Sequence stratigraphy and reservoir characterization of the lower Eocene rocks (Thebes Formation) along the Tethyan Ocean's southern margin: biostratigraphy and petrophysical parameter applications

The Egyptian lower Eocene carbonate rocks (Thebes Formation) are part of an extensive carbonate platform formed during an eustatic regression along the Tethyan Ocean's southern margin. These rocks are essential in developing Egypt's petroleum system because they can operate as vertical seals in specific basins or as source-reservoir rocks. However, few in-depth studies have been conducted to explore the diagenetic history, pore system, petrophysical characteristics, and sequence stratigraphic framework of these rocks and its relationship to the global sequences. Multiple datasets (foraminiferal assemblages, petrographic, and petrophysical data) from the lower Eocene strata exposed in Wadi El-Dakhl and El-Sheikh Fadl sections on the western side of the Gulf of Suez were integrated. The biostratigraphic examination of the planktic foraminifera shows that three Eocene biozones (E5–E7) were identified in the studied successions. The reservoir quality index and the flow zone indicator show that reservoir quality ranges from poor to fair and good. The primary reservoir porosity was resulted from diagenetic processes, including dissolution, fracturing, and meteoric diagenesis, which enhanced reservoir quality. The inferred benthic foraminifera and petrographic studies indicate a shallowing upward trend and deposition in fluctuating settings ranging from upper slope to middle shelf. This progressive reduction in the relative sea level corresponds to a gradual eustatic decline and demonstrates the enhanced control of tectonic activity and eustasy. Consequently, the Thebes Formation was deposited due to Tethyan sea-level changes, and its pore system was influenced by tectonics associated with the Syrian Arc system and Gulf of Suez rifting.


Introduction
The lower Eocene carbonates of Egypt provide new insights into the evolution of the extensive carbonate platform that developed along the Tethyan Ocean's southern margin (Said 1962). These rocks encompass 21% of Egypt's total surface area, however, their thickness gradually decreases in the southern Gulf of Suez, where they are represented by the Thebes Formation, made of limestone with alternating chert (Said 1990;Bosworth and McCaly 2001;Shaaban 2004;Muttoni and Kent 2007;Hewaidy et al. 2019;Farouk et al. 2020).
The Thebes Formation contributes approximately 1.1% of the Gulf of Suez's oil production potential (Alsharhan 2003;Radwan et al. 2020). Additionally, it serves as a source rock and vertical seals over Cretaceous sandstone reservoirs in 1 3 certain parts of the Gulf of Suez (Ungerer et al. 1986;Wever 2000;Radwan et al. 2020).
Despite its major importance, the Thebes Formation received little consideration; its paleoenvironmental condition is still poorly understood and lacks a coherent sequence stratigraphic framework liable for correlation with global sequences (King et al. 2017;Farouk et al. 2020;Deaf 2021). Furthermore, the Thebes production is primarily constituted of fractured limestone, which is known to be heterogeneous, chemically unstable, and the diagenesis has a significant impact on its reservoir quality (Murray 1960;Bosworth et al. 2014;Kiani et al. 2022). However, few detailed investigations of the Thebes Formation's diagenetic history have been performed, and few descriptions of the formation's fracture systems have been documented (Murray 1960;Bosworth et al. 2014;Kiani et al. 2022).
This emphasizes the importance of conducting comprehensive studies based on microfacies analysis, pore system analysis, quantitative analyses of benthic foraminiferal assemblages, sequence stratigraphic framework, and inferred sequence stratigraphy to explain the magnitude of relative sealevel fluctuations. The microfacies and pore system analyses may aid in revealing the evolution of the Tethyan southern margin in the early Eocene and had a significant impact on the zonation of high reservoir potential (Höntzsch et al. 2011b;Abd-Allah et al. 2019;Hewaidy et al. 2019;Kiani et al. 2022). At the same time, a quantitative examination of benthic foraminiferal assemblages would serve as a reliable proxy for inferring paleobathymetric changes (Van Der Zwaan et al. 1990; Katz et al. 2013;Farouk et al. 2020;Ghandour 2020a;Bazeen et al. 2021). Furthermore, sequence stratigraphic data are crucial and have proven helpful in unraveling global sealevel variations (Haq 1991;Willems et al. 1996;Hallam 1999;Özer et al. 2001;Simmons 2012;Youssef and Hefny 2015;Ghandour 2020b).
