutense de Madrid, 28040 Madrid, Spain g University of Mining and Geology, St. Ivan Rilski, Sofia 1700, Bulgaria h Environmental Engineering Research Centre, School of Civil Engineering, The Queen’s University of Belfast, Stranmillis Road, BT9 5AG, Ireland i Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada b c

a r t i c l e i n f o

a b s t r a c t

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

Sequence stratigraphy of fluvial deposits is a controversial topic because changes in relative sea level will eventually have indirect impact on the spatial and temporal distribution of depositional facies. Changes in the relative sea level may influence the accommodation space in fluvial plains, and hence have impact on types of fluvial system, frequency of avulsion, and style of vertical and lateral accretion. This study aims to investigate whether depositional facies and changes in the fluvial system of the Lower Triassic Petrohan Terrigenous Group sandstones (NW Bulgaria) in response to changes in the relative sea level have an impact on the spatial and temporal distribution of diagenetic alterations. Eogenetic alterations, which were encountered in the fluvial sandstones, include: (i) mechanically infiltrated clays, particularly in channel and crevasse splay sandstones towards the top of the lowstand systems tract (LST) and the base of the highstand systems tract (HST). (ii) Pseudomatrix, which resulted from mechanical compaction of mud intraclasts, occurs mainly in channel sandstones at the base of the LST and towards the top of the HST and thus led to porosity and permeability deterioration. (iii) Calcite (d18OVPDB ¼ 8.1& to 7.5& and d13CVPDB ¼ 7.8& to 6.3&) and dolomite (d18OVPDB ¼ 8.3& to 5.2& and d13CVPDB ¼ 8.3& to 7.1&), which are associated with palaeosol horizons developed on top of crevasse splay and channel sandstones of transgressive systems tract (TST) and LST. Such extensive eogenetic calcite cements may act as potential layers for the formation of reservoir compartments for underlying sandstones. Mesogenetic alterations include: (i) calcite (d18OVPDB ¼ 18.4& to 12.8& and d13CVPDB ¼ 8.6& to 6.8&) and dolomite (d18OVPDB ¼ 14.7& to 12.4& and d13CVPDB ¼ 8.0& to 7.0&), which were formed in all depositional facies and systems tract sandstones, (ii) illite, which is the dominant diagenetic clay mineral in all depositional facies and systems tracts, was associated with albitization of detrital K-feldspars, and (iii) quartz overgrowths, which are most abundant in TST rather than LST and HST sandstones, because of the presence of suitable infiltrated clays and pseudomatrix in the latter sandstones. Such cementation by calcite, dolomite, and quartz overgrowths and formation of illite led to porosity and permeability deterioration during mesodiagenesis. The results of this study revealed the importance of integration of diagenesis with depositional facies and sequence stratigraphy of fluvial sandstones in improving our ability to predict the spatial and temporal distribution of eogenetic alterations and their subsequent impact on mesogenetic alterations, and thus on reservoir quality modifications. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Diagenesis Fluvial sandstones Sequence stratigraphy Lower Triassic (NW Bulgaria)

* Corresponding author. Department of Petroleum Geosciences, Universiti Brunei Darussalam, Tungku Link, Gadong BE1410, Brunei. Tel.: þ673 2463001x1368; fax: þ673 2461502. E-mail addresses: [email protected], [email protected] (M.A.K. El-Ghali). 0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2008.08.003

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

1. Introduction Sequence stratigraphy techniques have been applied successfully to predict the spatial and temporal distribution of paralic and shallow-marine depositional facies, and hence also the distribution and geometry of reservoir and source rocks in sedimentary basins (e.g. Posamentier et al., 1988a, b; Van Wagoner et al., 1988; Posamentier and Allen, 1993; Posamentier and Allen, 1999; Catuneanu, 2002). These depositional settings are sensitive to changes in relative sea level, which, induce, in turn, considerable changes in parameters that have profound impact on distribution of diagenetic alterations in sandstones, such as pore water chemistry, detrital composition (e.g. intraclasts versus extraclasts), rate of sediment supply, organic matter, and bioturbation (Morad et al., 2000). The application of sequence stratigraphy techniques to unravel and predict the response of fluvial style to changes in relative sea level is less straightforward compared with paralic and shallowmarine environments. However, changes of the fluvial style from braided to high sinuosity, meandering (i.e. architecture of fluvial deposits) have been suggested by Wright and Marriott (1993), Shanley and McCabe (1994), and Posamentier and Allen (1999) to occur as consequence of changes in the depositional base level, which are controlled by changes in the relative sea level. Changes in the relative sea level, which also control the style of fluvial systems, have in conjunction with detrital sand composition, climatic conditions, and patterns of regional ground water flow, strong impact on eogenetic alterations in fluvial sediments (Morad et al., 2000). This study aims to elucidate and discuss the distribution of diagenetic alterations and of their potential impact on reservoir quality evolution in fluvial sandstones of the Petrohan Terrigenous Group (PTG), Lower Triassic, NW Bulgaria (Figs. 1 and 2) in the context of depositional facies and sequence stratigraphy. Diagenetic regimes used in this study are: (i) eodiagenesis (0–2 km depth and <70  C) that refers to alterations during which pore water chemistry is controlled by surface water, and (ii) mesodiagenesis (>2 km depth

1213

and >70  C) refers to diagenetic alterations that are mediated by evolved formation water (Morad et al., 2000). In this study we adopted the widely applied sequence stratigraphic model for fluvial systems available in the literature, which was summarized by Wright and Marriott (1993), and Shanley and McCabe (1994). According to these models, changes in relative sea level control changes in depositional base level, and thus in the fluvial style. The lowstand systems tract (LST) forms when the base level falls (corresponds to fall in the relative sea level) and is, therefore, accompanied by low rate of accommodation creation. As a result, channels migrate and accrete laterally, and form amalgamated coarse-grained, braided fluvial sandstones with an erosional bounding surface at the base (i.e. sequence boundary, SB; Shanley and McCabe, 1994). During deposition of the late stage of LST, accommodation starts to increase slowly owing to progressive base level rise. As a result, floodplain deposits may develop, but their 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), base level rise increases as a consequence of sea level rise. As a result, the accommodation space increases and leads to enhanced vertical rather than lateral accretion, and thus to an increase in the aggradations of floodplain deposits, with subordinate isolated channel and crevasse splay deposits. During deposition of the highstand systems tract (HST), base level rise and the increase in accommodation space slow down leading to reduction of aggradation rate on floodplains. Further decrease in the accommodation towards the top of the HST leads to lateral rather than vertical accretion of the fluvial deposits, and thus increases sinuosity, which results in lateral accretion of fluvial channels (Wright and Marriott, 1993). 2. Geological setting The Petrohan Terrigenous Group (PTG; z150 m thick) was  deposited during the early Triassic (Catalov, 1975; Tronkov and

Fig. 1. Location map of the study area in NW Bulgaria. Sampling of the Petrohan Terrigenous Group was focused on two locations, the Cerovo and the Oplinja outcrops (modified from Ajdanlijsky, 2002a).

1214

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

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

Fig. 3), comprise (i) amalgamated braided fluvial (lowstand systems tract, LST), (ii) mud-rich, anastomosing isolated fluvial channel (transgressive systems tract, TST), and (iii) high sinuosity meandering fluvial deposits (highstand systems tract, HST). Fluvial LST deposits were formed during the time of base level fall. During base level fall, the channels migrated laterally on preexisting floodplains or channel deposits, and thus formed a multistory and multilateral amalgamated sandstone complex within incised valley with an erosional basal surface (cf. Shanley and McCabe, 1994; Zhang et al., 1997; Fig. 3). These LST deposits are medium- to coarse-grained, stacked multistory, amalgamated channel sandstones with a small proportion of fine-grained, overbank deposits. The channel fill deposits are dominated by sandy bed-forms with a thickness that ranges between 3.6 m and 4.9 m, and they exhibit high width/thickness ratios (Ajdanlijsky, 2002a, b). Fluvial TST deposits are interpreted to have been formed during the successive rapid rise of the base level, which resulted in a high rate of aggradation and in lowering of the fluvial gradient, and thus in anastomosing fluvial deposits with predominantly fine-grained floodplain sediments and crevasse splay sandstones developed (cf. Shanley and McCabe, 1994; Zhang et al., 1997; Fig. 3). The fluvial TST deposits comprise isolated channel sandstones, and crevasse splay sandstones interbedded with overbank mudstones (Ajdanlijsky, 2002b). The channel deposits are composed of fine- to mediumgrained, ribbon sand bodies, whereas the crevasse splay or levee deposits are composed of fine-grained, tabular sand bodies. The overbank sediments are sheet-like in appearance, which are laterally more extensive than the channel sandstones, and comprise both massive and horizontally laminated interbedded siltstones and mudstones (Ajdanlijsky, 2002b). Fluvial HST was formed when the base level rise started to slow down, with reduction of aggradation rate, and this resulted in an increase in the lateral migration and the sinuosity of the river system (cf. Shanley and McCabe, 1994; Zhang et al., 1997). The HST deposits are multistory and multilateral, channel complex sandstones and overbank mudstones of meandering fluvial system. The overbank proportion of the HST is less and the channel-fills greater compared with fluvial TST deposits (Ajdanlijsky, 2002b; Fig. 3). 4. Samples and analytical procedure

