1 Introduction

Seismic interpretation and sequence stratigraphic analysis are essential tools for unraveling the geological framework and evaluating reservoir potential in sedimentary basins (Catuneanu et al. 2011; Abdel-Fatta and Slatt, 2013; El-Fawal et al. 2016; Sarhan 2019; Sarhan and Safa 2019; Fadul et al. 2020; Elhossainy et al. 2021a, b; Sarhan 2021a; Shalaby and Sarhan 2021; Shehata and Sarhan 2022; Imam et al. 2022; Kassem et al. 2022; Radwan et al. 2022a, b; Shehata et al. 2023a). They provide valuable insights into the basin's architecture, sedimentary processes, and reservoir characteristics, enabling effective exploration and production strategies while mitigating risks. Seismic interpretation plays a crucial role in petroleum exploration and development, aiding in subsurface structure identification, reservoir characterization, prospect evaluation, well planning, production optimization, and cost/time savings (Chopra and Marfurt 2016; Abdel-Fattah et al. 2021; Nabawy et al. 2023; Sarhan et al. 2023). On the other hand, sequence stratigraphy, a branch of stratigraphy that investigates genetically linked rock units separated by unconformities and their correlative surfaces, offers a robust framework for studying sedimentary sequences (Vail et al. 1977a, b; Van Wagoner et al. 1988; Embry 2009; Catuneanu et al. 2011).

The significance of sequence stratigraphy in the exploration and development of hydrocarbon resources cannot be overstated. It provides valuable insights into the geological history and depositional processes of sedimentary basins, facilitating the identification of potential reservoirs. The pioneering work of Exxon Corporation in introducing the sequence stratigraphy model revolutionized the field (Embry 2009; Abdel-Fattah et al. 2022; Shehata et al. 2021a, b, 2023a). This model, widely accepted and adopted, defines a sequence as a succession of three distinct sedimentary units that develop sequentially: a regressive (progradational) deposit, a transgressive (retrogradational) deposit, and a subsequent regressive (progradational) deposit. These units are separated by subaerial unconformities and their corresponding correlative surfaces, marking significant gaps in the geological record and influencing the boundaries of each sequence (Galloway 1989; Embry 2009). Within the Exxon model of sequence stratigraphy, each depositional sequence is further divided into three systems tracts: the lowstand systems tract (LST), the transgressive systems tract (TST), and the highstand systems tract (HST) (Hunt and Tucker 1992; Posamentier and Allen 1999; Embry 2009). These systems tracts represent distinct depositional environments and sedimentation patterns, aiding in the interpretation of sedimentary facies and the prediction of potential hydrocarbon reservoirs.

The Bahariya Formation is widely distributed all-over the Western Desert showing an unconformable relationship with the underlying Albian Kharita Formation and conformable relation with the overlying Abu Roash Formation (Said 1990; Mahmoud and Deaf 2007; El-Soughier et al. 2014; Shehata et al. 2019; Nabawy et al. 2021; Shehata and Sarhan 2022). Depositionally, the Bahariya Formation is developed initially in Fluviatile-deltaic to shallow marine environments. The Bahariya Formation in the South Umbaraka oilfields (Selkit, Khepri, and Sethos), located in the Western Desert of Egypt (Fig. 1), is a significant hydrocarbon-bearing unit with complex depositional patterns and heterogeneity (Abdelmaksoud et al. 2019; Abdelwahhab and Raef 2020; Sarhan et al. 2017; Sarhan 2021b; Elatrash et al. 2021; Abdelwahhab et al. 2022; Radwan et al. 2022a, b). The main objective of this study is to conduct a comprehensive analysis of the Bahariya Formation in the South Umbaraka oilfields, focusing on the integration of seismic interpretation and sequence stratigraphic analysis as well as the petrophysical evaluation. By integrating these techniques with seismic and well logs studies, the study aims to unravel the subsurface geological framework, sequence stratigraphic architecture, and petrophysical properties of the reservoir rocks. This integrated approach provides valuable insights into the reservoir distribution, connectivity, and heterogeneity within the Bahariya Formation, contributing to improved exploration and production strategies in this region. The integration of seismic interpretation and sequence stratigraphic analysis in this novel context will serve as a reference for future studies and industry professionals involved in the exploration and production of hydrocarbon resources. The outcomes of this research can help guide decision-making processes, mitigate exploration risks, and maximize hydrocarbon recovery not only in the South Umbaraka oilfields but also in similar geological settings globally. Therefore, the significance of this work lies in its contribution to the advancement of knowledge and practices in the field of reservoir characterization and exploration strategies, benefiting the global petroleum industry.

Fig. 1
figure 1

A location map displaying the geographical distribution of the accessible seismic (blue lines) and well data (green circles) within the South Umbaraka oilfields (Selkit, Khepri, and Sethos), situated in the Western Desert of Egypt

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2 Geologic setting

The stratigraphic column of the northern part of the Western Desert in Egypt provides a comprehensive record of sedimentary layers spanning from Cambrian rocks to recent deposits (Fig. 2). The thickness of the sedimentary successions exhibits variations, reaching over 10,700 m in the Abu Gharadig Basin and decreasing to around 2990 m over the Ras Qattara ridge, which serves as the northern boundary of the basin (Sestini 1984; Froidevaux 1987; Abdel-Fattah 2015). The sedimentary rock sequences in the Western Desert basins represent multiple cycles of sea transgression and regression and were subjected to significant deformation due to three distinct orogenic episodes (Metwalli and Bakr 2007). The Middle Paleozoic witnessed the Caledonian Orogeny, followed by the Hercynian Orogeny towards the end of the Paleozoic, after that the Alpine Orogeny took place during the Jurassic-Paleogene.

