Seismic interpretation and hydrocarbon assessment of the post-rift Cenomanian Bahariya reservoir, Beni Suef Basin, Egypt

The geophysical interpretations for a collection of high-quality 2D seismic data plus well logging data for six drilled wells exposed the structural controls and the hydrocarbon potentiality of the Beni Suef Basin, north Western Desert, Egypt. The created structural map on the top of the Early Cenomanian Bahariya Formation (the significant oil reserve in north Western Desert), exhibits a series of NW–SE normal faults as well as a master ENE strike-slip major fault. The extensional faults formed horst structures while the strike-slip fault created a negative flower structure in the central part of the study area. These structural features represent the main petroleum traps within Beni Suef Basin. The reservoir quality was assessed through the petrophysical analysis of the Bahariya Formation in wells; Azhar A2, Azhar E-2X, Azhar E3, Yusif-4X, Fayoum-2X and Beni Suef W-1X. This analysis proved that the Lower Bahariya unit has better reservoir quality than the Upper Bahariya reservoir, with shale volume less than 10%; effective porosity between 10 and 20%, water saturation between 25 and 75% and total pay thickness between 11 and 88 ft. The obtained results encourage drilling more exploratory wells in Beni Suef Basin to enhance the productivity from the sandstones of the Lower Bahariya reservoir.


Introduction
Rift basins are defined as prolific hydrocarbonaceous provinces with huge amounts of recoverable oil reserves (Lambiase and Morley 1999;Morley 1999;Withjack et al. 2002). Although structural evolution significantly affects the distribution of petroleum within rifts, the basin-fill as well as the stratigraphic characteristics define whether a basin is hydrocarbonaceous or not and provide the sources, reservoirs, and seals-rock units in the rift basin (Lambiase and Morley 1999;Withjack et al. 2002;Martins-Neto and Catuneanu 2010;Shehata et al. 2018Shehata et al. , 2019Shehata et al. , 2021aKassem et al. 2021Kassem et al. , 2022. The integration of different types of datasets such as well logging, seismic and core analysis in addition to petrophysical and geophysical interpretations will enable to trace and evaluate the reservoir rock units in addition to track their spatial distribution (Abdel-Fattah and Slatt 2013;Piggott and Abdel Fattah 2014;Dolson 2016;Radwan et al. 2021;Shehata et al. 2021b).
The Beni Suef Basin is NW rift basin developed in the Early Cretaceous, occurring at the NE corner of the African Plate with significant hydrocarbon potential that is put in the future plan of exploration in the north central Egypt. The basin is contiguous with known basins in the Nile Valley and North-western Desert (e.g., Shushan, Gindi, Asyut, Abu Gharadig Basin, Fig. 1a). Geographically, Beni Suef basin is divided into two main provinces (Fig. 1b) the western one which is called West of Nile (WON, which is our study area, Fig. 1c) and eastern one which is called East of Nile (EON).
In General, the north Western Desert basins showed different types of hydrocarbon entrapments, which are almost related to the latest Cretaceous tectonics of the Syrian-arc (Abu El Naga 1984;Moustafa 2008;Sakran et al. 2019). The resultant traps are three-and four-way closures, trap door, block faults trending NE-SW and NW-SE and horsts (Sultan and Abd El Halim 1988;Dolson et al. 2001;Salem and Sehim 2017;Sakran et al. 2019). The sandstone intervals in the Cretaceous sediments which are laterally-distributed in the majority of the North Western Desert basins, including the studied basin, formed the main reservoirs (Richardson et al. 1998;Nabawy et al. 2021;Sarhan 2017a;Sarhan et al. 2017a, b;Basal et al. 2019;Sarhan 2019;Elhossainy et al. 2021;Sarhan 2021a, b). These sandstones are encountered in Early Cretaceous Alam El-Bueib and Kharita formations, and Late Cretaceous Bahariya and Abu Roash formations.
The study area is situated in the western sector of Beni Suef basin (WON) between 31° 30′ & 29° 50′ E and between 29° 25′ & 28° 31′ N ( Fig. 1) and included numerous oil fields such as Azhar, Fayoum, Beni Suef and Yusif (Fig. 1b, c). This study aimed to evaluate the hydrocarbon potential of sandstone intervals on Bahariya reservoirs in the Western Province of Beni Suef basin (WON) based on the overall integration of seismic and well-logging datasets from Six wells, that is, Azhar A2, Azhar E-2X, Azhar E3, Yusif-4X, Beni Suef W-1X and Fayoum-2X. The objectives include the seismic facies interpretations, structural mapping and petrophysical assessment of the entirely different sandstone intervals of the Bahariya Formation in these wells.

