Introduction

Carbonate rocks are among the most common sedimentary types. They are also of supreme importance as petroleum source rocks because they contain roughly half of the world’s hydrocarbon reserves. Therefore, it is important to understand and study the nature of the formation of this class of rocks as well as the factors that control their deposition. Many research studies have shown that carbonate facies are usually the product of combined processes in their depositional setting. Factors, such as water depth, water chemistry, temperature, diagenesis, and biological activity, exercise an influence on carbonate rock formation (Bathurst 1975; Wilson 1975; Tucker and Wright 1990; Scholle and Ulmer-Scholle 2003; Flügel 2010). Carbonate facies are also characterized by cyclic or rhythmic changes in their stratigraphy, which may be due to changes in the base level (Reading 1978; Schwarzacher 2000; Strasser et al. 2006; El‑Younsy and Salman 2021). Study of these cyclic features has improved our understanding of the lateral and the vertical relationships of carbonate facies. This knowledge has been used by the petroleum industry to build better models for hydrocarbon exploration and production. The Gulf of Suez (NE of Egypt), which is known as a Clysmic Gulf, is an elongated shape separated Eastern Desert from Sinai Peninsula (Robson 1971; Bosworth et al. 1998). It is considered one of the most important hydrocarbon provinces in the world, containing several hydrocarbon fields (Chowdhary and Taha 1987; Barakat and Dominik 2010; El-Gendy et al. 2017, 2022; Radwan 2020; Barakat et al. 2022). The object of the present study, Ras Budran Oil Field, lies in the central part of the Gulf of Suez. It is regarded as the principal petroleum field in the Gulf (Fig. 1). The primary target sediments in the Ras Budran Field contain remarkable carbonate rocks laid down in the Upper Cretaceous Period. This group of rocks is considered to be one of the most important hydrocarbon sources in the region. Therefore, the main purpose of the present work is to aid the understanding of this important sequence through the identification and the analysis of the vertical and the lateral stacking patterns of its Campanian–Maastrichtian carbonates, principally termed the Brown Limestone and the Sudr Chalk. In addition, the architecture of the depositional environments of both the Brown Limestone and the Sudr Chalk is elucidated through the integration of log trends supported by the composite log description.

Fig. 1
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The Ras Budran Oil Field lies in the central part of the Gulf of Suez in the Egypt, as seen on a general map (A). A map showing the location of the wells investigated in the present study (B)

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Geological setting

The Gulf of Suez constitutes a rift basin extending from northwest to southeast, located at the northern end of the Red Sea rift system (Alsharhan 2003; Dolson 2020; Abdelwahab et al. 2022). The geology of the Ras Budran Field is characterized by numerous facies deposited under various sedimentary environments. These facies include siliciclastics, carbonates, and evaporites deposited during three geological eras, from the Paleozoic the Cenozoic. These deposits also span the three tectonic phases of the Gulf, from the Paleozoic–Eocene Pre-rift, Miocene Syn-rift, and Pliocene Post-rift (Said 1990), as shown in Fig. (2). The Pre-rift stratigraphic strata contain sand, shale, and carbonate facies that were deposited under terrestrial and marine platform habitats (Said 1990, Fig. 2). These deposits are divided into nine rock units starting at the base, including Nubia, Raha, Abu Qada, Wata, Matulla, Brown Limestone, Sudr, Esna, and Thebes. The Raha, Abu Qada, Wata, Matulla rock units belong to Nezzazat Group while the other units (Brown Limestone, Sudr, Esna, and Thebes) belong to El Egma Group. The Gharandal and Ras Malaab groups of the Miocene Syn-rift sequences were separated into six rock units, which were distinguished by a variety of strata, including sand, shale, carbonate, and evaporites. The Gharandal Group was separated into the Nukhul, Rudeis, and Kareem formations, while Zeit, South Gharib, and Belayim formations are part of the Ras Malaab Group. The Pliocene Post-rift layers include limestones, shales, and sands.