Fig. 2
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A stratigraphic column of the various geological formations that are present in the Western Desert region of Egypt (Abdel-Fattah 2015), providing a chronological record of the sedimentary rocks and petroleum systems in the Western Desert Basins

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During the Middle Jurassic, shallow marine deposits were prevalent, indicating the presence of a marine environment in the region at that time. However, it was during the Albian-Cenomanian period when the Bahariya Formation experienced a notable cycle of marine transgression. This cycle was characterized by a rise and subsequent fall in relative sea level, resulting in the deposition of clastic sequences (Sestini 1984; Dolson et al. 2001; Shehata et al. 2019, 2020, 2023b). The Neocomian stage marks the beginning of this cycle, where deltaic sediments were deposited in the region. These sediments were followed by near-shore-deltaic strata in the Lower Aptian and Albian stages. This indicates a transition from a more terrestrial environment to a near-shore marine environment during this period.

The marine transgression reached its peak during the Upper and Middle Aptian stages. It was during this time that Alamein carbonates were buried in a restricted marine/lagoonal environment (Dolson et al. 2001). These carbonates serve as evidence of the marine conditions that prevailed in the region during that period. Following the peak of the transgression, there was a transition back to continental sediments in the Upper Albian to Lower Cenomanian stages. These continental sediments represent a return to terrestrial conditions and mark the end of the marine transgression cycle within the Bahariya Formation (Sestini 1984; Dolson et al. 2001). The Bahariya Formation itself consists of white to pink, intermediate to coarse-grained quartz sandstones, which are interspersed with thin shale interbeds and carbonate sediments. These sediments make up the upper portion of the clastic sequence within the formation.

Beneath the Bahariya Formation lies the Abu Roash Formation, which is composed of marine carbonates and shales from the Turonian-Coniacian stages (Abdel-Fattah et al. 2015; Elhossainy et al. 2021a, b; Shehata et al. 2019, 2021a). This formation exhibits a fining-upward depositional phase, indicating a shift from coarser sediments to finer ones over time. The depositional environment of the Bahariya Formation changed over time. Initially, it was deposited in fluviatile (river-related) environments during the early to mid-Cenomanian stages. However, as the marine transgression persisted, the depositional environment transitioned to estuarine conditions. This indicates the influence of changing sea levels and the impact of flooding on the deposition within the formation. The Bahariya Formation is conformably underlain by the Abu Roash Formation, meaning there is a continuous and smooth transition between the two formations. However, it unconformably overlies the Albian Kharita Formation, indicating a significant time gap or erosion event between the two formations (Abdel-Fattah et al. 2018; Shehata et al. 2018).

The structural framework of Egypt can be best understood through the lens of plate tectonics (Meshref 1990; Hussein and Abd-Allah 2001; Hassan et al. 2023). The region's tectonic activity has led to the formation of distinctive features, including tectonic highs and reactivated Paleozoic faults, which have played a significant role in shaping Egypt's geology. The tectonic highs in the region, trending in an east to northeast direction, intersect with reactivated Paleozoic faults that run in a northwest-southeast orientation (Metwalli and Bakr 2007). The tectonic movements resulted in a complex pattern of graben-depocenters, which have evolved over the unstable shelf area. Notably, the Qattara-Abu Gharadiq basins in the southwest of Egypt stand out as prominent features resulting from these tectonic processes. The warping of basement fault blocks, occasionally reactivated, has had a profound impact on the region's geological features. This warping led to significant erosion and the formation of alternating sequences of shale and limestone. These sequences of differing rock types reflect the dynamic nature of the subsurface, influenced by the interaction between tectonic forces and the underlying structural framework. Local unconformities, overlapping wedges, and thickening of sedimentary layers towards nearby subsiding areas have been observed throughout the region. These geological phenomena are a result of the complex interplay between tectonic activity and the sedimentary processes occurring in response to changing environmental conditions. The presence of unconformities indicates periods of erosion or non-deposition, while overlapping wedges and thickened sedimentary layers suggest the accumulation of sediments in areas experiencing subsidence (Hussein and Abd-Allah 2001).

During the Maastrichtian-Paleocene period, extensive deformation took place, leaving a lasting imprint on the geological structures of Egypt (Metwalli and Bakr 2007; Sakran et al. 2019). Many of the structures encountered in drilling activities during this period consist of faulted anticlines. These structures serve as indicators of the intense tectonic activity that occurred during this time.

The South Umbarka area has undergone a series of significant tectonic events across different depths, ranging from deeper to shallower horizons (Meshref 1990; Abdel-Fattah et al. 2018). These events encompass various types of movements, including left and right lateral shears, as well as minor compressional movements. The complex tectonic nature of this region presents considerable challenges when it comes to identifying suitable drilling locations for both shallow and deeper targets. The Bahariya Formation, which is a key geological unit in the South Umbarka area, serves as the reservoir for both shallow and deep targets. In the case of the deeper sand reservoir, it consists of transitional sands that are interbedded with shale, siltstone, and limestone streaks (Hussein and Abd-Allah 2001). This heterogeneous composition adds to the complexity of the reservoir and necessitates a comprehensive understanding of its geological characteristics. It is important to note that the South Umbarka area primarily functions as an oil-producing region, situated within the unstable shelf area (Abdel-Fattah et al. 2018). The presence of ongoing tectonic activity contributes to the dynamic nature of the region, influencing the distribution and accumulation of hydrocarbon resources. Therefore, a thorough investigation and analysis of the geological framework are crucial to unlock the full potential of the reservoirs in this area.

