Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt

Cretaceous Research xxx (2013) 1e10

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Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt Sameh S. Tahoun a, Omar Mohamed b, * a b

Cairo University, Faculty of Science, Geology Department, 12613 Giza, Egypt Minia University, Faculty of Sciences, Geology Department, El-Minia, Egypt

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 March 2013 Accepted in revised form 16 June 2013 Available online xxx

Twenty-eight samples from the Bahariya Formation of the Salam-17 Well in the north Western Desert were palynologically investigated. These samples are of Cenomanian age. Fair diversity and fair to moderately preserved palynomorph assemblage has been recovered. Among them, the dinoflagellate cysts showed very poor diversity and abundance. Four miospore zones have been informally identified in the lower Cenomanian. Various palynofacies criteria, adopted from previous publications (e.g. relative particle abundance data, brown to black wood ratio, equi-dimensional to lath-shaped black wood ratio, average size of phytoclasts and spores/pollen ratio) are applied as alternative indicators to monitor the proximaledistal trends instead of the marine palynomorphs-based parameters. The method can be applied in the Egyptian Western Desert to overcome the rarity and absence of dinoflagellate cysts in the recovered organic residues. The palynofacies study of the section demonstrates a predominantly regressive phase, characterized by deltaic, distributary or tidal channels, interrupted by short-lived marine incursions. The palynofacies trends within the studied succession indicate six genetic sequences informally described as Genetic Stratigraphic Sequences A through F. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Genetic sequences Palynofacies Cenomanian Bahariya Formation north Western Desert

1. Introduction Owing to its importance as one of the significant oil reservoirs in the Western Desert of Egypt, the Bahariya Formation was the scope of most micropaleontologic studies including correlation, age determination, unconformity identification, sequence stratigraphic modelling and analysis, fingerprinting and characterization of the formations beside paleoenvironmental interpretations. On account of the mainly clastic composition of the Bahariya Formation, most of the inorganic microfossils are rare or absent making the results of palynological studies valuable in understanding the variations and characteristics of that important reservoir. The present work deals with two important items. The first is that this work defines the successive downhole palynoevents in the lower Cenomanian of the Western Desert territory. These bioevents

* Corresponding author. E-mail addresses: [email protected], (O. Mohamed).

[email protected]

will be very helpful in resolving some reservoir problems concerning correlation, age determination and unconformity identification. Moreover, any disconformity within the reservoir succession can be detected based on the absence of one or more of such palynoevent horizons. The second item is the usage of the palynofacies signals in tracing the proximaledistal trends. These trends reconstruct the genetic sequence stratigraphic framework. Many palynological studies have been carried out on the Cenomanian of the Bahariya Formation (e.g. El Beialy, 1993a,b,c, 1994a,b,c,d, 1995; El Beialy et al., 2010, 2011; El Shamma, 1991; Ibrahim, 1996, 2002; Mahmoud and Moawad, 2002; Schrank and Ibrahim, 1995; Tahoun, 2012, Tahoun et al., 2012, 2013; Zobaa et al., 2011, 2013) but all are lacking sequence stratigraphic interpretations. Unlike all such previous studies, the present study presents a first genetic sequence stratigraphy based on palynofacies indications in the Cenomanian of the Bahariya Formation. According to Tyson (1995), qualitative and quantitative studies of the sedimentary organic matter (SOM) contained within sediments permit the determination of a number of characteristics of the depositional environment. These characteristics include, for example, variations in proximaledistal location of the depocenter

0195-6671/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cretres.2013.06.004

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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with respect to the sedimentary source, regressiveetransgressive trends within a stratigraphic sequence and the nature of the environment of deposition (such as water column oxygenation, salinity, etc.). All of which have a great potential to aid sequence stratigraphic interpretations. The present work is an attempt to reuse five core samples previously investigated by El Shamma (1991), in addition, new six core samples and seventeen ditch cutting samples have been used for palynostratigraphic and paleoenvironmental purposes. In the present study, the poor diversity and recovery of the dinoflagellate cysts due to bad preservation status or a great dilution effect of the particulate organic matter, represent a big challenge to interpret the paleoenvironments and the results may be misleading. All the palynological parameters depend upon the dinoflagellates such as, palynological marine index, allochthonous versus autochthonous palynomorphs ratio could not be used with confidence. Instead, some other important parameters adopted from previous publications (see Pittet and Gorin, 1997; Götz et al., 2005, 2008; Tahoun et al., 2013) like amorphous organic matter percentage, phytoclasts percentage, brown to black wood ratio, equi-dimensional to lath-shaped black wood ratio, average size of phytoclasts, spores/pollen ratio, could be used as monitoring criteria to trace the transgressive and regressive trends. The method can be applied to overcome the rarity and absence of dinoflagellate cysts in the recovered organic residues. 2. Stratigraphic setting The clastic Bahariya Formation consists chiefly of sandstones and siltstones interbedded with various shale beds as well as very small limestone streaks. The shale is silty and calcareous in parts. The siltstones and sandstones are grayish to yellowish white and frequently pyritic. Previous miospores and dinoflagellates investigations indicated that this formation was assigned to Albian to Cenomanian ages (e.g. El Beialy, 1994a,b; El Beialy et al., 2010; Ibrahim, 2002; Tahoun, 2012; Zobaa et al., 2013). Generally, the Bahariya Formation is conformably overlain by shales and carbonate of Abu Roash “G” Member, representing the bottom member of the Abu Roash Formation (AeG members) in the majority of