Fig. 2
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General lithostratigraphic chart of the Ras Budran Field, Gulf of Suez (Said 1990)

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Materials and methods

The available data for this study comprise wireline logging, composite logs, and geological reports including two wells, the RB-A2 well across the depth interval from 9992 to 10,287 ft, and the RB-B2 well across the depth interval from 9737 to 10,038 ft (Fig. 1). These data are listed in Table 1. The wireline logs were gamma ray (GR), resistivity (ILD, ILM), neutron (NPHI), bulk density (RHOB), sonic (DT), and photoelectric factor (PEF). The Schlumberger Petrel ™ (ver. 2017) was used for the loading (as LAS files) and analysis of the well logs. The integrated logs were used to delineate the stacking patterns of the carbonate deposits. The integrated logs were then linked with the composite logs and the geological reports from the two wells to define the depositional environments. The GR log trends supplied by Kendall (2003) were used to interpret the stacking patterns and the depositional settings (Fig. 3). Lastly, the CorelDraw Graphics Suite 2022 was used to draw and export the figures presented in this article. The collected data have been supplied by the Suez Oil Company (SUCO) and the Egyptian General Petroleum Corporation (EGPC).

Table 1 The available well logging tools, supported by composite logs and geological reports, for the studied wells
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Fig. 3
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Carbonate stacking patterns and their depositional interpretation in relation to the gamma ray (GR) log trends introduced by Kendall (2003)

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Results and discussion

Lithofacies descriptions and characteristics

The present study focuses on the Campanian–Maastrichtian carbonate deposits (Brown Limestone and Sudr) in the Ras Budran Field, Gulf of Suez, Egypt (Fig. 2). These stratigraphic units belong to the pre-rift sediments and are the main petroleum source rocks in the region, the Brown Limestone in particular (Alsharhan 2003; Fig. 4). A detailed description of these strata follows, from the oldest to the youngest (Figs. 5, 6 & Tables 2, 3).

Fig. 4
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The average organic carbon content of pre-rift and syn-rift rock units’ (% TOC) in the Gulf of Suez (Redrawn after Alsharhan 2003)

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Fig. 5
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Descriptions, characteristics, stacking patterns, and depositional controls of the carbonate lithofacies at the RB-A2 well

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Fig. 6
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Descriptions, characteristics, stacking patterns, and depositional controls of the carbonate lithofacies at the RB-B2 well

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Table 2 A summary of lithofacies description, biofacies assemblage, and stratigraphic intervals of the rock strata in the studied wells
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Table 3 A summary of the electro-facies types, thicknesses, stacking patterns, and depositional environments of the rock strata in the studied wells
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Brown Limestone (Campanian)

This deposit consists mainly of dark brownish, grey to buff in color, microcrystalline, hard limestone with argillaceous material toward the top. As shown in Table 2, the stratigraphic interval of this unit attains about 201 ft (from 10,086 to 10,287 ft) at RB-A2 and about 183 ft at RB-B2 (from 9855 to 10,038 ft). Moreover, according to the composite logs and geological reports, this deposit includes a fauna assemblage comprising such species as Neobulimina canadensis and Lacosteina maquawilensis (Cushman and Wickenden 1928; Ansary and Fakhr 1958) indicating a shallow marine environment (Culver 1988). As shown in Figs. 5 and 6, the brown limestone shows high GR readings due to its high content of organic matter (Alsharhan 2003; Fig. 4). In addition, it shows sharp lithological contacts between the Matulla Sandstone below and the Sudr Chalk above. This is inferred from the contrast responses of the integrated logs via the GR, ILD, ILM, and DT readings. These formational contacts, the Matulla/Brown Limestone and the Brown Limestone/Sudr Chalk, may coincide with other well-known temporal contacts in Egypt, such as the Santonian–Campanian and the Campanian–Maastrichtian (El-Younsy et al. 2015, 2017; Obaidalla et al. 2017, 2020; Salman 2013, 2017, 2021).