3 Data and methods

In this study, we had access to thirty 2D seismic profiles and conventional well-log data from eight wells (KHEP-3, KHEP-5, KHEP-10, SELK-6, SELK-7, SETHOS-16, SETHOS-7, ETHOS-22) drilled within the South Umbaraka area. The used methodology includes a series of steps aimed at understanding the reservoir characteristics of the Bahariya Formation in the study area.

The first step in our methodology is the seismic interpretation. We carefully analyzed the seismic profiles and identified the seismic horizon corresponding to the top of the Bahariya Formation. By examining the seismic grid, we also analyzed the influencing faults in the study area. This process allowed us to create a structural map for the Bahariya Formation, providing insights into its subsurface geometry and structural features (Abdel-Fattah et al. 2023).

Next, we conducted a sequence stratigraphic analysis using the available borehole data from the eight wells. To ensure accuracy, we referred to established concepts and definitions provided by several references, including Vail (1987), Van Wagoner et al. (1988), Posamentier et al. (1988), Posamentier and Vail (1988), Vail et al. (1991), Posamentier and Weimer (1993), Posamentier and Allen (1999), Catuneanu (2006), and Catuneanu et al. (2011). This analysis allowed us to define the depositional sequences within the Bahariya Formation, providing valuable information about the sedimentary environment and the stratigraphic framework of the formation. Subsequently, the seismic interpretation process yielded a structural framework, which was then integrated with lithofacies data to create a 3D model of the Bahariya Formation within the study area. This model provides valuable insights into the variations in lithology within the structural context and their influence on the petrophysical properties of the reservoir facies.

Following the sequence stratigraphic analysis, we conducted a qualitative inspection of the wireline log data from the eight wells. The purpose of this inspection was to identify potential hydrocarbon-bearing zones within the Bahariya Formation in the South Umbaraka field. By examining the log responses, we looked for indicators such as variations in lithology, porosity, and fluid content that could suggest the presence of hydrocarbons. Once potential hydrocarbon-bearing zones were identified, we proceeded with a quantitative examination to assess the quality of these reservoirs. We calculated crucial petrophysical parameters using the well-log data. These parameters included shale content (Vsh), total porosity (ɸT), effective porosity (ɸe), water saturation (Sw), and hydrocarbon saturation (Sh). Shale content (Vsh) is an essential parameter as it indicates the presence of clay-rich rocks that can affect fluid flow within the reservoir. Total porosity (ɸT) represents the overall pore volume within the rock, while effective porosity (ɸe) refers to the interconnected and accessible pore space available for fluid flow. Water saturation (Sw) represents the fraction of pore space occupied by water, while hydrocarbon saturation (Sh) represents the fraction of pore space occupied by hydrocarbons.

To calculate these parameters, we utilized various well-log measurements, including gamma ray, density, neutron, and resistivity logs. The gamma ray log provided information on shale content, as shales typically exhibit higher gamma ray values compared to sandstones and carbonates. The density and neutron logs were used to estimate total porosity (ɸT), with the density log providing information on bulk density and the neutron log providing information on hydrogen index. The difference between these logs allows for porosity estimation. To determine effective porosity (ɸe), we applied shale correction techniques to account for the presence of clay minerals. Water saturation (Sw) was calculated using resistivity logs, as the resistivity of water is typically higher than that of hydrocarbons. Hydrocarbon saturation (Sh) was obtained by subtracting water saturation from unity.

To calculate these petrophysical parameters, we utilized specific equations. For shale volume (Vsh), we employed Asquith and Gibson's Eq. (1982) based on gamma ray logs. The equation allowed us to determine the quantity of clay in the investigated zones by considering gamma ray reading values (GR), lowest gamma ray value (GRmin), and maximum gamma ray value (GRmax).

$${\text{V}}_{{{\text{Sh}}}} = \left( {{\text{GR}} - {\text{GR}}_{{{\text{min}}}} } \right)/\left( {{\text{GR}}_{{{\text{max}}}} - {\text{GR}}_{{{\text{min}}}} } \right)$$
(1)

where Vsh is shale volume; GR is gamma ray reading value; GRmin is lowest gamma ray value; GRmax is maximum gamma ray value.

Total porosity (ɸT) was computed from neutron-density logs using a formula provided by Asquith and Gibson (1982). This formula involved adding neutron porosity (ɸN) and density porosity (ɸD) values, which were determined from the logs. Density porosity (ɸD) was derived using a formula that took into account matrix density (ρma), formation bulk density (ρb), and fluid density (ρfl).

$$\Phi_{{\text{T}}} = \left( {\Phi_{{\text{N}}} + \, \Phi_{{\text{D}}} } \right)/{2}$$
(2)

where ΦT is total porosity; ΦN is neutron porosity; ΦD is density porosity. The density porosity (ΦD) can be determined using the following formula:

$$\Phi_{{\text{D}}} = \left( {\uprho _{{{\text{ma}}}} -\uprho _{{\text{b}}} } \right)/\left( {\uprho _{{{\text{ma}}}} -\uprho _{{{\text{fl}}}} } \right)$$
(3)

where ΦD is density-derived porosity, ρma is matrix density, and ρb is formation bulk density (the log reading), ρfl represents fluid density.