Ajdanlijsky, 1998; Ajdanlijsky, 2002a, b) in NW Bulgaria where the complete succession of the Triassic system is well exposed (Fig. 1). During deposition of the PTG, northwestern Bulgaria was part of the Eurasian platform, which was situated on the passive margin of the Eurasian craton and positioned between 30 and 40 N palaeolatitude (Philip et al., 1996). Palaeoclimatic conditions were arid to semi-arid (Ajdanlijsky, 2002b). The PTG represents the initial stage of the Mesozoic transgression event, which is marked by clastic red beds at its base (Tronkov, 1981). The PTG disconformably overlies Palaeozoic basement rocks and is in turn overlain by tide-dominated deltaic and tidal flat deposits of the Svidol Formation (Fig. 2). The PTG consists of sandstones, siltstones and mudstones, which were 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 fluvial sequences (each ca. 35–45 m thick; Ajdanlijsky, 2002b), each marked at its base by a distinctive erosion surface incised into underlying deposits, with amplitudes varying from several meters to over 30 m. These sequences, which are interpreted to be generated during a base level fall (cf. Shanley and McCabe, 1994;

One hundred and ten sandstone samples were collected from two outcrop areas with well constrained depositional facies and sequence stratigraphic framework (Figs. 1 and 3). The samples cover the various depositional facies and systems tracts (Fig. 3). Detailed petrographic examination was performed on 110 thin sections, which were prepared subsequent to impregnation with blue epoxy resin under vacuum. Modal compositions were obtained from the 110 sandstone samples by counting 300 points in each thin-section (Table 1). A JEOL JSM-T330 scanning electron microscope (SEM) equipped with digital imaging system was used to investigate the habits and textural relationships of diagenetic minerals in 21 selected samples. The samples were coated with a thin layer of gold and examined under an acceleration voltage of 20 kV and a beam current of 0.6 nA. Carbon and oxygen stable isotope analyses (Table 2) were performed on 30 carbonate-cemented sandstone samples. Sampling using microdrilling techniques was attempted in order to obtain carbonate cements with different textures. However, the small size of the pores led to contamination among different generations of carbonate cements. Nevertheless, all analyses represent a certain dominant carbonate cement type in the samples. Sandstone samples cemented by calcite and dolomite were subjected to the sequential chemical separation treatment described by Al-Aasm

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

1215

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

et al. (1990). These samples were analyzed using a Finnegan–MAT Delta plus mass spectrometer. Calcite-cemented sandstone samples were reacted with 100% phosphoric acid at 25  C for 4 h, and dolomite-cemented samples were reacted at 50  C for 24 h. The gas was analyzed using a SIRA-12 mass spectrometer. The phosphoric acid fractionation factors used were 1.01025 for calcite and 1.01060 for dolomite. Precision of all analyses was better than 0.04&. Oxygen and carbon isotope data are presented in the d notation relative to the Vienna PDB (PeeDee Belemnite) and SMOW (Standard Mean Ocean Water) standards. Chemical analysis (Table 2) of the carbonate cements was performed on 34 carbon-coated, polished thin sections using a Cameca Camebax BX50 microprobe (EMP), equipped with a backscatter electron (BSE) detector, under accelerating voltage of 20 kV, beam current of 10–15 nA, and a beam spot size of 1–5 mm. Analytical totals of carbonates (97–110%) were normalized to 100% for comparison purposes. 5. Results 5.1. Sandstones: texture and composition The Petrohan Terrigenous Group (PTG) fluvial sandstones are subrounded to well rounded, poorly- to moderately-sorted, and fine- to coarse-grained quartzarenites and sublitharenites (av. Q95.8F1L3.2; Fig. 4 and Table 1). The dominant framework grain is quartz (range 21–78 vol.%; av. 61 vol.%), which is represented mainly by monocrystalline quartz (21–78%; av. 52%), and smaller amounts of polycrystalline quartz (1–50%; av. 11%). The feldspars (trace-3%; av. <1%) are K-feldspars and plagioclase (Table 1). The lithic fragments (trace-6%; av. 1%) are mainly acid volcanic, plutonic, and trace amounts of sedimentary and metamorphic rocks (Table 1). Micas (trace-19%; av. 4%), which are more abundant in TST than in LST and HST sandstones (Table 1), include muscovite (trace-19%; av. 3%) and biotite (trace-11%; av. 0.5%, Table 1). Mud intraclasts (trace-12%; av. 2%), which are more abundant in LST and HST sandstones than in TST sandstones (Table 1), were squeezed between the rigid quartz grains and resulted in the formation of pseudomatrix. Heavy minerals (trace-2%; av. <0.5%) include zircon, apatite and epidote.

5.2. Diagenetic alterations: petrology, geochemistry, and distribution patterns 5.2.1. Silicates 5.2.1.1. Clay minerals. Clay minerals (trace-35%; av. 15%) include grain-coating and/or pore-bridging clays (trace-7%; av. 2%), illite (trace35%; av. 15%) and chlorite (trace-9%; av. 0.5%). Grain-coating and/or pore-bridging clays occur as thin (z2–15 mm thick) platelets that are tangentially arranged on detrital grain surfaces or bridges between detrital grains (Fig. 5A and B, respectively). SEM examinations revealed that grain-coating clays display honeycomb-like texture (Fig. 5C), being most common in channel and crevasse splay sandstones towards the top of LST and the base of HST successions (trace-7%; av. 3% and trace5.1%; av. 2%, respectively; Table 1), compared with TST successions where they occur as trace amounts (Table 1). Grain-coating clays are thick (z10–15 mm) and continuous in LST and HST sandstones but are relatively thin (z2–4 mm) and discontinuous in TST sandstones. In some cases, clays occur as discontinuous coatings being thickest in the grain embayments (Fig. 5D). However, illite occurs mainly as pore-lining and replacement of grain-coating clays and detrital grains such as micas and pseudomatrix. 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 terminations. In some cases, illite extends and bridges the pores. The micas are replaced by fibrous, filamentous, and booklet-like illite that inflates into the adjacent pores (Fig. 6A and B). Illite is engulfed by, and thus pre-dates, quartz overgrowths. Illite is common in all depositional 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 thus pre-dates, quartz overgrowths. Chlorite occurs in small amounts in all depositional facies and systems tracts (Table 1). 5.2.1.2. Quartz. Quartz cement (trace-25%; av. 6%) occurs as euhedral, syntaxial overgrowths around detrital quartz grains (100– 150 mm thick) and, very rarely, as outgrowths. Quartz overgrowths are common near sites of intergranular contacts. The boundaries between the quartz overgrowths and the detrital quartz grains are

1216

Table 1 Modal 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 (n ¼ 6)

CH HST (n ¼ 22)

CS TST (n ¼ 14)

CH TST (n ¼ 6)

CS LST (n ¼ 10)

CH LST (n ¼ 52)

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 grains Monocrystalline quartz Polycrystalline quartz Potassium feldspars Plagioclase feldspars Volcanic lithic fragments Plutonic lithic fragments Sedimentary lithic fragments Metamorphic lithic fragments Muscovite Biotite Chert Heavy minerals Mud intraclasts