The main objective of this study is to develop a sequence stratigraphic framework for lower Eocene carbonate rocks (the Thebes Formation) on the Tethyan Ocean's southern margin by integrating their available petrographic data, petrophysical properties, and foraminiferal investigations. This aim will be achieved by evaluating the impact of diagenesis on the examined rock properties and inferring their paleoenvironment, which will then be used to reconstruct the relative sea-level. This could be beneficial in enhancing reservoir characteristics and predicting highquality zonation.

Geologic setting
During the early Eocene, the Tethys Ocean covered large areas of Egypt along its southern margin. Accordingly, a broad carbonate platform with bedded inner-platform facies was deposited, covering 21% of Egypt's entire surface area (Said 1990;El-Azabi 2006;Bosworth et al. 2014;Abd-Elhakim et al. 2021). The early Eocene in Egypt was an epoch of general regression, as evidenced from facies distribution, most likely caused by local tectonism and eustasy. The development of the Syrian Arc System affected the Egyptian Tethys margin this epoch through a sequence of different tectonic movements, resulting in a more complicated stratigraphic setting (Reches et al. 1981;Shahar 1994;Scheibner et al. 2003a;Höntzsch et al. 2011a;Boukhary et al. 2013;El Ayyat and Obaidalla 2016). The southern limit of the Syrian Arc System in the Eastern Desert is defined by the Southern Galala Plateau (Kuss et al. 2000;Scheibner et al. 2003b;Wilmsen and Nagm 2012), which is located in the northern part of the study area (Fig. 1). Furthermore, the lower Eocene rocks of Egypt are heavily weathered and fractured due to the Gulf of Suez rifting zone (Bosworth and McCaly 2001;Alsharhan 2003;Bosworth et al. 2014;Abu Sharib et al. 2017;Farouk et al. 2020).
Our study area is situated in the southern portion of the Galala Plateau in south of Zaafrana Area, approximately 200 km south of Suez in the Eastern Desert and approximately 28 km inland from the western side of the Gulf of Suez (Fig. 1). Moreover, the basement complex limits the Eocene sedimentary succession to the east of the study area (Fig. 1). The Eocene succession observed within the study area includes the Thebes Formation, which is the main objective of this study. Said (1960) originally proposed the Thebes Formation among the prominent limestone cliffs of Gebel Gurnah in the Nile Valley area. The Thebes Formation in the present study is 45-55 m thick and is formed chiefly of snow-white hard massive limestone and yellowish-white argillaceous limestone with abundant interbedded chert nodule horizons and a loaded cast of dolomitic concretions embedded in limestone groundmass (Fig. 2).

Material and methods
Forty-seven fresh rock samples were logged and sampled (at intervals spaced ~ 1 m a part approximately) from two exposed stratigraphic successions to construct the sequence stratigraphy and investigate the lower Eocene rocks' reservoir characterization and diagenetic features in the study area. The first succession is 45 m thick, with 26 samples representing the lower Eocene Thebes Formation at the Wadi El-Dakhl section (28° 39′ 19′′ N and 32° 33′ 14′′ E), while the second succession is 55 m thick, with 21 samples at the El-Sheikh Fadl section (28° 18′ 55′′ N and 32° 7′ 45′′ E), in Egyptian North Eastern Desert (Fig. 2). To achieve the study's goals, we integrated various datasets of 1 3 The micropaleontological analyses of the studied samples were carried out using the standard foraminiferal preparation techniques of Snyder and Huber (1996). Approximately 10 gm of dry and fresh rock samples were soaked and boiled in a suitable amount of H 2 O 2 until disintegration occurred, washed, and sieved through a 63 m sieve, and dried. We examined the dried residue and identified the foraminifera using a binocular stereomicroscope. The significant representatives of foraminiferal species were photographed using the Scanning Electron Microscope of the Egyptian Mineral Resources Authority and illustrated in Fig. 3.