To calculate effective porosity (ɸe), we applied the equation from Asquith and Gibson (1982), which involved multiplying total porosity (ɸT) by the complement of shale volume (Vsh).

$${\varphi }_{{\text{e}}} = {\varphi }_{{\text{T}}} * \left( {1 - {\text{V}}_{{{\text{Sh}}}} } \right)$$
(4)

where ϕe is effective porosity; ϕT is total porosity; Vsh is shale volume.

Water saturation (Sw) for the inspected zones was determined using the Indonesia equation proposed by Poupon and Leveaux (1971). This equation took into consideration shale volume (VSh), effective porosity (ɸe), shale resistivity (RSh), deep resistivity (Rt), resistivity of connate water (Rw), cementation exponent (m), saturation exponent (n), and tortuosity factor (a). For the Bahariya Formation in the South Umbaraka Field, we set the tortuosity factor "a" to 1.65 and the cementation exponent "m" to 1.

$$S_{w} = \left\{ {\left[ {\left( {\frac{{V_{{{\text{Sh}}}}^{{2 - V_{{{\text{Sh}}}} }} }}{{R_{{{\text{Sh}}}} }}} \right)^{\frac{1}{2}} + \left( {\frac{{\phi_{e}^{m} }}{{R_{{\text{w}}} }}} \right)^{\frac{1}{2}} } \right]^{2} R_{t} } \right\}^{{\frac{ - 1}{n}}}$$
(5)

where Sw is water saturation, VSh is shale volume, ϕe is effective porosity, RSh is shale resistivity, Rt is deep resistivity, Rw is resistivity of connate water, m is cementation exponent, n is saturation exponent, and a is tortuosity factor.

By quantitatively evaluating these petrophysical parameters, we gained insights into the reservoir quality of the hydrocarbon-bearing zones within the Bahariya Formation. This information is crucial for understanding the potential productivity and economic viability of the reservoirs in the South Umbaraka oilfields.

4 Results

4.1 Seismic interpretation

The seismic interpretations conducted for the Bahariya Formation in the South Umbaraka area have yielded valuable insights into the structural characteristics of the region, particularly in the Selkit, Khepri, and Sethos oilfields. The first task in seismic interpretation is linking seismic reflections with stratigraphy. Wells possess formation density and sonic logs, which enable creating a synthetic seismogram to compare predicted seismic responses with actual data (Fig. 3). Through the analysis of seismic data and structural interpretation, several interesting features have been identified (Figs. 4, 5, and 6). On seismic sections, the reflectors corresponding to the Bahariya Formation demonstrate variable amplitude, with a range of low to moderate continuity. These reflectors exhibit a configuration that can be described as chaotic to sub-parallel, with a wedge or sheet-like geometry (Figs. 4 and 5).

Fig. 3
figure 3

Synthetic seismogram for (Sethos-22) well and its seismic tie

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Fig. 4
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A N-S seismic profile that reveals the faulted anticline of the Bahariya Formation in the South Umbaraka area (Western Desert, Egypt)

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Fig. 5
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3D perspective of seismic lines intersecting the structure map of the Bahariya Formation showcases the positions of closures and the associated oil wells in the South Umbaraka area of the Western Desert, Egypt

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Fig. 6
figure 6

Depth structure map of the Bahariya Formation reveals fascinating structural characteristics in the South Umbaraka oilfields (Selkit, Khepri, and Sethos), located in Western Desert of Egypt. The Selkit field is situated on a 4-way dip closure, forming an east–west trending anticlinal dome. In contrast, the Khepri field occupies a faulted anticline with a west-northwest to east-southeast trend. The Sethos field, located to the south, is positioned on a 4-way dip closure, resulting in a slightly faulted anticlinal dome. A significant northwest-southeast trending fault acts as a major boundary, dividing the area into the North Block and the South Block

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In the Selkit oilfield, a 4-way dip closure was observed, forming an anticlinal dome with an east–west trend (Figs. 5 and 6). This structural feature indicates a folding pattern where the rock layers have been compressed, resulting in an upward arch. Similarly, in the Khepri oilfield, a 3-way dip closure was observed, representing a faulted anticline with a west-northwest to east-southeast trend (Figs. 5 and 6). This indicates the presence of a fault plane along the fold, suggesting movement and displacement of the rock layers. Moving southward to the Sethos oilfield, a 4-way dip closure was observed, forming an anticlinal dome that is slightly faulted by a northwest-southeast trending fault (Figs. 4 and 6). Notably, this fault acts as a major boundary dividing the area into the northern and southern blocks (Fig. 6). The formation of these folds and structural features has been discussed by previous studies conducted by Dolson et al. (2001), and Abdel-Fattah and Alrefaee (2014), which focused on tectonic events in the region. These studies indicate that the structural framework of the South Umbaraka area has been shaped by compressional movements during the Campanian–Maastrichtian to Paleogene tectonic event.