the drilled wells in the northern part of the Western Desert (Hantar, 1990). The upper boundary of the Bahariya Formation in the studied well is recorded at depth of 5790 ft, while the lower boundary is not reached due to the unavailability of the samples. 3. Materials and methods Extensive palynological analysis has been carried out for twenty-eight samples (eleven core samples and seventeen ditch cutting samples) from the Bahariya Formation of the Salam-17 borehole [30
410 1800 N and 26
580 2400 E] in the north Western Desert of Egypt (Fig. 1). The investigated samples cover the depth interval from 5790 ft to 6600 ft (Fig. 2). Standard palynological processing technique has been followed including heavy liquid separation. No oxidation treatments were done in order to keep the organic matter for palynofacies investigations. Two slides from each sample were prepared using Canada Balsam as a permanent mounting medium. Slides were examined using an Axioskop Zeiss light microscope. The study of the SOM is usually referred to as palynofacies analysis, a methodology defined by Tyson (1995) as “the palynological study of depositional environments and hydrocarbon source rock potential based upon the total assemblage of particulate organic matter”. This novel definition broadens the somewhat more restricted definitions proposed elsewhere (Traverse, 1988), which tried to delimit depositional environments exclusively from distinctive palynomorph assemblages (dinoflagellate cysts, spores, pollen grains, etc.) with little attention being paid to the plantderived detritus (phytoclasts). In all samples, approximately 500 organic particles were identified and counted according to the method of Tyson (1995), and their relative frequencies were used for stratigraphic and paleoenvironmental interpretations. All materials used in the present study were housed and stored in the Palynolab, Geology Department, Faculty of Science, Cairo University. Six constituent categories have been identified. The phytoclast group is subdivided into brown or translucent and black or opaque to semi-opaque particles. Brown (translucent) phytoclasts (category 1) comprise fresh woody fragments and cuticles, which are

Fig. 1. Location map of the Salam-17 Well in the north Western Desert, Egypt (base map after Zobaa et al., 2013).

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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Fig. 2. The lithostratigraphy of the Bahariya Formation, position of the samples, Palynozones, frequency of palynomorphs in order of highest occurrences from the Salam-17 Well.

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Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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much less oxidized. This fresh phytoclasts are inherently more biodegradable than the opaque fraction, (Tyson, 1995). The black (opaque and semi-opaque) fraction comprises charcoal and highly oxidized land plant tissues. This fraction is reworked and resistant to biodegradation. For hydrodynamic reasons, it is further subdivided into equi-dimensional (category 2) and lath-shaped fragments with a higher ease of transport (category 3) (Pittet and Gorin, 1997). The palynomorphs group is subdivided into two subgroups: (1) the continental palynomorphs (miospore subgroup) comprise pollen grains and spores (category 4), and (2) the marine palynomorphs comprise dinoflagellate cysts and foraminiferal test linings (category 5). Foraminiferal test linings are considered reliable indicators for shelf or slope conditions, and their frequency tends to decrease offshore (Tyson, 1995). The amorphous organic matter (AOM) (category 6) characterizes low-energy, stagnant and oxygen-depleted paleoenvironments (Tyson, 1987). In the present study, phytoclasts and miospores (categories 1e4) represent the allochthonous (land-derived) fraction, whereas dinoflagellate cysts, foraminifera linings and AOM constitute the relatively autochthonous (marine) fraction. Amorphous organic matter percentage, phytoclasts percentage, brown to black wood ratio, equi-dimensional to lath-shaped black wood ratio, average size of phytoclasts, spores/pollen ratio are among the several palynofacies distribution trends and parameters that may have been used effectively (cf. Tyson, 1993, 1995). These trends and parameters based on percentages of palynofacies categories are illustrated in Table 1. 4. Results The palynofacies analysis carried out herein is based on the moderately to well preserved organic residues yielded from most of the investigated samples, only samples number 1, 26, 27 and 28 show highly degraded and corroded particulate organic matter with bad preservation status. The recovered miospores show fair diversity and abundance. In contrary, the dinoflagellate cysts show poor diversity and abundance. 4.1. Palynostratigraphy The aims of the palynological investigation of the Salam-17 well is to document and delimit the recorded palynoevent succession illustrated in terms of successive first downhole appearance (equivalent to LADs) for some stratigraphically important taxa. The good separation of these first downhole appearance horizons will be more useful than lumping. Such idea permits the recognition whether there are unconformities or sedimentary breaks within the studied reservoir based on absence of one or