54.0 4.0 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

77.5 13.3 1.7 0.3 2.3 0.3 0.7 0.0 8.7 0.0 0.0 0.3 3.3

61.2 8.8 1.1 0.1 0.7 0.1 0.1 0.0 2.4 0.0 0.0 0.1 1.8

5.3 3.4 0.3 0.1 0.9 0.1 0.3 0.0 3.1 0.0 0.0 0.2 1.5

33.3 4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

75.1 30.0 1.7 1.0 4.7 5.0 0.0 0.7 15.7 1.0 0.0 2.0 12.1

50.5 14.5 0.5 0.1 1.3 0.5 0.0 0.1 2.9 0.2 0.0 0.4 1.1

7.7 8.5 0.6 0.2 1.3 1.1 0.0 0.2 4.4 0.3 0.0 0.6 1.5

44.7 3.3 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

58.3 18.0 1.3 0.0 3.3 2.3 0.7 0.0 7.3 0.0 0.0 1.3 2.3

49.1 9.9 0.9 0.0 0.8 1.0 0.1 0.0 4.2 0.0 0.0 0.7 0.5

5.8 4.4 0.4 0.0 1.0 0.9 0.2 0.0 2.3 0.0 0.0 0.5 0.8

18.3 2.3 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.0 0.0 0.0 0.0

61.0 8.3 1.0 0.0 2.3 3.0 0.0 0.0 9.7 1.3 0.0 0.7 3.7

39.6 4.2 0.5 0.0 1.4 0.5 0.0 0.0 3.6 0.2 0.0 0.1 1.1

20.0 2.2 0.4 0.0 0.9 1.2 0.0 0.0 3.4 0.5 0.0 0.3 1.7

48.7 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0 0.0 0.0 0.0

67.7 11.3 1.7 1.0 3.7 1.7 0.0 0.0 7.0 3.3 0.0 1.7 3.7

55.8 6.2 0.5 0.2 1.3 0.2 0.0 0.0 3.6 0.9 0.0 0.6 0.6

5.9 3.9 0.5 0.3 1.1 0.5 0.0 0.0 2.7 1.3 0.0 0.7 1.3

20.3 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

59.7 50.0 1.7 1.0 4.3 5.7 4.7 0.0 10.3 1.7 0.0 2.0 10.6

48.1 12.5 0.4 0.1 1.2 1.0 0.3 0.0 2.3 0.1 0.0 0.4 1.3

8.1 9.6 0.6 0.2 1.3 1.5 0.8 0.0 2.8 0.4 0.0 0.5 1.9

Diagenetic alterations Infiltrated clays Quartz overgrowths Pseudomatrix Illite Chlorite Albite in feldspars Calcite replaces detrital grains Microcrystalline calcite Blocky calcite Poikilotopic calcite Dolomite Pyrite Fe-oxide

0.0 0.0 0.0 7.3 0.0 1.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0

3.2 9.4 6.7 26.0 0.0 10.3 6.3 0.0 4.3 1.0 2.0 1.0 2.7

1.4 3.3 2.2 9.4 0.0 3.1 1.3 0.0 1.1 0.2 0.5 0.3 0.8

0.0 5.9 2.5 6.6 0.0 3.7 2.5 0.0 1.7 0.4 0.8 0.4 1.0

0.0 2.0 0.0 4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

5.1 8.7 12.3 23.7 3.3 6.3 8.3 0.0 15.3 6.3 6.7 1.7 2.0

1.7 3.6 2.2 11.3 0.4 1.5 1.8 0.0 2.7 0.4 0.6 0.4 0.6

0.7 5.3 3.9 5.9 0.9 1.9 2.5 0.0 4.2 1.4 1.6 0.6 0.7

Trace 1.0 0.0 11.7 0.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Trace 16.3 1.7 24.7 0.0 4.7 13.3 0.0 7.0 1.3 2.7 2.3 5.7

Trace 6.8 0.6 12.4 0.0 2.7 2.0 1.0 2.5 0.3 1.0 0.7 2.1

Trace 6.1 0.7 4.4 0.0 1.3 4.3 0.0 2.7 0.6 1.2 0.9 2.0

Trace 0.0 0.0 0.0 0.0 0.0 0.7 0.0 2.3 0.0 0.0 0.0 0.0

Trace 21.0 0.0 23.3 0.7 1.0 9.0 6.0 20.7 8.7 18.0 3.3 0.0

Trace 10.7 0.0 7.4 0.1 0.3 4.6 4.5 8.5 5.1 6.5 1.2 0.0

Trace 8.5 0.0 9.0 0.3 0.4 3.0 2.6 7.4 3.6 7.7 1.6 0.0

0.0 0.0 0.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

3.2 5.0 13.3 22.0 0.7 8.0 2.0 7.3 13.7 12.0 11.0 1.3 4.6

2.2 1.9 1.5 10.1 0.1 1.9 0.5 2.4 3.5 2.9 2.1 0.4 0.6

0.4 1.9 4.2 5.2 0.2 2.6 0.7 0.8 5.4 3.8 3.3 0.5 0.4

0.0 0.0 0.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

6.7 8.3 15.7 35.3 9.3 7.7 15.0 4.7 16.3 13.7 10.3 3.7 4.7

3.9 4.2 1.6 10.5 1.1 1.4 2.2 1.6 2.9 0.8 1.0 0.5 0.5

1.7 5.4 2.9 6.3 2.6 2.2 3.6 0.7 4.4 2.3 2.0 0.8 0.7

Porosity Intergranular porosity Intragranular porosity Moldic porosity Fracture porosity

0.0 0.0 0.0 0.0

0.0 0.3 0.0 0.0

0.0 0.1 0.0 0.0

0.0 0.1 0.0 0.0

0.0 0.0 0.0 0.0

2.0 2.0 5.0 1.3

0.2 0.2 0.5 0.1

0.6 0.6 1.5 0.3

0.0 0.0 0.0 0.0

2.3 2.0 1.7 0.0

0.3 0.2 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 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 1.0 0.0

0.0 0.0 0.1 0.0

0.0 0.0 0.3 0.0

0.0 0.0 0.0 0.0

5.7 0.0 0.3 2.0

0.2 0.0 0.0 0.0

0.9 0.0 0.0 0.3

0.8 0.7 0.6 0.0

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

Min.

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

1217

Table 2 Elemental 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&

Calcite PTG/OV-02/13 p1 PTG/OV-02/04 p1 PTG/OV-02/03 p2 PTG/OV-02/03 p3 PTG/OV-02/02 p1 PTG/OV-02/02 p2 PTG/OV-02/45 p1 PTG/CV-02/54 p1 PTG/CV-02/54 p2 PTG/CV-02/55 p3 PTG/CV-02/53 p1 PTG/CV-02/53 p2 PTG/CV-02/53 p3 PTG/CV-02/53 p5 PTG/CV-02/16 p1 PTG/CV-02/17 p2 PTG/OV-02/52 p4 PTG/OV-02/52 p1 PTG/OV-02/33 p1 PTG/CV-02/45 p4 PTG/CV-02/45 p6 PTG/CV-02/43 p1 PTG/CV-02/40 p1 PTG/CV-02/40 p2 PTG/CV-02/41 p3 PTG/CV-02/41 p4 PTG/CV-02/14 p3 PTG/OV-02/52 p1a PTG/OV-02/52 p1b PTG/OV-02/51 p2a PTG/OV-02/51 p2b PTG/OV-02/31 p1 PTG/CV-02/45 p5 PTG/CV-02/43 p2 PTG/CV-02/43 p3 PTG/CV-02/49 p5 PTG/CV-02/14 p4 PTG/CV-02/07 p4 PTG/CV-02/08 p5

HST HST HST HST HST HST TST TST TST TST TST TST TST TST TST TST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST LST

Channel Channel Channel Channel Channel Channel Crevasse Crevasse Crevasse Crevasse Crevasse Crevasse Crevasse Crevasse Crevasse Crevasse Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel Channel

Calcite replace quartz Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Poikilotopic calcite Micritic and microcrystalline Micritic and microcrystalline Micritic and microcrystalline Micritic and microcrystalline Poikilotopic calcite Coarse-crystalline calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Poikilotopic calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite Coarse-crystalline calcite

1.4 0.1 0.4 0.3 0.0 1.6 0.6 1.1 0.3 0.9 0.1 0.5 0.5 0.1 0.7 n.d. 0.5 0.5 0.2 0.1 1.0 1.1 0.4 0.3 0.1 n.d. 0.5 0.8 0.5 0.1 0.2 n.d. 0.6 0.1 1.2 1.7 3.3 n.d. 0.2

97.1 99.8 97.2 99.7 98.1 95.9 99.4 96.3 97.9 97.7 98.0 97.8 99.4 99.9 97.9 100.0 98.7 97.8 97.7 99.9 99.0 98.2 94.7 99.7 99.8 99.2 95.0 97.8 99.4 98.5 98.9 98.5 99.2 99.1 98.8 97.5 96.0 96.3 98.9

0.1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.2 n.d. 1.2 0.0 n.d. n.d. 2.1 n.d. 0.1 0.2 1.8 1.1 n.d. 0.4 0.1 0.3 n.d. 0.4 0.0 0.4 0.4 0.9 0.1

1.4 0.1 2.4 n.d. 1.9 2.5 n.d. 2.6 1.7 1.3 2.0 1.6 0.1 n.d. 1.4 n.d. 0.6 1.6 0.9 0.0 n.d. 0.7 2.8 n.d. n.d. 0.5 2.7 0.2 n.d. 1.0 0.8 1.2 n.d. 0.4 0.0 0.3 0.3 2.7 0.8