To gain a comprehensive understanding of the structural configuration, a 3D structural model was utilized to create structural cross-sections (Fig. 6). These cross-sections provide valuable information about the lateral extension, thickness variation of reservoir zones, and distribution of faults. By examining multiple cross-sections in different directions within the study area, a holistic view of the structural framework can be obtained. Interestingly, both the Upper and Lower Bahariya units exhibit similar structural characteristics. This similarity can be attributed to their limited vertical thickness, which generally does not exceed 150–200 feet. Furthermore, the entire Bahariya Formation has been influenced by tectonic movements during the Upper Cretaceous (Cenomanian) period, as described by Meshref (1990) and Hussein and Abd-Allah (2001).

The presence of the anticlinal domal structure in the Selkit and Sethos oilfields suggests favorable conditions for oil accumulation (Figs. 5 and 6). The folding creates traps within the reservoir where hydrocarbons can accumulate. On the other hand, the fault plane observed in the Khepri field serves as an effective sealing mechanism. This fault plane acts as a barrier, preventing the migration of hydrocarbons and enabling the accumulation of oil below the fault plane. This has been demonstrated through the drilling of wells such as Khepri-3 and Khepri-5, strategically positioned to target the oil accumulation beneath the fault plane.

To further support the structural configuration and identify potential hydrocarbon traps within the study area, seismic lines in the north–south direction has been examined (Figs. 4 and 5). This seismic line provides additional evidence of the structural characteristics and aids in the identification of potential reservoirs.

4.2 Sequence stratigraphic analysis

The Cenomanian Bahariya Formation exhibits a depositional succession consisting of mixed clastic and carbonate units, which display variations in the logging datasets (Figs. 7, 8 and 9).

Fig. 7
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Sequence stratigraphic subdivisions of the Cenomanian succession (BAH.SQ-1) of the lower unit of Bahariya Formation in well SELK-7

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Fig. 8
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Sequence stratigraphic subdivisions of the Cenomanian succession (BAH.SQ-2) of the lower unit of Bahariya Formation in well SELK-7

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Fig. 9
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Sequence stratigraphic framework of the Cenomanian Bahariya Formation succession (BAH.SQ-1 & BAH.SQ-1) through well log correlation in the South Umbaraka area of the Western Desert, Egypt

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In the study area, the Bahariya Formation is further subdivided into two distinct depositional sequences: the Lower Bahariya sequence (BAH.SQ-1) and the Upper Bahariya sequence (BAH.SQ-2) (Figs. 7, 8 and 9). Each sequence exhibits unique characteristics in terms of wireline-log responses, and lithofacies. These subdivisions allow for a more detailed analysis and understanding of the Bahariya Formation. The Lower Bahariya sequence (BAH.SQ-1) and Upper Bahariya sequence (BAH.SQ-2) can be distinguished based on their respective wireline-log characteristics, seismic signatures, and lithofacies variations. These distinctive features provide valuable insights into the depositional environment and the geological processes that influenced the formation of the Bahariya Formation. These observations aid in the interpretation of the depositional environment, sedimentary facies distribution, and the identification of potential hydrocarbon reservoirs.

4.2.1 Depositional sequence (BAH.SQ-1)

The lowermost depositional sequence within the Cenomanian Bahariya Formation is referred to as BAH.SQ-1, which corresponds to the Lower Bahariya sediments in the study area (Fig. 7). BAH.SQ-1 primarily consists of mixed siliciclastic-dominated units with some carbonate components, exhibiting a thickness ranging between 400 and 850 ft (Figs. 7 and 9).

The base of BAH.SQ-1 is marked by an unconformity called as sequence boundary SB-1. This boundary is characterized by a sudden increase in gamma ray and neutron porosity values, accompanied by a sharp decrease in bulk density log readings (Figs. 7 and 9). The logging data reveals a lithological transition from fluviatile siliciclastic deposits of the Albian Kharita Formation to fluviomarine/shallow marine sediments of the Cenomanian Bahariya Formation. As for the upper sequence boundary (SB-2), the well-log data and lithological facies variations indicate a conformable transitional surface. This surface represents the transition from sandstone and siltstone deposits, likely associated with shallow marine conditions during the maximum sea-level drop at the end of BAH.SQ-1, to shale and limestones, indicating a relatively deeper marine shelf environment accompanying the initiation of BAH.SQ-2 (Figs. 7 and 9).

On the well-logs, the transgressive surface ts-1 can be identified by the changes in log-facies from low gamma ray sand-dominated sediments of the LST-1 (lowstand) to the high gamma ray readings of the overlying TST-1 (transgressive systems tract) dominated by shale (Fig. 6). Similarly, the maximum flooding surface mfs-1 can be identified on the well-logs as a transition from low gamma ray limestones at the end of TST-1 to higher gamma ray shale and siltstone units of the highstand systems tract HST-1 (Fig. 7).

LST-1 primarily consists of stacked sandstone parasequences with a relatively thin nature. These parasequences display low gamma ray readings and a box-car ray pattern, with multiple stacking of approximately 50 ft thick fining-upward units (Fig. 7). Based on available datasets, sedimentological characteristics, and well-log data, it is inferred that these parasequences were initially developed within a possible fluvial-deltaic setting and subsequently evolved under shallow marine conditions.

TST-1, on the other hand, represents a relatively thick unit characterized by fining and deepening upward parasequences composed of mixed siliciclastic and carbonate materials (Fig. 7). The upper portions of TST-1 show an enrichment in limestone content. This is evident from the significant increase in gamma ray and neutron porosity readings above ts-1, indicating higher mud content. The lower levels of the parasequences exhibit high gamma ray and neutron porosity readings, suggesting shallow marine and inner shelf facies. As sea levels continued to rise, the accommodation volume gradually surpassed the sediment supply (A > S), resulting in a clean-upward gamma ray response and indicating the development of deeper marine shelf limestone facies in an upward-deepening trend.