Table 1 Some important parameters used in the palynofacies analysis (adapted and modified after Tyson, 1993, 1995). Parameters

Proximaledistal trend

% Sand

% Phytoclast/kerogen % AOM/kerogen % Palynomorphs of kerogen % Brown wood/kerogen % Black wood/kerogen Brown wood:black wood ratio Equi-dimensional black:lathshaped black ratio Spores:pollen ratio Phytoclast coarse fraction % Plankton of palynomorphs

High-low Low-high Low-high Low-high-low High-low-high Low-high-low High-low

Increases Decreases Decreases Decreases Increases Decreases May increases

High-low High-low Low-high-low

Increases Increases Decreases

more of such LADs successive bioevents/horizons. The successive first downhole appearance data is very effective in cases of ditch cutting samples and cavings. The last downhole appearance data may be altered due to caving but the first downhole appearance data are not. The palynomorphs group is poor in most of the studied samples. Few index spores and pollen grains are recorded and are adequate enough to be used in age assessment and biozonation. 26 palynofloral species belonging to 23 genera are identified. These consist of 8 genera and 10 species of spores, 12 genera and 13 species of pollen grains, 3 species belonging to 3 genera of dinoflagellate cysts. The stratigraphic distribution of such palynomorphs in the studied samples is illustrated in Fig. 2. The marker and/or well preserved species have been depicted in Fig. 3. Imperative correlation of the proposed miospore zones with their equivalents published in north Western Desert of Egypt is presented in Table 2. The zones is as follows from top to bottom: Zone 1: Afropollis jardinus e Elaterosporites klaszii interval zone Definition: Interval from the last occurrence (top occurrence) of Afropollis jardinus to the last occurrence of Elaterosporites klaszii. Occurrence: From 5796 to 5832 ft (36 ft thick), samples 1e2, the upper part of the Bahariya Formation. Age assignment: The stratigraphically important miospores Afropollis jardinus and Crybelosporites pannuceus discriminate the Cenomanian deposits and their highest occurrences were accounted to define the top of lower and/or middle Cenomanian of the north Western Desert of Egypt by Aboul Ela and Mahrous (1992); El Beialy (1994a,b); El Beialy et al. (1990, 2010, 2011); Ibrahim (1996, 2002); Schrank and Ibrahim (1995); Schrank and Mahmoud (1998); and Tahoun (2012). Suggested age: early Cenomanian. Zone 2: Elaterosporites klaszii e Cretacaeiporites densimurus interval zone Definition: Interval from the last occurrence of Elaterosporites klaszii to the last occurrence of Cretacaeiporites densimurus. Occurrence: From 5832 to 6034 ft (202 ft thick), samples 3e8, the middle part of the Bahariya Formation. Age assignment: Elatersporites klaszii is valuable in early Cenomanian biostratigraphy. The highest occurrence of this taxon was noted within the topmost part of the lower Cenomanian deposits of Egypt (Aboul Ela and Mahrous, 1992; El Beialy, 1993a,b, 1994c; El Beialy et al., 2010, 2011; Ibrahim, 1996; Mahmoud and Moawad, 1999, 2000, 2002; Mahmoud et al., 1999; Sultan, 1978, 1987; Tahoun, 2012). Suggested age: early Cenomanian. Zone 3: Cretacaeiporites densimurus e Trilobosporites laevigatus interval zone Definition: Interval from the last occurrence of Cretacaeiporites densimurus to the last occurrences of Trilobosporites laevigatus. Occurrence: From 6034 to 6340 ft (306 ft thick), samples 9e18, the lower part of the Bahariya Formation. Age assignment: Aboul Ela and Mahrous (1992), El Beialy (1993c, 1994a) and El Shamma (1991) placed the highest occurrence of Cretacaeiporites densimurus within the lowest part of the lower Cenomanian. While El Beialy et al. (2010), Mahmoud and Moawad (2000), and Tahoun (2012) utilized its top occurrence to delineate the lower Cenomanian in the Western Desert of Egypt. Suggested age: early Cenomanian. Zone 4: Trilobosporites laevigatus interval zone Definition: The upper boundary is defined by the last occurrence of Trilobosporites laevigatus while the lower boundary was not reached in the present study. Occurrence: From 6340 to 6600 ft (260 ft thick), samples 19e28, the lowest part of the Bahariya Formation.