Planktic foraminiferal species obtained from the studied sections were employed to establish a geologic-time framework and provide biostratigraphic zonal framework of the Thebes Formation. The inferred biozones are identified using a combination of standard biostratigraphic zonal schemes for the Paleogene planktic foraminifera (e.g., Berggren et al. 1995;Wade et al. 2011;Fig. 4). To provide clues into paleoenvironmental changes, we performed a cluster analysis of Ward's method and Pearson correlation on the examined assemblages of benthic foraminifera (only common species more than 2% in at least one sample) using Origin-Lab software. The benthic foraminifera were recognized using the atlas of Holbourn et al. (2013) and Van Morkhoven et al. (1986) and the systematic criteria laid out by Loeblich and Tappan (1988).
The planktic/benthic (P/B) ratios were calculated and expressed as P × 100/(P + B) to infer the paleobathymetry of the examined successions with some cautions, as the oxygen level in bottom waters and more unique paleoproductivity settings could lead to misleading paleodepth interpretation (Berger and Diester-Haass 1988;Van Der Zwaan et al. 1990;Farouk et al. 2020;Ayyad et al. 2022). Furthermore, species diversity indices such as Fisher's α and Species Richness (S) are also helpful tools for estimating the paleobathymetry of the examined successions and are derived using the PAST Software Package of Hammer et al. (2001). When these proxies are combined with the percentages of the common benthic foraminiferal species (> 2% in one sample), the prevailing paleodepths should be better estimated. Furthermore, detailed petrographic analyses (including microfacies and diagenetic examination) were performed to investigate sedimentary facies and the diagenetic mechanisms governing the distribution of reservoir units in the Thebes Formation. The petrographic examinations include the thin sections' textural features, cementation, grain size, sorting, and bioclasts. As a result, thirty-six thin sections were prepared in the petrology laboratory of Al-Azhar University's Geology Department. Thin sections were examined under a polarizing microscope, in which preparation included vacuum impregnation with a blue epoxy to distinguish between different types of porosity (Dickson 1965). The identified carbonate rocks were categorized using the classification system of Dunham (1962), and the equivalent environment of each microfacies type was determined following Wilson (1975) and Flügel (2010).
Moreover, forty-seven samples were prepared as a 1-inch diameter core plug using a diamond drilling machine for petrophysical measurements (Fig. 2). The porosity values of the investigated samples were calculated using helium porosimeter equipment to precisely estimate the effective porosity applying Boyle's Law of gas expansion. On the other hand, the permeability values were determined according to Darcy's law using a Hassler-type core holding apparatus. The porosity and permeability influence the reservoir quality index (RQI), which is determined using the equation described by Amaefule et al. (1993) through the following equation: RQI = 0.0314 √ k∕ , with; k permeability in mD and Φ porosity in percentage. The flow zone indicator values (FZI), which mainly depend on permeability, were then determined using Gunter et al. (1997) approach. As a result, a combination of petrographical investigations, porosity, and permeability measurements allowed the study area's reservoir zones to be distinguished from non-reservoir intervals of the Thebes Formation.

Micropaleontology
The microscopic examination of the obtained samples revealed a moderately to well-preserved planktic and benthic foraminifera, leading to the identification of 35 planktic foraminifer species belonging to seven genera. On the other hand, the study of benthic foraminifera led to the identification of 56 species that fit in 39 genera. Figure 3 depicts the SEM of the most crucial planktic foraminifera that provided reliable biostratigraphic data for the analyzed successions. Three planktic foraminiferal biozones (E5, E6, and E7) were detected in the studied sections (Fig. 4). The top and base events of the biostratigraphically relevant species are used to distinguish the biozones (Fig. 3).