7.6 7.1 8.6

18.1 18.4 16.5

7.7

17.0

7.1 7.0

18.0 12.8

6.8 7.1 6.3 7.8 6.9 7.9

15.3 7.8 7.5 8.1 7.7 14.2

8.1

17.2

7.7 7.1

17.2 17.1

8.1

12.8

7.4

13.8

7.2

15.0

8.2

17.2

7.0 6.9 7.6

17.3 12.9 17.1

7.5 7.3 8.1 8.2

18.3 15.6 17.5 17.5

Dolomite PTG/CV-02/53 p4 PTG/CV-02/53 p6 PTG/CV-02/53 p7 PTG/CV-02/53 p8 PTG/CV-02/53 p9 PTG/CV-02/53 p10 PTG/CV-02/48 p4 PTG/CV-02/14 p2

TST TST TST TST TST TST LST LST

Crevasse Crevasse Crevasse Crevasse Crevasse Crevasse Channel Channel

44.8 43.9 44.7 43.7 44.1 45.1 42.7 41.6

54.3 54.5 54.4 55.2 54.6 53.6 53.6 54.7

0.3 0.2 0.0 0.3 n.d. 0.2 1.9 1.7

0.5 1.4 0.8 0.8 1.2 1.2 1.8 2.0

7.1 7.4

5.2 5.6

8.3

6.1

7.2 7.0 8.0

8.3 14.7 12.4

splay splay splay splay splay splay splay splay splay splay

splay splay splay splay splay splay

Rhombic Rhombic Rhombic Rhombic Rhombic Rhombic Rhombic Rhombic

dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite

well defined by fluid inclusions, clay coatings and/or iron oxides. Quartz overgrowths engulf, and thus post-date, illite, but are engulfed by, and thus pre-date, coarse-crystalline calcite and dolomite II. Quartz overgrowths are most common in channel and crevasse splay TST sandstones (trace-21%; av. 8%) compared with LST (trace-11%; av. 3%) and HST (trace-9%; av. 2%) sandstones. 5.2.1.3. Feldspars. Feldspars (trace-10%; av. 2%) occur as albitized detrital 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 feldspars occurs as small prismatic crystals arranged along the twinning planes of the detrital feldspars. The albitized feldspars, which are similar petrographically to those described by Morad (1988) and Morad et al. (1990), occur in all depositional facies and systems tracts (Table 1). 5.2.2. Carbonates 5.2.2.1. Calcite. Calcite (trace-29%; av. 4%) occurs as micritic to microcrystalline (50 mm) and coarse-crystalline (up to 200 mm),

calcite calcite calcite calcite

pore-filling cement, which replaces partly and/or totally detrital grains. Micritic and microcrystalline calcite are commonly associated with palaeosol horizons that developed on crevasse splay and channel TST and HST sandstones. Micritic and microcrystalline calcite occur as grain-coatings around detrital grains (Fig. 7A), as dense texture within which scattered detrital grains are embedded (Fig. 7A), and as local patches (200–800 mm). Rarely, microcrystalline calcite 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 in tightly 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 and microcrystalline calcite (Table 2) reveals a narrow range of d18OVPDB values (8.1& to 7.5&) and d13CVPDB (7.8& to 6.3&), whereas

1218

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

Quartz

highstand systems tract transgressive systems tract lowstand systems tract

Quartzarenite 5

5

e

Su

nit

bar

are

kos e

th bli Su

25

25 Lithic sub-arkose Arkose

Feldspar

Litharenite

Lithic fragments

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 (intergranular volume 30–48%). Dolomite I is most common in palaeosol horizons that developed on crevasse splay and channel sandstones of TST and HST successions. Conversely, dolomite II occurs as scattered rhombic crystals that tend to fill small pores (Fig. 9B) in tightly packed framework grains (intergranular volume 5–15%). Dolomite II engulfs, and thus post-dates, illite and quartz overgrowths. Dolomite II is common in all depositional facies and systems tracts. Dolomites I and II are somewhat Ca-rich (53.6–55.2%; av. 54.4 mole %; 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 isotope analysis 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 reveals a narrow range of d18OVPDB values (14.7& to 12.4&) and d13CVPDB values (8.0& to 7.0&). The cross plot of d18O and d13C of dolomite I and dolomite II displays no correlation (r ¼ 0.1; Fig. 8).

coarse-crystalline calcite (Table 2) reveals a fairly wide range of d18OVPDB values (18.4& to 12.8&) and a narrow range of d13CVPDB values (8.6& to 6.8&). The cross plot of d18O and d13C of the micritic and microcrystalline calcite and coarse-crystalline calcite displays relatively weak correlation (r ¼ þ0.4; Fig. 8).

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

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

5.2.4. Other diagenetic alterations Pyrite (trace-4%; av. 0.5%) occurs as small, scattered euhedral crystals (z20 mm) that fill intergranular pores or occurs within

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

Fig. 5. (A) Photomicrograph (crossed polarizers) showing mechanically infiltrated clays (arrow) coating detrital grains. (B) SEM image showing grain-coating clays and bridges (arrow) of clays between detrital grains. (C) SEM image showing close-up view of grain-coating clays, typical for smectite. (D) Photomicrograph (crossed polarizers) showing inherited clay coatings (black solid arrow) engulfed by quartz overgrowths (white solid arrow).

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

1219

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 illitized micas. (C) Photomicrograph (crossed polarizers) showing platelets of chlorite (chl) within framework grains. (D) SEM image showing platelets of chlorite (chl) coating detrital grain.

pseudomatrix as well as in the vicinity of mica flakes. Pyrite is engulfed by, and thus pre-dates, calcite and quartz overgrowths. Pyrite is more common in crevasse splay TST sandstones (trace-3%; av. z1.5%) than in crevasse splay LST and HST sandstones (trace-2%; av. z0.5% and trace4%; av. z0.5%, respectively). Fe-oxide (trace-5%; av. 0.5%) occurs as intergranular pore-filling and detrital grain-coatings, being closely associated with micas. Fe-oxide is abundant in crevasse splay TST (trace-5%; av. 2%) and less common in crevasse splay LST and HST sandstones, where it occurs in trace amounts (Table 1). 5.3. Compaction and sandstone porosity The sandstones of the Petrohan Terrigenous Group display variable degrees of mechanical and chemical compaction, particularly when the early cements were lacking. Mechanical compaction is evidenced by bending of micas (Fig. 10B) and pseudoplastic deformation of mud intraclasts into pseudomatrix (Fig. 10A). Chemical compaction, which occurs along detrital quartz grains, has resulted in the development of concave–convex and sutured contacts. Chemical compaction is most extensive when the quartz grains are coated with thin illite or when the mica occurs at the interface along quartz grain contacts (Fig. 10B). Total thin-section porosity of the sandstones, which includes both primary and secondary pores, reveals a narrow range from trace to 6% (Table 1). The primary intergranular porosity reveals a range from trace to 6% (Table 1), whereas the secondary intragranular and moldic porosity, which was derived from partial to complete dissolution of feldspars, reveals a range from trace to 5% (Table 1). Microporosity was difficult to quantify under the petrographic microscope but it does exist mainly as intragranular micropores within albitized feldspars and between clay crystals. A plot of total intergranular volume versus total intergranular cement and pseudomatrix (Fig. 11) indicates that the loss of depositional porosity was greater due to compaction than to cementation. The distribution of porosity within different depositional facies and systems tracts of fluvial sandstones does not show any significant variation, owing to its small amounts.

6. Discussion Linking diagenesis to sequence stratigraphic framework of fluvial deposits is fraught with difficulties and uncertainties owing to the merely indirect impact of changes in the relative sea level on the architecture of such deposits. However, such linking would in principle reflect changes in the rate of accommodation, which, in turn, influence the fluvial depositional system (Shanley and McCabe, 1994). Another difficulty arises from the complex spatial and temporal distribution of eo- and mesogenetic alterations in the Triassic fluvial deposits. Nevertheless, eogenetic grain-coating clays, calcite and dolomite and pseudomatrix, as well as mesogenetic quartz overgrowths and illite, show fairly systematic spatial and temporal distribution patterns with depositional facies and systems tracts. Conversely, mesogenetic albite, calcite, and dolomite, display no such distribution patterns. Although it is not possible to determine the precise timing of diagenetic alterations due to the lack of a burial history curve, the textural relationships between diagenetic alterations combined with the isotopic composition enabled us to determine the relative timing of an overall paragenetic sequence (Fig. 12). 6.1. Distribution of diagenetic silicates in contexts of depositional facies and sequence stratigraphy 6.1.1. Clay minerals Grain-coating clays, which occur in the Triassic fluvial sandstones as platelets that are tangentially arranged around the detrital grain surfaces, are typically formed by mechanical infiltration (cf. Matlack et al., 1989; Moraes and De Ros, 1990, 1992). The honeycomb-like texture of these grain-coating clays is possibly mixed layer illite/smectite. Smectitic, infiltrated grain-coating clays preferentially form during weathering under arid to semi-arid climatic conditions (Keller, 1970; Moraes and De Ros, 1992; De Ros et al., 1994), such as those prevailed during deposition of the Triassic sandstones (Ajdanlijsky, 2002a, b). Grain-coating clays are common in fluvial deposits (Morad et al., 2000; Ketzer et al., 2003;

1220

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

Fig. 7. Photomicrographs (crossed polarizers) showing: (A) microcrystalline calcite coats (solid arrow), micritic concretionary calcite, and microcrystalline calcite replacing the host sediments (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 quartz overgrowths (solid arrow).