The uppermost tract within BAH.SQ-1 is known as HST-1, which comprises mixed shale, siltstone, sandstone, and some limestone units. HST-1 is characterized by prograding (shallowing upward) parasequences displaying a general coarsening upward pattern (Fig. 7). The rapid landward shift of the shoreline led to a higher deposition rate compared to the accommodation volume (S > A). Consequently, the deeper marine shelf limestone of the underlying TST-1, indicated by low gamma and neutron porosity readings (indicative of lower mud content), was prograded upward by the shallower shelfal and shallow marine facies of HST-1, as reflected by higher gamma and neutron porosity readings (indicative of higher mud content). Thin streaks of shallow marine sandstone are reported at the top parts of this tract, marking the maximum sea-level drop during BAH.SQ-1, just prior to SB-2.

4.2.2 Depositional sequence (BAH.SQ-2)

The uppermost depositional sequence within the Cenomanian sediments in the study area is referred to as BAH.SQ-2. This sequence encompasses the entire upper unit of the Bahariya Formation and is primarily composed of mixed siliciclastic/carbonate units with a thickness ranging between 180 and 200 ft (Figs. 8 and 9).

BAH.SQ-2 is overlain by the sequence boundary SB-2 mentioned earlier (Figs. 8 and 9). The upper sequence boundary (SB-3) is characterized by a conformable transitional surface, indicated by the well-log data, and facies variations. This surface marks the transition from sandstone and siltstone deposits, possibly associated with shallow marine conditions during the maximum sea-level drop at the end of BAH.SQ-2, to shale and limestones, suggesting a relatively deeper marine shelf environment accompanying the earliest marine invasion with the onset of the Abu Roash "G" Member above (Figs. 8 and 9).

On the well-logs, the maximum flooding surface (MFS-2) can be identified based on the transition from limestones with low gamma ray readings at the end of the transgressive systems tract (TST-2) to shale and siltstone units with higher gamma ray readings in the highstand systems tract (HST-2) (Fig. 8).

TST-2 is characterized by relatively thick parasequences exhibiting a fining upward (deepening upward) trend. These parasequences consist of a mixed siliciclastic/carbonate unit with an enrichment of limestone towards the upper portions (Fig. 8). In the lower levels, the parasequences display high gamma ray and neutron porosity readings, indicative of higher mud content and suggesting shallow marine and inner shelf facies. Due to the continuous rise in sea level, the accommodation volume gradually exceeded the sediment supply (A > S). This led to a clean-upward gamma ray response, indicating the development of deeper marine shelf limestone facies in an upward-deepening trend.

HST-2 comprises a mixed unit of shale, siltstone, sandstone, and limestone. It is primarily characterized by prograding (shallowing upward) parasequences (Fig. 8). The rapid landward shift of the shoreline resulted in a higher deposition rate compared to the accommodation volume (S > A). As a result, the deeper marine shelf limestone of the underlying TST-2, characterized by low gamma and neutron porosity readings (indicative of lower mud content), was prograded upward by the relatively higher gamma and neutron porosity readings (indicative of higher mud content) associated with shallower shelfal and shallow marine facies of HST-2. Thin streaks of shallow marine sandstone are reported at the top portions of this tract, marking the maximum sea-level drop during BAH.SQ-2, just prior to SB-3 (Figs. 8 and 9).

4.3 Petrophysical appraisal

The analysis of several wells, namely KHEP-3, KHEP-5, KHEP-10, SELK-6, SELK-7, SETHOS-16, and SETHOS-7, has allowed for a comprehensive evaluation of the reservoir zones within the Lower and Upper Bahariya Formation. Through qualitative and quantitative assessments, the characteristics of these reservoirs have been determined, revealing distinct differences between the Lower and Upper Bahariya Formation in terms of their quality and potential.

Upon comparing the reservoir characteristics, as illustrated in Fig. 10 and summarized in Table 1, it becomes evident that the Lower Bahariya Formation exhibits more favorable qualities than the Upper Bahariya Formation. Specifically, the Lower Bahariya Formation displays a net-pay thickness ranging from 10 to 45 ft, a shale volume between 10 and 15%, effective porosity ranging from 12 to 18%, and water saturation levels between 25 and 50%. In contrast, the Upper Bahariya Formation in the examined wells reveals a net-pay thickness varying from 5 to 15 ft, a shale volume between 15 and 25%, effective porosity ranging from 10 to 13%, and water saturation levels between 20 and 55%.

Fig. 10
figure 10

Wire-line logs with interpretation for the reservoir intervals within the Upper and Lower Bahariya Formation in SETHOS-7 well, KHEP-5 well and SELK-6 well

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Table 1 A summary table presenting the calculated petrophysical parameters for the most prospective intervals of the Lower and Upper Bahariya Formation in the wells examined within the South Umbaraka oilfields (Selkit, Khepri, and Sethos)
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These results indicate that the Lower Bahariya Formation generally possesses more desirable petrophysical characteristics compared to the Upper Bahariya Formation (Table 1). The key differentiating factors contributing to this observation can be attributed to various geological aspects and depositional processes. One of the significant factors influencing the contrasting petrophysical characteristics between the Lower and Upper Bahariya Formation is the sedimentary environment in which they were deposited. The Lower Bahariya Formation primarily consists of fluvial deposits, which typically exhibit enhanced reservoir qualities (Fig. 11). Fluvial systems are characterized by well-connected sandstone channels and point bar deposits, which offer higher porosity and permeability values, thereby facilitating improved fluid flow and storage capacity. These favorable characteristics contribute to the higher quality reservoirs observed in the Lower Bahariya Formation.