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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Fig. 3. Photomicrograph of the palynomorphs sequentially attended by corresponding depth, slide designation beside light microscopic citation coordinates. PTERIDOPHYTIC SPORES: A. Cyathidites australis Couper, 1953; depth 5838-44 ft, slide 1, corrd. 114.9/6.2. B. Cyathidites minor Couper, 1953; depth 5974-79 ft, slide 2, corrd. 107.8/22.1.C. Dictyophyllidites harrisii Couper, 1958; depth 6064-69 ft, slide 1, corrd. 108.2/15.7. D. Todisporites major Couper, 1958; depth 5802-08 ft, slide 1, corrd. 111.8/5.2. E. Cicatricosisporites orbiculatus Singh, 1964; depth 5802-08 ft, slide 2, corrd. 89.7/11.6. F. Cicatricosisporites spp.; depth 5980-86 ft, slide 2, corrd. 100.5/7.3. G. Cicatricosisporites spp.; depth 6064-69 ft, slide 1, corrd. 96.9/20.2. H. Trilobosporites laevigatus El Beialy, 1994; depth 6500 ft, slide 1, corrd. 119.3/6.8. GYMNOSPERM POLLEN GRAINS: I. Classopollis brasiliensis Herngreen, 1975; depth 5974-79 ft, slide 1, corrd. 110.7/18.4. J. Classopollis classoides Pflug, 1953 emend. Pocock and Jansonius, 1961; depth 6040-46 ft, slide 1, corrd. 120.1/8. K. Ephedripites sp.; depth 5838-44 ft, slide 2, corrd. 88.7/11.2. L. Elaterosporites klaszii (Jardiné and Magloire) Jardiné, 1967; depth 5974-79 ft, slide 2, corrd. 90.7/6.4. M. Inaperturopollenites dubius (Potonié and Venitz) Pflug and Thomson, in Thomson and Pflug, 1953; depth 5826-32 ft, slide 1, corrd. 106.7/19. N. Araucariacites australis Cookson, 1947 ex Couper, 1953; depth 6090 ft, slide 1, corrd. 100.9/21.3. ANGIOSPERM POLLEN GRAINS: O. Cretacaeiporites densimurus Schrank and Ibrahim, 1995; depth 6040-46 ft, slide 1, corrd. 92.8/10.7. P. Afropollis jardinus (Brenner) Doyle, Jardiné and Doerenkamp, 1982; depth 5826-32 ft, slide 2, corrd. 118.5/14.5. DINOFLAGELLATE CYSTS: Q. Subtilisphaera sp.; depth 5802-08 ft, slide 2, corrd. 118.3/5.6.

Age assignment: Trilobosporites laevigatus is considered as an important marker species which has its last appearance in the lower Cenomanian of Egypt. It is worthy mentioning to note that the highest occurrences of Trilobosporites laevigatus characterize and define relatively lower levels in the lower Cenomanian than those of top ranges of Elaterocolpites castelainii Form ‘B’,

Elatersporites klaszii, Elateroplicites africaensis and Sofrepites legouxae. The last appearance datum of Trilobosporites laevigatus was reported from the lower Cenomanian of Egypt (El Beialy et al., 2010; Ibrahim, 2002; Schrank and Ibrahim, 1995; Sultan, 1987; Sultan and Aly, 1986). Suggested age: early Cenomanian.

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

C. densimurus interval subzone Zone V

2 e Trilobosporites laevigatus Zone

E. klaszii interval subzone Zone IV

A number of different palynological criteria have been used in previous studies to infer changes in relative sea level. (see Pittet and Gorin, 1997; Götz et al., 2005, 2008; Tahoun et al., 2013). A review of the predicted trends in palynofacies characteristics through a thirdorder siliciclastic sequence was given by Tyson (1993, 1995). These trends have been used in the present work (Table 1; see Fig. 4). Four palynofacies approaches have been utilized to interpret relative sea-level change in the studied section:

1 e Elaterocolpites castelainii e Elatersporites klaszii Zone 2 e zone “C” 2 e upper part of Cretacaeiporites scabratus zone “A”

1 e Cretacaeiporites scabratus Zone

Afropollis jardinus interval zone Zone III 2 e Afropollis jardinus Zone

4.2. Palynofacies and genetic stratigraphic sequences

1 e Elatersporites klaszii Zone (CE4) 3, 4 e Elatersporites klaszii Zone (CE3)