Geologic setting
The Beni Suef Basin is one of the intercontinental rift basins that formed in the Early Cretaceous and due to the breakup of Western Gondwana and South and Equatorial Atlantic Oceans-opening (Moustafa 2008;Shehata et al. 2018, Bosworth et al. 2008Sakran et al. 2019). Accordingly, activerifting and the clockwise' rotation of the north-central Africa plate is happened (Guiraud and Maurin 1992;Guiraud et al. 2005). This basin is located just in the beginning of the unstable shelf of Egypt (Said 1962) within the Nile Valley ( Fig. 1). Tectonically, the basin is subjected to two major tectonic phases; the syn-rift extensional one in the Early Cretaceous and the post-rift one on the latest Cretaceous which is affected by the Santonian Syrian-arc compressional regime (Salem and Sehim 2017;Shehata et al. 2018Shehata et al. , 2020Sakran et al. 2019).
The Beni Suef basin has a relatively thick lithostratigraphic succession that aged Cretaceous-Paleogene (Fig. 2a), starting with the deposition with thick fluvial siliciclastic that belongs to the Early Cretaceous Kharita Formation, then siliciclastic-dominated sediments belong to Cenomanian Bahariya Formation, followed by intercalations of clastic/carbonate units of Cenomanian-Santonian Abu Roash Formation and turned upward to carbonates and chalk of Campanian-Maastrichtian Khoman Formation (Zahran et al. 2011;Shehata et al. 2018Shehata et al. , 2019Shehata et al. , 2020. The Cenomanian Bahariya Formation (Said 1962) overlies un-conformably the fluviatile sediments of Kharita Formation and is conformably superimposed by the Abu Roash Formation of marine nature. Lithologically, it is composed  of sandstone/siltstone interbeds with some limestone and shale intercalations (Fig. 2b). The lower part of the formation is composed of sand-dominated units, displaying repetition patterns of blocky gamma-ray (Fig. 2b). On the other hand, the upper parts showing remarkable increase in the gamma ray/neutron porosity readings, which reflect the increase in upward mud-content (Fig. 2b). In the studied area, Bahariya Formation' thickness ranges from 708 to 801 feet with relative thickening toward the Southeastern parts from the NE, NW and SW parts (Fig. 2c). Depositionally, the Bahariya Formation is developed initially in Fluviatile-deltaic environments, and gradually changed to shallow marine setting up-section (Said 1990;Mahmoud and Deaf 2007;El-Soughier et al. 2014;Nabawy et al. 2021).

Data and methodology
Thirty seismic profiles (SEGY) in depth domain within the western province of Beni Suef Basin (Fig. 1c) were carefully examined and detailed interpreted. These seismic profiles comprising fifteen inline trending NE-SW profiles and fifteen xline trending NW-SE profiles and are in good quality which satisfying our interpretations. It covers an area around 300 km 2 and encompasses four main productive fields in the basin namely Azhar, Beni Suef, Yusif and Fayoum oil fields (Fig. 1b). Sex deep drilled wells are scattered in the study area and intersecting and tying with the seismic profiles ( Fig. 1c). Azhar oil field comprises three wells namely: Azhar A2, Azhar E-2X and Azhar E3, Yusif field contain Yusif-4X well, Fayoum field has Fayoum-2X well and Beni Suef W-1X well is encountered in Beni Suef oil field.
The applied procedures for assessing the Bahariya sandstones in Beni Suef Basin started with the seismic interpretation stage. In this step, the seismic horizon corresponding to the top of the Bahariya Formation was picked and mapped with the related faults over the grid of the available seismic lines.
The next step was the qualitative examination for the well-log data of the accessible wells in order to delineate the hydrocarbon-bearing zones located within the Bahariya Formation. This was followed by the quantitative assessment for the petrophysical parameters of the promising zones by using Techlog software. The investigated parameters comprise; effective porosity (ΦE), total porosity (ΦT), water saturation (Sw) and shale volume (V Sh ). The parameters of V Sh , ΦT and ΦE were calculated using formulae of Asquith and Gibson's (1982) as follow: where V Sh is the shale volume; GR, is the gamma ray reading value; GR min , is the lowest gamma ray value; and GR max , is the maximum gamma ray value.
where ΦT is the total porosity, ΦN is the neutron porosity, and ΦD is the density porosity.
where ΦE is the effective porosity, ΦT is the total porosity, and V Sh is the shale volume. (1) However, the water saturation for the reservoir intervals was computed using Archie model (Archie 1942) since the sandstones which encompassing these zones in Bahariya Formation exhibit low shale content (V Sh < 10%) as follow: where S w is the water saturation; Ф is the porosity; R t is the deep resistivity; R w is the connate water resistivity.