Fig. 11
figure 11

3D facies model of the Cenomanian Bahariya Formation succession based on seismic interpretation and sequence stratigraphic analysis in the South Umbaraka area (Western Desert, Egypt)

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Additionally, the provenance and sediment composition of the two formations play a crucial role in their petrophysical disparities. The Lower Bahariya Formation is believed to have originated from mature sediment sources, indicating the delivery of coarser-grained, better-sorted, and more quartz-rich sediments (Figs. 7 and 9). Such sediment properties are associated with higher reservoir quality, including improved porosity and permeability. Conversely, the Upper Bahariya Formation may have experienced a different provenance and sediment supply, resulting in the deposition of sediments with lower quality reservoir characteristics (Figs. 8, 9 and 11).

Furthermore, diagenetic processes have likely contributed to the divergent petrophysical properties between the Lower and Upper Bahariya Formation. Diagenesis refers to the physical and chemical alterations that occur in sediments after deposition. Studies suggest that the Lower Bahariya reservoirs have undergone fewer diagenetic changes compared to the Upper Bahariya Formation, leading to less cementation and compaction in the former. Consequently, the primary porosity of the Lower Bahariya reservoirs is better preserved, ultimately enhancing their overall reservoir quality.

Stratigraphic variations within the Bahariya Formation also influence the observed petrophysical differences. The formation comprises distinct lithological variations, depositional environments, and sedimentary facies. The lower sections of the Bahariya Formation, including the Lower Bahariya Formation, often consist of more extensive sandstone bodies as shown in the 3D facies model (Fig. 11). Sandstones generally exhibit superior reservoir properties compared to shale and siltstone intervals, which are more commonly found in the upper sections of the formation (Fig. 10). This lithological contrast contributes to the enhanced petrophysical characteristics observed in the Lower Bahariya Formation.

5 Discussion

Seismic interpretation of the Bahariya Formation in the South Umbaraka oilfields, specifically the Selkit, Khepri, and Sethos fields, has provided significant insights into the reservoir characteristics and structural setting of the region. These findings have important implications for understanding the hydrocarbon potential and exploration strategies in this area (Figs. 7, 8 and 9). The Sethos oifield stands out due to its distinct reservoir characteristics, including higher porosity and hydrocarbon saturation values. This can be attributed to the presence of a fault-assisted anticlinal closure that trends in a northwest-southeast direction. The faulting and folding processes that have influenced the structural setting on top of the Bahariya Formation in this area are key factors in the formation of this reservoir characteristic. The fault-assisted anticlinal closure acts as a trap, facilitating the accumulation of hydrocarbons in the Sethos oilfield.

In addition to the Sethos oifield, the Selkit and Khepri fields exhibit unique structural configurations. The Selkit oilfield is identified as a 4-way dip closure, indicating a structural formation resembling an anticlinal dome with an east–west trend. Similarly, the Khepri field is characterized as a 3-way dip closure, representing a faulted anticline with a west-northwest to east-southeast trend. These structural configurations have important implications for hydrocarbon accumulation and exploration strategies in each field. The identification of the Selkit and Sethos fields as 4-way dip closures suggests favorable conditions for oil accumulation. The folding and faulting processes associated with these closures create potential traps for hydrocarbon reservoirs. Conversely, the Khepri field, with its faulted anticline and 3-way dip closure, serves as an effective sealing mechanism. The fault plane in the Khepri field acts as a barrier that prevents hydrocarbon migration, enhancing the potential for oil accumulation beneath it. This structural characteristic has been advantageous for the exploration of attic oil, as demonstrated by successful drilling in wells such as Khepri-3 and Khepri-5.

The findings from the seismic interpretations are supported by geological studies and tectonic events in the region. Dolson et al. (2001) and Abdel-Fattah et al. (2015) have documented the tectonic events that occurred during the Late Cretaceous to Early Paleogene period, which have played a significant role in shaping the structural framework of the area. These events include compressional movements that resulted in folding, faulting, and the formation of the observed structural closures in the South Umbaraka oilfields. The seismic data and structural interpretations have provided valuable information for understanding the reservoir characteristics and exploration potential in the study area. By analyzing seismic horizons and fault patterns, a suitable structural map for the Bahariya Formation has been generated. This map allows for the identification of potential hydrocarbon traps and aids in the planning of drilling and exploration activities.

In sedimentary basins, sequence stratigraphic components from systems tracts are of significant importance in understanding hydrocarbon plays and reservoirs. The Lowstand Systems Tracts (LST) generally comprise fluviomarine and shallow marine deposits, which are known to form good reservoirs. Numerous studies have emphasized the significance of sequence stratigraphy in the determination of hydrocarbon potential (Catuneanu et al. 2011; Catuneanu 2006; Abdel-Fattah and Slatt 2013; Shehata et al. 2018, 2021a, b; Kassem et al. 2021, 2022). These studies have demonstrated that LST deposits are favorable reservoirs due to their specific sedimentary characteristics.