Worden and Morad, 2003; El-Ghali et al., 2006). Conversely, discontinuous grain-coating clays, which are restricted to grain embayments in the fluvial sandstones, are likely inherited clays (Wilson and Pittman, 1977; Wilson, 1994). The distribution of infiltrated clays within channel and crevasse splay sandstones, particularly in LST, HST and, rarely, in TST successions, was most likely related to percolation of mud-rich surface waters (cf. Moraes and De Ros, 1992; Ketzer et al., 2003). However, clays infiltration in channels and crevasse splay deposits towards the top of LST sandstones most likely occurred during late stages of LST and possibly subsequent TST. During deposition of late stage LST and subsequent TST, accommodation space was progressively created as a result of base level rise, allowing deposition of considerable amounts of floodplain muds (Wright and Marriott, 1993; Shanley and McCabe, 1994). Deposition of these muds produced ideal conditions for mud-rich surface waters, which resulted in the formation of infiltrated clays (Moraes and De Ros, 1992). The abundant and thick infiltrated clays in LST sandstones is probably attributed to their high depositional permeability (cf. Moraes and De Ros, 1992). Infiltrated clays in the fluvial TST sandstones most likely occurred by crevassing (Kirschbaum and McCabe, 1992) during

aggradations of floodplains owing to increasing of accommodation space, which occurs as a result of base level rise (Wright and Marriott, 1993). Crevassing during aggradations of floodplain promotes the development of mud-rich surface waters and thus ideal conditions for clay infiltration into crevasse splays and channels sands. The presence of relatively small amounts and the thin nature of infiltrated clays in TST sandstones are attributed to the rapid sealing of the underlying crevasse splay and channel sand by floodplain mud (cf. Ketzer et al., 2003) and/or due to the low depositional permeability in crevasse splay sands. Infiltrated clays in channels and crevasse splays sand at the base of HST are interpreted to be formed during early stage of the HST (El-Ghali et al., 2006). Progressive decrease in accommodation creation as a consequence of slow rise in the base level (i.e. slow rise in the relative sea level) in early stage of the HST resulted in deposition of floodplain muds, but less copiously compared to the TST (cf. Wright and Marriott, 1993; Miall, 1997). Mud-rich surface waters, which prevailed during deposition of these floodplain muds, would percolate into underlying channel and crevasse splay sandstones and thus produced ideal conditions for infiltration of clays into the sandstones towards the base of the HST (Ketzer et al., 2003; El-Ghali et al., 2006).

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

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

-5.0 -5.5 -6.0

key micritic and microcrystalline calcite coarse-crystalline calcite dolomite I dolomite II

-6.5 -7.0 -7.5 -8.0 -8.5 -9.0 -20

-18

-16

-14

-12

1221

-10

-8

-6

-4

Fig. 8. d13CVPDB& versus d18OVPDB& plot of bulk calcite and dolomite cements showing: (i) very weak positive correlation (r ¼ þ0.4) for calcite cements, which is attributed partly to slightly increasing input in 12C from thermal alteration of organic matter during progressive burial and increasing temperature, and (ii) no correlation (r ¼ 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.

The fibrous, hair-like, flaky and honeycomb-like and with spiny terminations and booklet-like habits of illite indicate diagenetic origin (Morad et al., 2000; Lemon and Cubitt, 2003). Illite, which typically forms during progressive burial (i.e. mesodiagenesis) under high temperature (90–130  C; Morad et al., 2000), required þ high aþ K /aH ratio in the pore waters (Ehrenberg et al., 1993; Morad þ et al., 1994). The high aþ K /aH ratio required to achieve the illitization process in the fluvial sandstones is attributed partially to the simultaneous albitization of detrital K-feldspars, which contributed the required Kþ ions to the pore waters (cf. Morad, 1988). The presence of grain-coating, honeycomb-like illite crystals with spiny terminations, suggests transformation of infiltrated clays, which were originally smectite (Moraes and De Ros, 1992), into illite via mixed layer illite/smectite (Keller et al., 1986; Morad et al., 2000). Conversely, the booklet-like illite crystals with fibrous and spiny termination 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 of kaolinite within the illitized micas has been reported from the Silurian–Devonian Furnas Formation of the Parana Basin in Brazil (De Ros, 1998). The distribution of illite (excluding illitized micas) is controlled by the spatial and temporal distribution of eogenetic infiltrated

6.1.2. Quartz The presence of quartz cement as syntaxial overgrowths near sites of intergranular dissolution and around closely packed detrital quartz grains indicates mesogenetic origin (McBride, 1989; Worden and Morad, 2000). The distribution of quartz overgrowths was mainly controlled by the spatial and temporal distribution of graincoating, infiltrated clays and pseudomatrix. Silica required for quartz cements was partly sourced internally from pressure dissolution of quartz grains, which was enhanced by the presence of thin and discontinuous clay coatings and the occurrence of micas at the interface along quartz grain contacts (Giles et al., 1992; Gluyas et al., 1993). The TST sandstones were extensively cemented by quartz overgrowths during mesodiagenesis due to the presence of thin and discontinuous nature of grain-coating clays and to the small volumes of pseudomatrix. Conversely, LST and HST sandstones, in which the grains are pervasively coated with infiltrated clays and contain more pseudomatrix, were less cemented by quartz overgrowths during mesodiagenesis (Moraes and De Ros, 1992). The thick nature of the grain-coating clays and occurrence of pseudomatrix prevented chemical compaction and rendered nucleation sites of quartz overgrowths scarce. 6.2. Distribution of pseudomatrix in contexts of depositional facies and sequence stratigraphy The increase in the amounts of pseudomatrix, which resulted from 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 sandstones during early LST and late HST events. During deposition of early LST, base level fall was associated with decreasing accommodation space (Shanley and McCabe, 1994), and resulted in lateral migration of channels and erosion of floodplain deposits. As a result of lateral channel migration, considerable amounts of mud intraclasts were incorporated into channel sandstones (Ketzer et al., 2003), which were transformed into pseudomatrix during burial. Likewise, deposition of the late HST, which was concomitant with base level fall, there was decrease of the accommodation space towards the top of HST (cf. Shanley and McCabe, 1994; Miall, 1997). As accommodation space decreases, channels will migrate laterally, resulting

Fig. 9. (A) BSE image showing small rhombic dolomite crystals (arrow) in palaeosol horizon. (B) Photomicrograph (crossed polarizers) showing rhombic dolomite crystals (arrow) filling small pores in tightly packed framework grains.

1222

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

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

in erosion of adjacent floodplain fines and thus incorporation of mud intraclasts into channel sandstones towards the top of HST. 6.3. Distribution of diagenetic carbonates in contexts of depositional facies and sequence stratigraphy 6.3.1. Calcite The micritic, microcrystalline, and coarse-crystalline textures of calcite cement, and its paragenetic relations with other diagenetic minerals, suggest precipitation in various diagenetic regimes. Micritic and microcrystalline calcite occur in palaeosol horizons, which developed on crevasse splay and channel sandstones of LST and TST. Such a distribution pattern of micritic and microcrystalline calcite related to spatial and temporal distribution of palaeosols is known 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 in palaeosol have presumably formed in the vadose zone (Hall et al., 2004) during near-surface diagenesis, as evidenced by dense micritic 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 HST refer to lowstand, transgressive and highstand systems tracts, respectively. Porosity was destroyed more by mechanical compaction than by cementation, except for the samples associated with palaeosol horizons where the porosity was destroyed by early carbonate cementation.