Sudr Chalk (Maastrichtian)

Recognized by its chalky limestone facies, this deposit is white, soft, cryptocrystalline, and fossiliferous. As also shown in Table 2, its stratigraphic interval is about 94 ft (9992–10,086 ft) at RB-A2 and about 118 ft at RB-B2 (9737–9855 ft). Moreover, according to the composite logs and geological reports, this deposit includes foraminiferal fauna, such as Coryphostoma incrassatum (Reuss 1851) and Globotruncanita stuarti (Lapparent 1918), indicating a deep-marine environment (Culver 1988). As shown in Figs. 5 and 6, the Sudr Formation shows low GR values since it usually contains little organic carbon. This is consistent with the rule that most deep-sea carbonates are less promising hydrocarbons sources than shallow-water carbonates (Emery and Myers 1996). The Sudr Formation terminates with the Sudr/Esna formational contact, which indicates the erosional unconformity boundary between the Cretaceous and the Paleogene (K–Pg) boundary (Shreif 2019; Obaidalla et al. 2020; Elhossainy et al. 2021; Salman et al. 2021).

Carbonate cyclic stacking patterns

The well logs are used to create models in which vertical trends occur in cyclic order, to define the frame of the sequence and its response to changing water depth. Also, log data can indicate the penetrating lithology and the depositional setting in which these rocks have accumulated. The data include indications of organic richness (low to high) and can be used to identify and correlate the primary source rocks (Serra 1989; Goldhammer et al. 1990; Emery and Myers 1996; Rider 1996; Kendall 2003; Elhossainy et al. 2022). In the present study, the integrated well logs (GR, LLD, LLM, NPHI, RHOB, DT, and PEF) demonstrated clear cyclic stacking patterns in the carbonate rocks of the RB-A2 and RB-B2 wells. From these observations, the brown limestone and Sudr deposits of the Ras Budran Field were divided into three electro-facies (Figs. 5, 6 & Table 3). Below is a detailed description of these electro-facies:

Electrofacies 1

This facies covers the lower part of the brown limestone (Figs. 5, 6). As shown in Table 3, the interval ranges from 154 ft (10,133–10,287 ft) in the RB-A2 well to 143 ft (9895–10,038 ft) in the RB-B2 well. A cyclic order is evident from the well logs. The GR log readings are generally moderate, fluctuating from 50 to 75 API in several rhythmic bundles (12–14). The resistivity values and DT readings (2000 Ω-m and 65–70 μs/ ft., respectively) remain constant or nearly so at the RB-A2 well. Conversely, at the RB-B2 well, these logs display a distinctive fluctuation from higher values (2000.0 Ω-m and 130.3 us/ ft., respectively) to lower values (30 Ω-m and 65.7 μs/ft., respectively) constitute two major rhythmic bundles. The NPHI, RHOB, and PEF logs were only recorded in the RB-A2 well and show little fluctuation. These fluctuations coincide with the stacking pattern revealed by GR. Overall, this electro-facies is present in a cyclic order and displays a cylindrical (aggrading) stacking pattern. This trend indicates a keep-up carbonate depositional setting in a heterogeneous facies accumulating in shallow water (Fig. 3; Kendall 2003). It also reflects an affinity with the early Campanian transgressive deposits associated with eustatic sea-level rise.

Electrofacies 2

This facies covers the upper portion of the Brown Limestone (Figs. 5, 6). Its thickness ranges from 47 ft (10,086–10,133 ft) in the RB-A2 well to about 40 ft (9855–9895 ft) in the RB-B2 well as shown in Table 3. The responses of the well logs reflect a cyclic stacking pattern. The GR log readings are generally high, due to the high organic matter content with sedimentation influx in the basin; these values fluctuate from 75 to 150 API. These fluctuations are grouped into three rhythmic bundles. The resistivity and the DT readings (2000 Ω-m and 70–75 μs/ ft., respectively) remain constant or nearly so at RB-A2 well. On the other hand, at the RB-B2 well, these logs show a distinctive fluctuation from higher values (2000 Ω-m and 112.8 μs/ ft., respectively) to lower values (33.5 Ω-m and 56.6 μs/ft., respectively) in one major rhythmic bundle. The NPHI, RHOB, and PEF logs in the RB-A2 well show slight fluctuation. These fluctuations are consistent with the stacking pattern revealed by GR. In general, this electro-facies is present in a cyclic order and displays a funnel (prograding) stacking pattern. This trend indicates a shallow shoreline depositional setting (Fig. 3; Kendall 2003). It also reflects an affinity with the late Campanian regressive deposits associated with eustatic sea-level fall.