In the South Umbarka area, the Bahariya Formation is found to be composed of two depositional sequences, which deviates from previous studies conducted in the North Western Desert. Shehata et al. (2019) considered the formation as a single depositional sequence. This discrepancy suggests regional variations in the stratigraphic architecture and highlights the importance of conducting detailed studies in different areas.

The Lower Bahariya Formation, particularly the LST-1, is characterized by sandstone-dominated units (Figs. 7, 9 and 11), making it a favorable reservoir in the study area. Petrophysical analyses have revealed several promising reservoir properties within this unit, including a net-pay thickness ranging from 10 to 45 feet, a shale volume of 10–15%, an effective porosity of 12–18%, and a water saturation of 25–50% (Table 1). In contrast, the upper Bahariya Formation displays different reservoir characteristics, with a net-pay thickness ranging from 5 to 15 feet, a shale volume of 15–25%, an effective porosity of 10–13%, and a water saturation of 20–55% (Table 1). The sandstone pay zone within the lower Bahariya Formation, specifically the LST-1, stands out due to its thickness, which ranges from 10 to 45 feet. The presence of a higher proportion of sand deposits in this zone enhances its reservoir potential. On the other hand, the pay zone within the upper Bahariya Formation (LST-1) exhibits a relatively smaller thickness, ranging from 5 to 15 feet, indicating limited reservoir potential.

Different studies have indicated that the Lower Bahariya reservoirs in the Western Desert of Egypt generally exhibit better petrophysical characteristics compared to the Upper Bahariya Formation. The Lower Bahariya Formation is predominantly composed of fluvial deposits, which are known to have favorable reservoir characteristics. Fluvial systems often contain well-connected sandstone channels and point bar deposits, which exhibit higher porosity and permeability values, allowing for better fluid flow and storage capacity (Basheer et al. 2023). The lower Bahariya Formation is believed to have been derived from well-exposed and mature sediment sources, resulting in the delivery of coarser-grained, better-sorted, and more quartz-rich sediments. These sediment properties contribute to higher reservoir quality, including improved porosity and permeability (Yasser et al. 2021). The diagenetic alterations experienced by the reservoir rocks can significantly impact their petrophysical characteristics. Studies suggest that the Lower Bahariya reservoirs have undergone fewer diagenetic changes compared to the upper Bahariya Formation. This results in less cementation and compaction in the lower reservoirs, preserving higher primary porosity and enhancing reservoir quality (Kassab et al. 2017). The Bahariya Formation exhibits stratigraphic variations in lithology, depositional environments, and sedimentary facies. The Lower Bahariya Formation often comprises more extensive sandstone bodies, whereas the Upper Bahariya Formation contains more shale and siltstone intervals. These stratigraphic variations can influence petrophysical characteristics, with sandstones generally exhibiting better reservoir properties compared to shale and siltstone intervals (Dally et al. 2023). The considerable presence of sand-rich sediments in the lower Bahariya Formation can be attributed to the redistribution of underlying sediments from the Kharita Formation. The Cenomanian Sea, during the deposition of the Bahariya Formation, played a significant role in reworking and distributing these sediments at the base of the formation. As the Cenomanian Sea transgressed, the percentage of shale-dominated sediments increased, resulting in the observed differences in reservoir properties between the Lower and Upper Bahariya Formation.

The findings of this study regarding the reservoir characteristics and stratigraphic division of the Bahariya Formation in the South Umbarka area contribute to our understanding of the hydrocarbon potential in the region. The presence of fault-assisted anticlinal closure and the influence of faulting and folding processes provide favorable conditions for the accumulation of hydrocarbons with higher porosity and saturation values in the Sethos oilfield. Furthermore, the identification of depositional sequences and the characterization of reservoir properties within the Lower Bahariya Formation highlight the importance of detailed sequence stratigraphic analysis for the exploration and production of hydrocarbons in sedimentary basins.

6 Conclusions

  • This study investigates the seismic interpretation and sequence stratigraphic analysis of the Bahariya Formation in the South Umbaraka oilfields, specifically the Selkit, Khepri, and Sethos fields, located in the Western Desert of Egypt.

  • Through seismic interpretations, various structural closures have been identified in the region, including a 4-way dip closure in the Selkit oilfield trending east–west, a 3-way dip closure in the Khepri oilfield trending west-northwest to east-southeast, and a 4-way dip closure in the Sethos oilfield slightly faulted by a northwest-southeast trending fault. Additionally, a major fault separates the area into northern and southern blocks.

  • The Bahariya Formation is divided into two depositional sequences: the lower Bahariya sequence (BAH.SQ-1) and the upper Bahariya sequence (BAH.SQ-2). BAH.SQ-1 comprises units dominated by mixed siliciclastics with some carbonate units, while BAH.SQ-2 consists of mixed siliciclastics and carbonates.

  • The Lower Bahariya Formation exhibits superior reservoir characteristics compared to the Upper Bahariya Formation in terms of net-pay thickness, shale volume, effective porosity, and water saturation.

  • The sandstone-dominated LST-1 within the Lower Bahariya Formation is identified as a favorable reservoir zone with positive petrophysical properties. Conversely, the Upper Bahariya Formation demonstrates limited reservoir potential.

  • Overall, this study enhances the understanding of hydrocarbon plays and the distribution of reservoirs in the in the South Umbaraka oilfields. The findings underscore the importance of considering faulting, folding, and depositional environments in the exploration and production of hydrocarbons in this area.