rhizocretionary structure (Retallack, 1988; Monger et al., 1991; Hall et al., 2004). Using the d18OVPDB values of micritic and microcrystalline calcite (8.1& to 7.5&), the fractionation equation of Friedman and O’Neil (1977), and assuming pore water with d18OVSMOW values (7& to 5&) which are equivalent to those of meteoric waters of the basin during the Lower Triassic (Craig and Gordon, 1965), precipitation would have occurred at temperatures of 19  C and 32  C (Fig. 13), which supports the inferred nearsurface, vadose diagenetic origin. The d13CVPDB values of micritic and microcrystalline calcite (7.8& to 6.3&) indicate that dissolved carbon was derived mainly from the decay of C3 plants and from atmospheric CO2 (cf. Cerling, 1984; Garcia et al., 1998; Morad, 1998). Coarse-crystalline calcite, which fills small pores in tightly packed framework grains (intergranular volume 5–15%) and engulfs quartz overgrowths, is interpreted to have precipitated from evolved formation water during deep burial diagenesis. Using the d18OVPDB values of calcite II (18.4& to 12.8&), the fractionation equation of Friedman and O’Neil (1977), and assuming the d18OV-SMOW values for the pore waters (2& to 0&), which are common for evolved formation water relative to the contemporary Lower Triassic meteoric water (7& to 5&, Lundegard and Land, 1986), coarse-crystalline calcite would have precipitated at temperatures between 75  C and 140  C (Fig. 13). These temperatures agree well with the inferred deep burial origin for precipitation of coarse-crystalline calcite, based on the petrographic examination, and with the post-quartz overgrowths paragenetic sequence; quartz overgrowths are typically of mesogenetic origin (80–130  C; McBride, 1989; Worden and Morad, 2000). The d13CVPDB values of coarse-crystalline calcite (8.6& to 6.9&) are similar to the d13CVPDB values of the micritic and microcrystalline calcite, which may suggest derivation of carbon from the dissolution of these eogenetic calcites. Slight input of 12C during progressive burial and increasing temperature from the maturation of organic matter is indicated by the weak correlation between d18O and d13C (Fig. 8; Irwin et al., 1977; Surdam et al., 1984; Morad, 1998). 6.3.2. Dolomite Based on the textural characteristics, presence of dolomite I in palaeosol horizons, and occurrence as local patches with microcrystalline textures and as small rhombs that fill large pores between loosely packed framework grains (intergranular volume 30–48%), precipitation is inferred to have occurred at near-surface during eodiagenesis. Dolomite precipitation is attributed to an increase of the Mg2þ/Ca2þ ratio in pore waters due to evaporative ionic concentration (Made´ et al., 1994; Garcia et al., 1998) under arid to semi-arid climatic condition, which prevailed during deposition of the fluvial sandstones (Ajdanlijsky, 2002a, b). Using the d18OVPDB values of dolomite I (8.3& to 5.2&), the fractionation equation of Land (1983), and assuming the d18OV-SMOW values

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

1223

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

(7& to 5&), which are equivalent to meteoric waters during the Lower Triassic (Craig and Gordon, 1965), dolomite I would have precipitated at temperatures of 20  C and 50  C (Fig. 13). These temperatures are compatible with the inferred near-surface to shallow burial diagenetic origin and based on the petrographic results. The d13CVPDB values of dolomite I (8.3& to 7.1&) indicate that 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 framework grains (IGV ¼ 5–15%) and engulfs quartz overgrowths, is interpreted 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 d18OVSMOW values for the pore waters (2& to 0&), which is equivalent to the evolved formation water relative to the contemporary Lower Triassic meteoric water (7& to 5&, Lundegard and Land, 1986), dolomite II would have precipitated at temperatures of 100  C and 145  C (Fig. 13). These calculated temperatures are typical for

dolomite precipitation during deep burial diagenesis and are in line with 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 from multiple sources, such as thermal alterations of organic matter and from the dissolution of eogenetic carbonate cements, which is supported 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 and reservoir quality evolution of fluvial sandstones within a sequence stratigraphic framework A general model for the distribution of diagenetic alterations in the fluvial sandstones within a sequence stratigraphic framework is given in Fig. 14. The sandstones have undergone various eo- and mesogenetic alterations that can be linked to variable extents to

1224

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

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

depositional facies and systems tracts. Prediction of these diagenetic alterations in relation to depositional facies and systems tracts has led to better understanding of the spatial and temporal distribution of reservoir quality evolution. Mechanically infiltrated clays, which are more abundant in sandstones toward the top of LST and the base of HST than in TST, were developed by percolation of mudrich surface waters as a result of increasing accommodation space and deposition of floodplain muds. Infiltrated grain-coating clays have resulted in the reduction of intergranular porosity and permeability by blocking pore throats (Fig. 5A and B), and increased the volume of ineffective micropores. Porosity and permeability in TST sandstones were less affected compared with sandstones of LST and HST owing to the occurrence of small amounts of mechanically infiltrated clays. Pseudomatrix, which is more abundant in sandstones toward the base of LST and the top of HST than in TST, has resulted from mechanical compaction of mud intraclasts. These intraclasts were incorporated mainly into channel and crevasse splay sandstones by erosion of underlying and/or adjacent floodplain muds, during lateral migration of channels as a consequence of base level fall. Accordingly, the occurrence of abundant pseudomatrix in sandstones toward the base of LST and the top of HST sandstones has strongly reduced original intergranular macroporosity by filling the adjacent pores, and lowered the permeability by blocking the pore throats (cf. Howard, 1992; Bloch, 1994; Smosna and Brune, 1997). Conversely, the TST sandstones, which contain small amounts of pseudomatrix, were less affected, and thus retained more intergranular macropores. Despite the presence of mechanically infiltrated clays and pseudomatrix, precipitation of extensive eogenetic calcite and dolomite was associated with palaeosol horizons that developed on top of crevasse splay and channel sandstones of LST and TST. Occurrence of such extensively cemented horizons with very low porosity is significant for reservoir quality evaluation, because such horizons act as barriers for fluid flow, and thus form potential reservoir compartments in underlying sandstones. Distribution of mesogenetic quartz overgrowths was controlled by the distribution of mechanically infiltrated clays and

pseudomatrix. The presence of limited amounts of quartz overgrowths in the LST and HST sandstones is attributed to the presence of abundant grain-coating mechanically infiltrated clays and pseudomatrix. These diagenetic clays have presumably prevented chemical compaction and rendered nucleation sites of quartz overgrowths unavailable. In contrast, the formation of porosity–permeability deteriorating, quartz overgrowths in the TST sandstones is attributed to the discontinuous and thin nature of the grain-coating, infiltrated clays. The dominance of illite over chlorite in all depositional facies and systems tracts is attributed to the dioctahedral nature of the infiltrated clays (Moraes and De Ros, 1992) and the presence of eogenetic kaolinite (Morad et al., 2000). These clay minerals are susceptible to illitization during mesodiagenesis (Morad et al., 2000) in the presence of Kþ sources. Potential source of Kþ in the Triassic fluvial sandstones includes albitization of detrital K-feldspars. Sandstones in all depositional facies and systems tracts were, subsequently, pervasively cemented by calcite and dolomite, which resulted in deterioration of the remaining porosity. Thus, the Triassic fluvial sandstones serve as analogs for tight gas reservoirs in which most of the intergranular porosity (and hence permeability) has been eliminated by extensive mechanical and chemical compaction as well as by cementation.

8. Conclusions Linking diagenesis to sequence stratigraphy of meandering and braided fluvial deposits should be done cautiously owing to the indirect control of changes in the relative sea level on the architecture of fluvial systems. This study revealed, however, that the spatial and temporal distribution of diagenetic alterations and of their impact on reservoir quality evolution in the Lower Triassic (Bulgaria) can be linked to depositional facies and sequence stratigraphic framework. Eogenetic infiltrated clays, calcite, dolomite and pseudomatrix as well as mesogenetic quartz overgrowths, and illite, show relatively systematic distribution patterns within depositional facies and sequence stratigraphic units. Conversely, mesogenetic albite, dolomite, and calcite, do not show such distribution patterns. Thick, extensive mechanically infiltrated clays, which deteriorated reservoir quality, are more common in sandstones towards the top of LST and the base of HST sandstones. These mechanically infiltrated clays were formed by increase of the accommodation space and deposition of floodplain muds, which allowed mud-rich surface waters to percolate into these sandstones. Precipitation of extensive eogenetic calcite and dolomite cements on top of crevasse splay and channel sandstones of LST and TST successions has the potential to induce reservoir compartmentalization in such fluvial successions for underlying sandstones. Pseudomatrix is more common towards the base of LST and the top of HST sandstones owing to the incorporation of mud intraclast into these sandstones by erosion of floodplain muds as a result of channels avulsion during base level fall. Quartz overgrowths are more common in the TST sandstones than in LST and HST sandstones owing to the presence of thinner, incomplete grain-coating mechanically infiltrated clays in the former sandstones. The dominance of illite over chlorite within all depositional facies and systems tracts is attributed to the presence of precursor dioctahedral smectite and kaolinite in the fluvial sandstones. Potassium needed for the illitization reactions was presumably derived partly from the albitization of detrital K-feldspars. The remaining pores in sandstones of all depositional facies and systems tracts were filled totally by calcite and dolomite cements during deep mesodiagenesis. Thus, the Triassic fluvial sandstones are suggested to serve as analogs for deep, tight

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

1225

Fig. 14. Schematic diagenetic model displaying the evolution pathways and spatial and temporal distribution of diagenetic minerals in the Triassic fluvial sandstones within a 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 the upper 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.