Electrofacies 3

This facies covers the Sudr Chalk. Its thickness ranges from 94 ft (9992–10,086 ft) in the RB-A2 well to about 118 ft (9737–9855 ft) in the RB-B2 well (Figs. 5, 6 & Table 3). The GR log readings are generally low due to its lithological content (chalky limestone), fluctuating from 10 to 20 API. The resistivity log displays a characteristic fluctuation from a higher value of 2000 Ω-m to a lower value of 17 Ω-m in three major rhythmic bundles. The DT log remains approximately constant (57–60 μs/ft) at the RB-A2 well, whereas at the RB-B2 well, it displays a distinctive fluctuation from 113 μs/ft to 60 μs/ft in three major rhythmic bundles. The NPHI, RHOB, and PEF logs are consistent with the stacking pattern revealed by the resistivity and DT trends. Electrofacies 3 has a serrated or saw-tooth shape indicating an aggradational stacking pattern (Figs. 5, 6). This represents a trend toward a distal deep-marine depositional mode (Fig. 3; Kendall 2003). It also reflects an affinity with the Maastrichtian transgressive deposits associated with eustatic sea-level rise.

Carbonate depositional controls

Most carbonate rocks are generated in situ, as opposed to being transported into place as clastic sediments. Therefore, they reflect all the conditions surrounding their generation during the different phases of their sedimentation (Goldhammer et al. 1990). Furthermore, carbonate rocks are characterized by the presence of cyclic or rhythmic stacking patterns. Such patterns may be attributed to factors, such as sea-level fluctuation, tectonic subsidence/uplift, climate, and sediment volume (Tucker and Wright 1990; Salman et al. 2021). In addition, the cyclical stacking of carbonate rocks on a metric scale has been interpreted as being related to sea-level fluctuations caused by perturbation of the Earth’s orbit in the Milankovitch frequency band and associated climate change (Drummond and Wilkinson 1993; Raspini 2001).

The high- and the low-frequency changes in the carbonate stratal patterns and lithofacies distribution that were observed in the present study are mainly controlled by eustatic sea-level changes (as evidenced by aggradational, and progradational stacking patterns). These layers have also been associated with a supply of clastic sediments, thus creating a good opportunity for a better understanding of the lateral and vertical relationships of the carbonate sedimentary facies. The deposition of these facies was also accompanied by several tectonic episodes (T1, T2, and T3), which played a major role in the morphology of the Gulf of Suez Basin during the Campanian and Maastrichtian ages (Figs. 5, 6). Sedimentation models of the carbonate accumulation and deposition in the basin were drawn, with reference to their location on the late Cretaceous paleo-geographic map presented by Scotese (2014; Fig. 7 a, b, c).

Fig. 7
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Block diagrams a–c illustrating the depositional circumstances and controls for the rock strata studied in the Gulf of Suez Basin, with concerning their situation along the late Cretaceous paleo-geographic map presented by Scotese (2014)

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During the pre-Campanian age (Santonian–Campanian), the basin affected by the first tectonic episode (T1) formed a major unconformity throughout the area of the present study. In stratigraphic terms, this episode took place at the basal part of the Brown Limestone. The T1 episode is recognized by sharp changes in the responses of the GR, LLD, LLM, DT, NPHI, and RHOB logs coincident with contact between the Matulla Formation and the overlying Brown Limestone. This transition also features a sudden change in the sedimentation pattern from siliciclastics to the overlying carbonates. The T1 episode is believed to be related to the Syrian Arc movement (Kerdany and Cherif 1990).