1 e II Afropollis jardinus Zone 2 e Afropollis jardinus Zone

1 e III upper part of Elaterocolpites castelainii e Elatersporites klaszii Zone 1 e Cretacaeiporites scabratus subzone

Upper part of assemblage zone III Zone 1: Afropollis jardinus interval Zone Zone 2: Elaterosporites klaszii interval Zone Zone 3: Cretacaeiporites densimurus interval Zone Zone 4: Trilobosporites laevigatus interval Zone

1 e CE1 2, 3 e Cretacaeiporites scabratus Zone (CE2) 2, 4 e Matonisporites simplex Zone (CE1)

1 e El Shamma (1991) 2 e El Shamma and Baioumi (1992) 3 e Bassiouni et al. (1992) 4 e El Shamma et al. (1999) 1 e Afropollis jardinus Zone (CE5) 2, 3, 4 e CE4 1 e Aboul Ela and Mahrous (1992) 2 e El Shamma and Arafa (1992) Sultan & Aly (1986) Present study

Table 2 Illustrates the correlation of the present miospore zones with their equivalents erected in north Western Desert of Egypt.

1 e Abd El Shafy and Abd El Moneim (1991) 2 e Aboul Ela et al. (1996)

El Beialy et al. (2010) Tahoun (2012)

S.S. Tahoun, O. Mohamed / Cretaceous Research xxx (2013) 1e10

1 e El Beialy (1993a) 2 e El Beialy (1994a)

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(i) The relative abundances of phytoclast categories including brown and black wood particles compared to the relative abundance of amorphous organic matter (AOM) category. (ii) The relative abundances of large equi-dimensional and small lathshaped black wood particles categories. (iii) Variations in dominant size of brown phytoclast particles estimated roughly from 0 to 7 (where 1e2 very small, 2e3 small, 3e4 medium, 4e5 large, 5e7 very large). (iv) Variations in the count of the palynomorphs. The results of the palynofacies study are summarized in Fig. 4. The studied succession probably shows a complete six distinct genetic stratigraphic sequences (A, B, C, D, E and F) and two incomplete ones, each of them are bounded by candidate sequence boundary surface. The Genetic Sequence A (6550e6410 ft) is in the lower part of Trilobosporites laevigatus interval zone. The Genetic Sequence B (6410e6260 ft) is in the upper part of Trilobosporites laevigatus interval zone and lower part of Cretacaeiporites densimurus e Trilobosporites laevigatus interval zone. The Genetic Sequence C (6260e6120 ft) is in the upper part of Cretacaeiporites densimurus e Trilobosporites laevigatus interval zone. The Genetic Sequence D (6120e6016 ft) is in the uppermost part of Cretacaeiporites densimurus e Trilobosporites laevigatus interval zone and the lowermost part of Elaterosporites klaszii e Cretacaeiporites densimurus interval zone. The Genetic Sequence E (6016e5979 ft) is in the lower part of Elaterosporites klaszii e Cretacaeiporites densimurus interval zone. The Genetic Sequence F (5979e5808 ft) is in the upper part of Elaterosporites klaszii e Cretacaeiporites densimurus interval zone and most of Afropollis jardinus e Elaterosporites klaszii interval zone. The Genetic Sequence A is bounded by the sequence boundary 1 (Sb 1) and the sequence boundary 2 (Sb 2). It is generally a proximal trend but characterized by a brief episode of considerable marine incursion at a depth of 6500 ft, where the AOM reaches its peak in this sequence. This relatively deeper horizon with stronger marine affinity is followed by a relatively long regressive phase. The probable Milankovitch-controlled alternating coarse and fine clastics (sandstone e silt to mudstones) of this cycle appear to exert a strong control over the palynofacies assemblages. The Sb 1 horizon contains high relative abundances of black wood particles (42%) with equi-dimensional black debris (26%) and brown to black ratio (0.6) in addition to equi-dimensional black particles to lathshaped ratio (1.6) and has a brown phytoclast size index of 5 (moderate to large), the total count of miospores is around 12. Relatively increased percentage of AOM at the 6500 ft depth and below characterizes poor-oxygenated, relatively dysoxic to anoxic depositional conditions. Sediments lying stratigraphically above the 6500 ft horizon, show generalized regressive characteristics, for example a decreased brown/black phytoclast particles ratio from 1.8 to 0.8 in the Sb 2 horizon. The Genetic Sequence B begins at Sb 2 in the upper part of Trilobosporites laevigatus interval zone. Deposition throughout this horizon (Sb 2) appears to have occurred under relatively falling sea

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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Fig. 4. The palynofacies parameters distribution and the genetic sequences in Salam-17 Well.