1226

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

reservoirs in which the intergranular porosity was nearly completely eliminated by compaction and cementation. This case study demonstrates that a predictable conceptual model for distribution of diagenetic alterations and reservoir quality evolution in fluvial sandstones can be constructed by combining the knowledge of diagenesis into depositional facies and sequence stratigraphy. Additionally, this study can serve as an analog for fluvial tight gas reservoir rocks. Acknowledgments The authors thank Hans Harryson for aiding with microprobe analysis. Ihsan Al-Aasm would like to acknowledge the continuous support from the Natural Science and Engineering Research Council of Canada (NSERC). The authors thank the anonymous reviewers and the Editor-in-Chief of the Journal of Marine and Petroleum Geology for their critical and constructive comments on the manuscript. References Ajdanlijsky, G., 2001. Synsedimentary deformations in the transitional interval between the Petrohan Terrigenous Group and the Svidol Formation near Sfrazen, Western Bulgaria. Annual University Mining Geology. Geology 43–44 (I), 27–30. Ajdanlijsky, G., 2002a. Palaeopedogenic occurrences in the Petrohan Terrigenous Group of the Berkovitza Unit, NW Bulgaria. In: GSB Geological Conference, 11– 13 October 2000, Sofia, pp. 315–317. Ajdanlijsky, G., 2002b. Facies architecture and cyclostratigraphy of the fluvial to shallow-marine lower Triassic successions in part of NW Bulgaria. In: Facies Analysis and High-Resolution Sequence and Cyclostratigraphy of Fluvial to Shallow Marine Successions. International Association of Sedimentologists, short course, Sofia, Bulgaria, p. 40. Al-Aasm, I.S., Taylor, B.E., Southmm, B., 1990. Stable isotope analysis of multiple carbonate samples using selective acid extraction. Chemical Geology 80, 119–125. Beckner, J., Mozley, P.S., 1998. Origin and spatial distribution of early phreatic and vadose calcite cements in the Zia Formation, Albuquerque Basin, New Mexico, U.S.A. In: Morad, S. (Ed.), Carbonate Cementation in Sandstones. Special Publication, International Association of Sedimentologists, vol. 26, pp. 27–51. Bloch, S., 1994. Effect of detrital mineral composition on reservoir quality. In: Wilson, M.D. (Ed.), Reservoir Quality Assessment and Prediction in Clastic Rocks. Society of Economic Palaeontologists and Mineralogists, (Short Course Special Publication), vol. 30, pp. 161–182.  Catalov, G., 1975. Facies analysis of the Svidol Formation (Lower Triassic) in Teteven Anticlinorium (central Fore-Balkan). Geologica Balcanica 5, 67–86. Catuneanu, O., 2002. Sequence stratigraphy of clastic systems: concepts, merits, and pitfalls. Journal of African Earth Sciences 35, 1–43. Cerling, T.E., 1984. The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth Planet Science Letters 71, 229–240. Craig, H., Gordon, L.I., 1965. Deuterium and oxygen 18 variations in the oceans and marine atmosphere. In: Proceedings of the Spoleto Conference on Stable Isotopes in Oceanographic Studies and Paleotemperatures. Laboratorio di Geologia Nucleare, Pisa, Italy, pp. 9–13. De Ros, L.F., 1998. Heterogeneous generation and evolution of diagenetic quartzarenites in the Silurian–Devonian Furnas Formation of the Parana Basin, southern Brazil. Sedimentary Geology 119, 99–128. De Ros, L.F., Morad, S., Paim, P.S.G., 1994. The role of detrital composition and climate on the diagenetic evolution of continental molasses: evidence from the Cambrio-Ordovican Guaritas sequence, southern Brazil. Sedimentary Geology 92, 197–228. Ehrenberg, S.N., 1989. Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones: discussion: compaction and porosity evolution of Pliocene sandstones, Ventura Basin, California – discussion. American Association of Petroleum Geologist, Bulletin 73, 1274–1276. Ehrenberg, S.N., Aagaard, P., Wilson, M.J., Fraser, A.R., Duthie, D.M.L., 1993. Depthdependent transformation of kaolinite to dickite in sandstones of the Norwegian Continental shelf. Clay Minerals 28, 325–352. El-Ghali, M.A.K., Mansurbeg, H., Morad, S., Al-Aasm, I., Ajdanlisky, G., 2006. Distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework: evidence from the Petrohan Terrigenous Group and the Svidol Formation, Lower Triassic, NW Bulgaria. Sedimentary Geology 190, 299–321. Esteban, M., Klappa, C.F., 1983. Subaerial exposure environments. In: Scholle, P.A., Bebount, D.G., Moore, C.H. (Eds.), Carbonate Depositional Environments. Mem. American Association of Petroleum Geologists, Tulsa, vol. 33, pp. 1–54. Friedman, I., O’Neil, J.R., 1977. Composition of stable isotopic fractionation factors of geochemical interest. In: Fleisher, M. (Ed.), Data of Geochemistry. 6th U.S. Geological Survey, Professional paper, vol. 440, pp. 1–12.