Seismic interpretation
The Bahariya Formation is displaying wedge/sheet-like geometry and characterized by variable reflectors' amplitude with low/moderate continuity and chaotic/sub-parallel configuration (Figs. 3, 4, 5, 6 and 7).  (Figs. 3 and 4). Interaction between these faults resulted in the development of horst structures in the northwestern and central parts of the study area which affecting the Bahariya and Abu Roash formations (Figs. 3 and 4). The central horst is bounded by F1 and F2 that throwing SE and NE (Fig. 3) while the northwestern horst is bounded by F5 and F6 and become more narrower (Fig. 4). On the other hand, the interpretation of the NW-SE xline profiles showed the presence of master ENE-oriented strike-slip major fault (FS) in the central part of the study area (Figs. 5, 6 and 7). ENE Strike-slip fault is associated with some splays of normal faults (F1, F3, F7 and F8) and other segments of strike faulting (S1), which is forming a negative flower structure (Figs. 5, 6 and 7).
Normal NW faults and plunging/doubly plunging NE folds are the main prevailing structures in the depth structural map (Fig. 8). The ENE strike-slip fault is segmented to two right-stepped strike fault' segments which form a structural-low area called releasing bend in between (Fig. 8).
The releasing bend is associated with some WNW-trending normal faults with basin-ward throw (Fig. 8). Most of the normal faults are formed in the Early Cretaceous while the strike-slip and their associated structures are developed in the latest Cretaceous.
The Normal faulting and the strike-slip and their associated structures are responsible for the development of different entrapment types and closures in the basin. NW-SE horst closure is dominant in Yusif by (bounded F9 and F10) and Fayoum oil field (bounded F5 and F6), Trap-door block is encountered in Beni Suef oil field (FS and F11) and Azhar oil-field is characterized by the existence of three-way dip closure that formed due to the effect of the strike-slip major fault, its segments (S1 and S2) and the normal faults F1, F4 and F8 (Fig. 9).