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Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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levels which progressively introduced more proximal characteristics to the particulate organic matter assemblages. Palynofacies data indicate that this relative sea-level fall by reduced amount of brown phyoclasts compared to black ones beside a very large (7) recorded phyoclast size conjugated with total count of miospores is around 11. However, the abundance maxima of miospore (59), generally a good indicator of decreasing sand content (i.e. most marine incursion cf. Tyson, 1993), occurs at 6340 ft. This peak is considered to be representative of a period of relatively low-energy marine invasion. The succeeding palynofacies assemblages reflect a subsequent relative sea-level fall which persists from 6340 to 6260 ft. According to the size of phytoclasts, count of miospores and relative abundance of phytoclasts, the Sb 3 is probably situated at 6260 ft with a decrease in the relative abundance of the small lath black debris fraction from 12% (6340 ft) to 7% (6260 ft), the equidimensional black particles to lath-shaped ratio is around 3.3 in addition to the large size of phytoclasts (6). This Genetic sequence is characterized by the dominance of brown over black wood in most of the sequence and not restricted only to the marine incursion horizon which can not be explained. The 6340 ft horizon can be interpreted as a flooding surface where the AOM is at its highest value (22%) and the equi-dimensional black particles to lathshaped ratio is the smallest (1.5) around the values above and below within the sequence. The dominance of phytoclasts over AOM throughout this sequence characterizes well-oxygenated, relatively oxic depositional conditions. The proposed Genetic Sequence C is bounded by the sequence boundary 3 (Sb 3) and the sequence boundary 4 (Sb 4). It is generally a regressive phase but it is characterized by a mild marine incursion which followed also by a relatively long regressive phase. The 6320 ft horizon can be interpreted as a flooding surface where the equi-dimensional black particles to lath-shaped ratio is very small (0.2), the brown wood percentage is around 57%. Sediments lying stratigraphically above the 6320 ft horizon, show generalized regressive characteristics, for example, declining percentage of brown phytoclasts from 57 to 31% while the black phytoclasts percentage increases in a reverse manner up to 63% at 6120 ft. (the Sb 4). This would be expected during relative sea-level fall. The Genetic Sequence D is bounded by the Sb 4 and the Sb 5. The Sb 4 horizon contains low relative abundances of well preserved brown wood particles (31%) with equi-dimensional black debris (41%) and brown to black ratio (0.5) in addition to equi-dimensional black particles to lath-shaped ratio (1.9) and has a brown phytoclast size index of 6 (large), the total count of miospores is around 6. Relatively the low percentage of AOM throughout this sequence illustrates well-oxygenated, relatively oxic depositional conditions. Sediments below the 6069 ft horizon, illustrate obvious deepening characters while, sediments lying stratigraphically above this horizon, show generalized regressive characteristics supporting a probable brief episode of marine incursion. The declining of brown/ black phytoclasts ratio from 4.4 to 0.1 and the decrease in the count of sporomorphs from 265 to only 6 in the Sb 5 horizon (6016 ft) beside the increasing size of phytoclasts from small to mediumlarge in addition to the increasing trend of equi-dimensional black wood particles to lath-shaped particles ratio from 1 to 3.6 are all good evidence of such a regressive event. The Genetic Sequence E commences at the Sb 5 (6010 ft) in the lower part of Elaterosporites klaszii e Cretacaeiporites densimurus interval zone. Deposition throughout this horizon (Sb 5) appears to have occurred under relatively falling sea level with similar palynofacies criteria to those discussed in the last sentence of the previous paragraph. Such falling sea-level status progressively introduced more proximal characteristics to the SOM assemblages. Nevertheless, the abundance maxima of miospore (241) in addition to a decreasing in the equi-dimensional black wood to lath-shaped