Garcia, A.J.V., Morad, S., De Ros, L.F., Al-Aasm, I.S., 1998. Palaeogeographical, palaeoclimatic and burial history controls on the diagenetic evolution of reservoir sandstones: evidence from the Lower Cretaceous Serraria sandstones in the Sergip–Alagoas Basin, NE Brazil. In: Morad, S. (Ed.), Carbonate Cementation in Sandstones. Special Publication, International Association of Sedimentologists, vol. 26, pp. 107–140. Giles, M.R., Stevenson, S., Martin, S.V., 1992. The reservoir properties and diagenesis of the Brent Group: a regional perspective. In: Morton, A.C., Haszeldine, R.S., Giles, M.R., Brown, S. (Eds.), Geology of the Brent Group. Special Publications of the Geological Society of London, vol. 61, pp. 289–327. Gluyas, J.G., Grant, S.M., Robinson, A., 1993. Geochemical evidence for a temporal link control on sandstone cementation. In: Horbury, A.D., Robinson, A.G. (Eds.), Diagenesis and Basin Development. Association of Petroleum Geologists studies in Geology, vol. 36, pp. 23–33. Hall, J.S., Mozley, P., Davis, J.M., Roy, N.D., 2004. Environments of formation and controls on spatial distribution of calcite cementation in Plio-Pleistocene fluvial deposits, New Mexico, U.S.A. Journal of Sedimentary Research 74, 643–653. Houseknecht, D.W., 1988. Intergranular pressure solution in four quartzose sandstones. Journal of Sedimentary Petrology 58, 228–246. Howard, J.J., 1992. Influence of authigenic clay minerals on permeability. In: Houseknecht, D.W., Pittman, E.D. (Eds.), Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstones. Society of Economic Palaeontologists and Mineralogists, (Special Publication), vol. 47, pp. 257–264. Irwin, H., Curtis, C., Coleman, M., 1977. Isotopic evidence for source of diagenetic carbonates formed during burial of organic rich sediments. Nature 269, 209–213. Keller, W.D., 1970. Environment aspects of clay minerals. Journal of Sedimentary Petrology 40, 788–813. Keller, W.D., Reynolds, R.C., Inoue, A., 1986. Morphology of clay minerals in the smectite-to-illite conversion series by scanning electron microscopy. Clays and Clay Minerals 34, 187–197. Ketzer, J.M., Morad, S., Amorosi, A., 2003. Predictive diagenetic clay–mineral distribution in siliciclastic rocks within a sequence stratigraphic framework. In: Worden Richard, H., Morad, S. (Eds.), Clay Mineral Cements in Sandstones. Blackwell. Oxford, International Association of Sedimentologists, (Special Publication), vol. 34, pp. 42–59. Kirschbaum, M.A., McCabe, P.J., 1992. Controls on the accumulation of coal and on the development of anastomosed fluvial systems in the Cretaceous Dakota Formation of south Utah. Sedimentology 34, 581–598. Land, L.S., 1983. The application of stable isotopes to studies of dolomite and to problems of diagenesis of clastic sediments. In: Stable Isotopes in Sedimentary Geology. Society of Economic Palaeontologists and Mineralogists, (Special Publication, Short Course), vol. 10, pp. 4.1–4.22. Lemon, N.M., Cubitt, C.J., 2003. Illite fluorescence microscopy: a new technique in the study of illite in the Merrimelia Formation, Cooper Basin, Australia. In: Worden, R.H., Morad, S. (Eds.), Clay Mineral Cements in Sandstones. International Association of Sedimentologists, (Special Publication), vol. 34, pp. 411–424. Lundegard, P.D., Land, L.S., 1986. Carbon dioxide and organic acids: their role in porosity enhancement and cementation, Palaeogene of the Texas Gulf Coast. In: Gautier, D.L. (Ed.), Roles of Organic Matter in Sediment Diagenesis. Society of Economic Palaeontologists and Mineralogists, (Special Publication), vol. 38, pp. 129–146. Mack, G.H., James, W.C., Monger, H.C., 1993. Classification of palaeosols. Geological Society of America Bulletin 105, 129–136. Made´, B., Ben Baccar, M., Fritz, B., 1994. Geochemical modeling of diagenetic reactions: a thermodynamic and kinetic approach. Mineralogy Magazine 58A, 549–550. Matlack, K.S., Houseknecht, D.W., Applin, K.R., 1989. Emplacement of clay into sand by infiltration. Journal of Sedimentary Petrology 59, 77–87. McBride, E.F., 1963. A classification of common sandstones. Journal of Sedimentary Petrology 33, 664–669. McBride, E.F., 1989. Quartz cementation in sandstones – a review. Earth Science Reviews 26, 69–112. Miall, A.D., 1997. The Geology of Stratigraphic Sequences. Springer, pp. 433. Monger, H.C., Daugherty, L.A., Gile, L.H., 1991. A microscopic examination of pedogenic calcite in an aridisol of southern New Mexico. In: Nett Leton, W.D. (Ed.), Occurrence, Characteristics, and Genesis of Carbonate, Gypsum, and Silica Accumulations in Soils. Soil Science Society of America, Special Publication, vol. 26, pp. 37–59. Mora, C.I., Fastovsky, D.E., Driese, S.G., 1993. Geochemistry and stable isotopes of palaeosols. In: University of Tennessee, Department of Geological Sciences, Studies in Geology, vol. 23, pp. 65. Morad, S., 1988. Albitized microcline grains of post-depositional and probable detrital origins in Brottum Formation sandstones (upper Proterozoic), Sparagmite region of Southern Norway. Geological Magazine 125, 229–239. Morad, S., 1998. Carbonate cementation in sandstones; distribution patterns and geochemical evolution. In: Morad, S. (Ed.), Carbonate Cementation in Sandstones. International Association of Sedimentologists, (Special Publication), vol. 26, pp. 1–26. Morad, S., Ben Ismail, H., De Ros, L.F., Al-Aasm, I.S., Serrhini, N.E., 1994. Diagenesis and formation water chemistry of Triassic reservoir sandstones from southern Tunisia. Sedimentology 41, 1253–1272. Morad, S., Bergan, M., Knarud, R., Nystuen, J.P., 1990. Albitization of detrital plagioclase in Triassic reservoir sandstones from the Snorre Field, Norwegian North Sea. Journal of Sedimentary Petrology 60, 411–425.

M.A.K. El-Ghali et al. / Marine and Petroleum Geology 26 (2009) 1212–1227 Morad, S., Ketzer, J.M., De Ros, F., 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology 47, 95–120. Moraes, M.A.S., De Ros, L.F., 1990. Infiltrated clay in fluvial Jurassic sandstones of Reconcavo Basin, northeastern Brazil. Journal of Sedimentary Petrology 60, 809–819. Moraes, M.A.S., De Ros, L.F., 1992. Depositional infiltrated and authigenic clays in fluvial sandstones of the Jurassic Sergi Formation, Reconcavo Basin, northeastern Brazil. In: Houseknecht, D.W., Pittman, D.E. (Eds.), Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstones. Tulsa, OK, United States, SEPM, Society for Sedimentary Geology, vol. 47, pp. 197–208. Philip, J., Masse, J.P., Camoin, G., 1996. Tethyan carbonate platforms. In: Nairn, A.E.M., Ricou, L.E., Vrielynck, B., Dercourt, J. (Eds.), The Ocean Basins and Margins. The Tethys Ocean. New York, London, Plenum Press, vol. 8, pp. 239–266. Posamentier, H.W., Allen, G.P., 1993. Variability of the sequence stratigraphic model: effects of local basin factors. Sedimentary Geology 86, 91–109. Posamentier, H.W., Allen, G.P., 1999. Siliciclastic sequence stratigraphy – concepts and application. In: Society of Economic Palaeontologists and Mineralogists, concepts in Sedimentology, Tulsa, vol. 7, pp. 210. Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988a. Eustatic controls on clastic deposition, I – conceptual framework. In: Wilgus, C.K. (Ed.), Sea Level Changes: An Integrated Approach. Society of Economic Palaeontologists and Mineralogists, Special Publication, vol. 42, pp. 109–124. Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988b. Eustatic controls on clastic deposition, II – sequence and systems tract models. In: Wilgus, C.K. (Ed.), Sea Level Changes: An Integrated Approach. Society of Economic Palaeontologists and Mineralogists, Special Publication, vol. 42, pp. 125–154. Retallack, G.J., 1988. Field recognition of palaeosols. In: Reinhardt, J., Sigleo, W.R. (Eds.), Palaeosols and Weathering Through Geologic Time; Principles and Applications. Geological Society of America, Special Paper, vol. 216, pp. 1–20. Shanley, K.W., McCabe, P.J., 1994. Perspective on the sequence stratigraphy of continental strata. American Association of Petroleum Geologist Bulletin 78, 544–568. Smosna, R., Brune, K.R., 1997. Depositional controls over porosity development in lithic sandstones of the Appalachian Basin: reducing exploration risk. In: Kupecz, J.A., Gluyas, J., Bloch, S. (Eds.), Reservoir Quality Prediction in Sandstones and Carbonates. American Association of Petroleum Geologist Memoir, vol. 96, pp. 249–265.

1227

Surdam, R.C., Boese, S.W., Crossey, L.J., 1984. The chemistry of secondary porosity. In: McDonald, D.A., Surdam, R.C. (Eds.), Clastic Diagenesis. American Association of Petroleum Geologist Memoir, vol. 37, pp. 127–149. Tronkov, D., 1981. Stratigraphy of the Triassic system in a part of the western Srednogorie (West Bulgaria). Geologica Balcanica 11 (1), 3–20. Tronkov, D., Ajdanlijsky, G., 1998. Sequence stratigraphy of the transition from continental to marine lower Triassic sediments in Western Balkan, NW Bulgaria (abstract). In: 16th Congress of Carpatho-Balkan Geological Association, Vienna, Austria, Abstract volume, p. 607. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy and key definitions. In: Wilgus, C.K. (Ed.), Sea Level Changes: An Integrated Approach. S.E P.M. Special Publication, vol. 42, pp. 39–46. Wilson, M.D., Pittman, E.D., 1977. Authigenic clays in sandstones: recognition and influence on reservoir properties and paleoenvironmental analysis. Journal of Sedimentary Petrology 47, 3–31. Wilson, M.D., 1994. Non-compositional controls on diagenetic processes. In: Wilson, M.D. (Ed.), Reservoir Quality Assessment and Prediction in Clastic Rocks. Society of Economic Palaeontologists and Mineralogists, (Short Course Special Publication), vol. 30, pp. 161–182. Worden, R., Morad, S., 2000. Quartz cement in oil field sandstones: a review of the critical problems. In: Worden, R.H., Morad, S. (Eds.), Quartz Cementation in Sandstones. International Association of Sedimentologists, Special Publication, vol. 29, pp. 1–20. Worden, R., Morad, S., 2003. Clay minerals in sandstones: controls on formation, distribution and evolution. In: Worden, R.H., Morad, S. (Eds.), Clay Mineral Cementation in Sandstones. International Association of Sedimentologists, Special Publication, vol. 34, pp. 3–41. Wright, V.P., Marriott, S.B., 1993. The sequence stratigraphy of fluvial depositional systems: the role of floodplain sediment storage. Sedimentary Geology 86, 203–210. Wright, V.P., Tucker, M.E., 1991. Introduction. In: Wright, V.P., Tucker, M.E. (Eds.), Calcretes. International Association of Sedimentologists, Reprint Series, vol. 2, pp. 1–22. Zhang, Z., Sun, K., Yin, J., 1997. Sedimentology and sequence stratigraphy of the Shanxi Formation (Lower Permian) in the northwestern Ordos Basin, China: an alternative sequence model for fluvial strata. Sedimentary Geology 112, 123–136.