By the early part of the Campanian age, a slight transgression had taken place generating a unique accumulation of Brown Limestone in which relatively shallow clastic sediments are included. This resulted in the deposition of the lower part of the Brown Limestone under shallow marine carbonate sediments in an aggradational stacking pattern (cylindrical shape) to form electro-facies 1 (Fig. 7a). Thereafter, during the latter part of the Campanian Age, the accommodation space diminished because of a reduced rate of subsidence and a high influx of sediments. This led to the formation of a progradational stacking pattern (funnel shape) in the upper part of the Brown Limestone. Those layers were deposited under the conditions of a gradual relative sea-level fall, to form electro-facies 2 (Fig. 7b). At the end of the Campanian Age, the Brown Limestone sequence was terminated by a second tectonic episode (T2) that formed a major unconformable surface throughout the study area. The T2 episode coincides with the formational Brown Limestone/Sudr contact, recognizable by an abrupt transition in the GR, LLD, LLM, DT, NPHI, and RHOB logs. This transition closely matches the disconformity contacts reported in the deserts of Egypt, by Obaidalla et al. (2017) and Salman (2021) in the Eastern Desert and by Obaidalla et al. (2020) in the Western Desert. These depositional conditions and tectonic controls for the Brown Limestone are suggestive of a petroleum source rock, where optimal sedimentation rates and enclosed seaways have both acted to promote high rates of organic matter preservation (Alsharhan 2003).

During the Maastrichtian age, a major incursion of the Tethys Sea was linked to the initiation of deep subsidence in the basin. As a result, a layer of thick chalky limestone (the Sudr Chalk) was accumulated under deep waters, with normal marine salinity and warm tropical surface water enriched in planktonic and benthonic foraminiferal fauna. Carbonate sediments were uniformly deposited throughout the basin resulting in an aggradational stacking pattern (serrated/saw teeth shape) several feet thick, to form electro-facies 3 (Fig. 7 c). This widespread marine transgression seems to coincide with a eustatic sea-level rise (Haq 2014). Later, at the end of the Maastrichtian age, the uppermost part of the Sudr Chalk was affected by a third tectonic episode (T3) that formed a major unconformable surface at the Cretaceous–Paleogene (K–Pg) boundary throughout the study. The T3 episode is believed to have developed as a consequence of the late Cretaceous Syrian Arc System during the closing phase of the Tethys Sea (Sestini 1984; Scheibner et al. 2001; Salman et al. 2021).

Conclusions

On the findings of integrated well logging, composite logs, and geological reports, the stacking patterns of the Upper Cretaceous carbonate rocks (Brown Limestone and Sudr) of the Ras Budran Field in the Gulf of Suez, Egypt, were categorized into three types of electro-facies. The first exhibits a cylindrical (aggrading) stacking pattern and is found in the lower portion of the Brown Limestone. This pattern points to a keep-up carbonate depositional setting, generating heterogeneous shallow-water facies. It reflects the influence of eustatic sea-level rise during the early Campanian age. The second has a funnel (prograding) stacking pattern and is found in the upper portion of the Brown Limestone. It accumulated in the form of shoreline shallow carbonate facies, signifying the regressive deposits brought on by the eustatic sea-level decline in the late Campanian age. The third, covering the Sudr Chalk, shows a serrated, stacking pattern characteristic of aggradation. This facies reflects the existence of transgressional deposits associated with the eustatic sea-level rise during the Maastrichtian age and indicates deposition in a distal and deep-marine setting.

Furthermore, the variability in the resulting cyclic layer thickness and type reflects changes in the accommodation space that may be caused by eustatic sea-level oscillations, changes in sedimentation influx, and changes in subsidence rate. Along with such changes, three tectonic episodes (T1, T2, and T3) also occurred, episodes which had a significant impact on the morphology of the basin during the Campanian and Maastrichtian ages. The first tectonic episode (T1) coincided with the contact between the Matulla Formation and the overlying Brown Limestone. The second episode (T2) coincided with the contact between the Brown Limestone and the Sudr Chalk above. These two episodes (T1 and T2) bracket the Brown Limestone, a formation that was deposited in shallow marine conditions with high organic matter sedimentation, i.e., its preservation as detritus fall under anoxic conditions. These circumstances led to the Brown Limestone becoming the primary source of petroleum rocks in the Gulf of Suez Basin. The final episode (T3) generated an unconformable surface at the Cretaceous–Paleogene (K–Pg) boundary at the upper margin of the Sudr Chalk throughout the study area. Therefore, changes in eustatic sea-level and local tectonic activity in the area combined to produce the Upper Cretaceous carbonate stacking patterns of the Gulf of Suez Basin and the deposition of the Ras Budran Field.