particles ratio from 3.6 to 0.5 with medium phytoclasts size compared to equivalent large sizes above and below, generally are good indicators of decreasing sand content (i.e. the most marine incursion cf. Tyson, 1993) which occurs at 5986 ft. This peak is considered to be representative of a relatively low-energy. The subsequent palynofacies event reflects a sudden sea-level fall from 5986 to 5979 ft. According to the increasing size of phytoclasts, decreasing count of miospores and rising abundance of black phytoclasts, the Sb 6 is probably located at 5979 ft with decline in relative abundance of the brown wood debris fraction from 57% (5986 ft) to 36% (5979 ft), the equi-dimensional black particles to lath-shaped ratio turned from 0.5 to 1.9 while the brown to black wood ratio decreased dramatically from 2 to 0.7. The dominance of phytoclasts over AOM throughout this sequence indicates oxic depositional conditions. The Genetic Sequence F, bounded by the Sb 6 and the Sb 7, is distinguished by a short-lived marine invasion which was followed by a relatively long regressive phase. The 5844 ft horizon can be interpreted as a flooding surface where the equi-dimensional black particles to lath-shaped ratio is the smallest value (0.9) within the genetic sequence F, the total count of miospores is 288, the black wood percentage is around 21% (minima) while the brown wood percentage is 56% (maxima). The interval above the 5844 ft horizon, shows generalized regressive characteristics, for example, increasing percentage of black phytoclasts from 21 to 73%. While the brown phytoclasts percentage declines in a reverse manner down to 21% at 5808 ft. (the Sb 7). The dominance of phytoclasts over AOM throughout this sequence indicates oxic depositional conditions. In contrary, in the sample representing the depth of 5796 ft, the supremacy of AOM with approximately 96% throughout the palynofacies indicates dyoxic to anoxic depositional conditions. 5. Discussion 5.1. Black wood interpretations based on variable depositional settings Two different interpretations of the black wood have been encountered in the previous palynological literature. These interpretations depend essentially upon the setting which the concerned section lies within. The first scenario, for the proximal to distal marine basin, the brown to black ratio decreases far shore in addition to the decreasing of the large equi-dimensional black to small lath-shaped black wood (Götz et al., 2005, 2008; Habib, 1982; Summerhayes, 1987; Tyson, 1989). Tyson (1993) demonstrates that the deep marine sediments encompass kerogen assemblages usually have been dominated by small size (<20 mm), equi-dimensional, opaque particles particularly throughout the transgressive periods representing low terrestrial organic matter input. These oxidized woody material is characteristic to continental slope facies as well. Summerhayes (1987) and recently Götz et al. (2005) show that in marine deposits, there is frequently an offshore increase in the ratio of opaques to total phytoclasts. Also, this is established in a general offshore increase in the ratio of opaques to well-preserved translucent woody material (Tyson, 1989, 1993). In proximal settings, the opaque material is diluted by the overall greater supply of fresh phytoclasts (Tyson, 1993). The second scenario, for the deltaic basin, distributary channels, a shoreface, tidal channels with relatively coarse-grained clastic facies, the brown to black wood ratio decreases as the content of the sand increases. Opaque phytoclast material is mainly derived from the oxidation of translucent woody material during a prolonged transport or a post-depositional alteration. There is an understandable association between elevated opaque phytoclasts

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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percentages and the comparatively coarse-grained, high energy, organic-poor facies (Tyson, 1993). This relationship is regularly ascribed to the opaque phytoclast’s minimal buoyancy and its resultant hydrodynamic equivalence to sand-sized clastics (Tyson, 1993). “The opaques material (inertinite) is certainly more dense than translucent material (vitrinite)” (Tyson, 1993, page 163, section 4). Tyson (1993) indicates a plenty of non charcoal opaque phytoclasts in the deltaic facies. This opaque material possibly depicts the in situ post-depostional bio-oxidation of wood fragments during seasonal oscillations in water table conditions. Moreover, that oxidation occurs in the littoral deposits of tidally fluctuating water tables (Tyson, 1993). “The correlation with sandy sediments is at least partially a reflection of their higher permeability and the increased potential for in situ oxidation” (Tyson, 1993, page 164, section 4). In the present study, the recorded palynofacies events are characterized by a small count of the palynomorphs (mainly spores and pollen grains with no dinoflagellate cysts), a high percentage of large-sized phytoclasts and increasing percentage of sand. Therefore, the second scenario, illustrating general regressive phase (proximal) interrupted with short-lived marine incursions, is supported in our case. 5.2. Palynofacies and the sequence stratigraphic interpretation Analyses of sedimentary organic matter have been employed as tools to aid sequence stratigraphic interpretations (Prauss, 1993; Steffen and Gorin, 1993). Moreover, the term “sequence palynology” was introduced for the first time by Prauss (1993). The interpretation of the palynofacies response to relative sea-level change forms the foundations of the sequence palynology. A Genetic sequence stratigraphic interpretation is based essentially upon the recognition of the key bounding surfaces to each individual sequence. In the present study, each palynological residue represents a single discrete horizon which may be compared only with other spot samples directly above or beneath that level. The candidate stratigraphic surfaces bounding sequences may be detected based upon palynofacies signatures’ fluctuations. Sequence boundaries related to relative sea-level minima and increased sand content horizons display the highest abundances of large equi-dimensional black wood particles and the lowest brown to black wood ratio (Boulter and Riddick, 1986; Bustin, 1988; Whitaker, 1984). In the present study, the sequence boundaries are integrally positioned according to different co-occurring palynofacies events including the large to very large size of the recorded phytoclasts in addition to the lowest counts of the recovered palynomorphs. The large-sized phytoclasts are hydrodynamically equivalent to medium to fine sand particles. Owing to the hydrodynamic equivalence, the palynomorphs are strongly associated with the silt and clay particles and not with the larger sand particles. Thus, the lowest amounts of palynomorphs are greatly linked with the increasing of the sand content. 6. Paleoenvironment Herein, we will discuss the paleoenvironment in terms of salinity and water column oxygenation. From all the obtained data, a deltaic, distributary channels or tidal channels with relatively coarse-grained, high-energy dominated depositional environment could be interpreted for such collective palynofacies signals (Boulter and Riddick, 1986; Bustin, 1988; Götz et al., 2005; Tyson, 1993, 1995; Whitaker, 1984). The salinity for all the studied Bahariya section could be easily expected to be highly reduced due to rarity of dinoflagellate cysts in some samples and the complete absence of them in most samples