Well logging interpretation
The Bahariya Formation sandstones in the investigated wells were detailed in the mud log' report ( Fig. 10) as; white/off-white, fine to very fine-grained, sub-rounded to sub-angular, moderately sorted, moderately hard to friable with silty cement, with traces of pyrite and kaolinite. All of the previous characters reveal the relatively good textural maturity of the Bahariya sandstones. Furthermore, these sandstones show uniform oil stain and occasionally spotted oil stain with pale yellow fluorescence with no stream-cut.
The visual investigation of the well-log data corresponding to the Bahariya Formation in well Azhar A2 revealed that the oil reservoir intervals vary between 7475 and 7675 ft depth (Fig. 11). In well Azhar E-2X, the oil-bearing intervals locate between 8050 and 8275 ft (Fig. 12) and in well Azhar E3, the best intervals were found among depths 7775 and 7850 ft (Fig. 13). However, the potential zones in Beni Suef W-1X well are varying in depths; 7650-7925 ft (Fig. 14). The oil pay zones in Fayoum-2X well are between depths 6950 and 7375 ft (Fig. 15). Furthermore, in well Yusif-4X, the oil-bearing intervals delineated among depths 7625 and 7750 ft (Fig. 16).
The superiority of the investigated intervals as possible oil reservoirs was judged based on several characteristics encompasses; low shale content, as evidenced by low gamma-ray and high deep-resistivity curves, the density curve is frequently to the left of the neutron curve (indicating the sandstone matrix), and the neutron curve's high porosity values. The total net pay thickness for the interpreted reservoir intervals in Azhar field reaches 43ft in Azhar A2 well, 26 ft in Azhar E3 and only 14ft in Azhar E-2X. However, the total thickness of the pay zones in Beni Suef W-1X well reaches 32 ft and only 11 ft in Yusif-4X, whereas the maximum net pay thickness was recorded in Fayoum-2X well with 88 ft thick.
The petrophysical calculations for the promising intervals within the Bahariya Formation in the studied wells show that the shale volume is usually lower than 10% and the effective porosity ranges between 10 and 20%. The calculated water saturation varies between 50 and 60% in Azhar A2 well; however in Azhar E-2X well it ranges between 25 and 60% and differs among 40 and 70% in Azhar E3 well. But, the computed water saturation of the pay zones in Beni Suef W-1X and Fayoum-2X wells fluctuates between 50 and 75%. While in Yusif-4X well, the water saturation is mostly around 50%.
The Pickett cross-plots (Pickett 1972) for the studied zones (Figs. 11,12,13,14,15 and 16) which demonstrate the relationship between deep resistivity (x-axis) and effective porosity (y-axis) in logarithmic scales were created. The constructed plots exhibit that the popular of the points describing the oil zones in the inspected wells are clustered above the Sw = 25% line and below the Sw = 75% line, reflecting the good potential of these intervals and confirming the performed petrophysical calculations.

Discussion
The Upper Cretaceous deposition in the northern Western Desert encompasses the studied basin reflects an actual evolution of a continuous transgressive-period (Kerdany and Cherif 1990;Guiraud and Bosworth 1999;Guiraud et al. 2005;Mahmoud et al. 2017;Shehata et al. 2019Shehata et al. , 2020. This has been noticed from the deposition of the Cenomanian Bahariya Formation clastics in fluvio-deltaic to shallow marine nature, that upward-grade into deltaic to marine setting of alternating limestone and fine clastics in the Abu Roash Formation (Hantar 1990;Guiraud et al. 2005;Sarhan 2017b;Shehata et al. 2020Shehata et al. , 2021b. By the Early Cenomanian, a significant land subsidence associated with wide marine invasion started to push its way southwards over the entire northern territories of Egypt, including Beni Suef Basin, and redistributed the Lower Cretaceous fluvial coarse-grained Kharita Formation sandstones, yielded deposition of thick mixed siliciclastic/carbonate succession of Bahariya Formation (Fig. 17) (Said 1990;Guiraud et al. 2005;Mahmoud et al. 2017;Shehata et al. 2018Shehata et al. , 2019Shehata et al. , 2020. Accordingly, the lower Bahariya units commences a numerous and multi-levels of sandstones with considerable total thickness of oil reservoir intervals ranged from 11 to 88 ft and resemble a good reservoir and pay zones with effective porosity varies from 10 to 20% and water saturation fluctuates between 25 and 75%. While, the upper Bahariya sediments have only few sandstone-streaks which considered as nonpromising pay zones. Moreover, the lesser volume of the shale content in the lower Bahariya sediments than the upper unit, raises the quality as well as the sandstone pay zones in the lower unit (Fig. 18).

• The Cenomanian Bahariya Formation within Beni Suef
Basin exhibiting wedge/sheet-like geometry intersected with NW-SE-oriented normal faults in addition to a master ENE-oriented strike-slip major fault.  • The oil pays intervals in the studied wells located within the lower Bahariya Formation with total thickness varies within 11-88 ft; effective porosity within 10-20%; shale volume less than 10%; and water saturation ranges between 25 and 75%. • The upper Bahariya Formation in Beni Suef Basin displays limited sandstone-streaks which considered as less efficient pay zones.
• As a recommendation, future exploration in Beni Suef Basin should target the lower Bahariya sandstones rather than the upper Bahariya sandy intervals.

Declarations
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