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beside the large phytoclasts size which supports the vicinity to fluvial source or its mouth in the basin. For the water oxygenation regime, most of the investigated section is supposed tohave been deposited in oxic conditions, except the intervals from 6600 to 6570 ft (basal part of the studied section) and 5790e5796 ft (the topmost core sample, the uppermost part of the studied section) which represent relatively dyoxic to anoxic depositional conditions. Anoxic conditions are suggested in the topmost part of the section, by the dominance of AOM. 7. Conclusions This study demonstrates that a detailed palynofacies investigation can help in the construction of a sequence stratigraphic framework for a paralic sedimentary succession, where the dinoflagellate cysts and sporomorphs are rare to absent in the recovered organic residues. Four miospore zones have been informally identified for the lower Cenomanian. The palynofacies study of the Bahariya section demonstrates a predominantly regressive phase, mostly deposited in oxic conditions, characterized by deltaic, distributary or tidal channels, interrupted by short-lived marine incursions. For the Bahariya section, it has been possible to recognize six discrete palynofacies based genetic sequences for the Cenomanian, informally designated as Genetic Stratigraphic Sequences A through F. The placement of the bounding surfaces of these sequences have been suggested by the palynofacies interpretation. This is only a leading independent trial opens the door for further and extensive studies where well covered interval with relatively high resolution sampling interval is available. The results may be corroborated by further integrated studies of the Bahariya sequence based on the comparison of detailed sedimentological and palynofacies aspects. Such comparisons enhance the validity of the Bahariya sequence palynological approach to sequence stratigraphic interpretation. Paleoenvironmentally, a deltaic, distributary channels or tidal channels with relatively coarse-grained, high-energy dominated depositional environment could be interpreted for such collective palynofacies signals. The estimated salinity for the studied Bahariya section is expected to be highly reduced. The depositional oxygenation condition is supposed to have been predominantly oxic, except the intervals from 6600 to 6570 ft and from 5790 to 5796 ft which represent relatively dyoxic to anoxic conditions. Acknowledgements We are indebted to the authorities of the Egyptian General Petroleum Corporation EGPC, for providing the samples for this study. Two anonymous reviewers are gratefully acknowledged for their helpful corrections, valuable comments and suggestions on an earlier version of this manuscript which improved its subject matter. Grateful thanks is due to Dr. Amir M. Hassan, Geology Department, Faculty of Science, Cairo University for critical reading and revising the pre-final manuscript of the present work. Also, The authors gratefully acknowledge the supporting of the location map by Dr. Mohamed K. Zobaa, Department of Geology, Faculty of Science, Benha University, Egypt. References Abd El Shafy, E., Abd El Moneim, A. Bakr, 1991. Palynostratigraphy and Paleoenvironment of Abu Roash formation in Abu Gharadig basin, Western Desert, Egypt. Bulletin Faculty of Science Zagazig University 13 (1), 306e335. Aboul Ela, N.M., Mahrous, H.A.R., 1992. Albian-Cenomanian miospores from the subsurface of the north Western Desert, Egypt. Neues Jahrbuch für Geologie und Paläontologie 10, 595e613. Aboul Ela, N.M., Shaw, D., Ragab, S.E., 1996. Palynostratigraphy of the Bahariya Formation in the subsurface of the Salam Oil Field, north Western Desert, Egypt. Proceedings of the 13th Petroleum Conference, E.G.P.C. 1, 381e400.

Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004

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Please cite this article in press as: Tahoun, S.S., Mohamed, O., Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt, Cretaceous Research (2013), http://dx.doi.org/10.1016/ j.cretres.2013.06.004