As a result, the Morozovella aragonensis/Morozovella subbotinae Concurrent-range Zone (E5) was found to the lowermost part of the Thebes Formation (samples 1-5 in Wadi El-Dakhl section; Fig. 4). The co-occurrence of Morozovella subbotinae with Morozovella aragonensis in this interval corroborate this hypothesis (Fig. 4). Following the geologic-time scale of Gradstein et al. (2020), the estimated duration for this interval is 1.65 myr, from the lowest occurrence (LO) of M. aragonensis at 52.46 Ma to the highest occurrence (HO) of Morozovella subbotinae at 50.81 Ma (Fig. 4).
Furthermore, the Acarinina pentacamerata Partial-range Zone (E6) is assigned to the middle part of the Thebes Formation (samples 6-9 in Wadi El-Dakhl and samples 1-4 in El-Sheikh Fadl section; Fig. 4). The E6 Zone is defined as partial range of Acarinina pentacamerata between the HO of Morozovella subbotinae and the LO of Acarinina cuneicamerata. This zone lasted for 0.41 myr, measured from the HO of M. subbotinae to the LO of Acarinina cuneicamerata (Fig. 4).
Moreover, the LO of A. cuneicamerata is assigned to the E7 Zone, extending from samples 10 to 26 in the Wadi El-Dakhl section and samples 5 to 21 in the El-Sheikh Fadl section, encompasses the upper part of the Thebes Formation (Fig. 4). This part spans 0.66 myr, from the LO of A. cuneicamerata at 50.40 Ma to the upper Ypresian maximum flooding surface (PaYp9) at 49.74 Ma (Fig. 4).

Cluster analysis
The multivariate cluster analysis classifies the inferred benthic foraminiferal species into four distinct clusters. Each cluster is characterized by a particular foraminiferal composition that often reflects paleoenvironmental patterns (Fig. 5). Figure 5 shows the quantitative characteristics of benthic foraminiferal species most suggestive of paleoenvironments within each cluster.
Cluster A The dominant benthic foraminiferal species in cluster A account for 15% of the entire benthic foraminiferal assemblage (Fig. 5c). Anomalinoides affinis, Pullenia jarvisi, Oridorsalis plummerae, and Cibicidoides cf. pesudoperlucidus are the most abundant species in cluster A. (Fig. 5A,B). The abundance of these species is restricted to the lower part of the Thebes Formation and includes samples 1-2 from the Wadi El-Dakhl section (Fig. 6). These species are often documented from the outer shelf to upper slope environments (Berggren and Aubert 1975;Alegret and Thomas 2004;Holbourn et al. 2013;Bazeen et al. 2021). Based on these faunal components, the paleodepth 1 3 for cluster A is proposed as an outer shelf to the upper slope depositional environment. Cluster C Cluster C has a higher percentage of benthic species than any other cluster (45%; Fig. 5c). These species are frequently found throughout the Wadi El-Dakhl section, except for the basal part that is infrequently found (Fig. 6). In the El-Sheikh Fadl section, they are primarily found in the lower part of the Thebes Formation, encompassing samples 1-2. (Fig. 7). Cluster C is distinguished primarily by several notable paleoecological indicators such as Cibicidoides dutemplei, Anomalinoides susanaensis, and Cibicidoides eocaenus (Fig. 5b). These representatives are widely established in settings ranging from the outer shelf to the upper slope (Van Morkhoven et al. 1986;Saint-Marc and Berggren 1988;Speijer and Schmitz 1998;Holbourn et al. 2013). Therefore, we regarded this cluster as evidence of an outer shelf depositional environment.
Cluster D Cluster D is sporadically represented in the analyzed successions, with percentage values never exceed 10% of the entire benthic assemblage (Fig. 5c). The abundance of cluster D benthic species is restricted to the upper part of the El-Sheikh Fadl section (Fig. 8). Cibicidoides pseudoungerianus represent the most common species in this cluster ( Fig. 5A,B). This benthic species was reported to dominate in depositional environments ranging from the outer shelf to the upper slope Holbourn et al. 2013). Consequently, cluster D is considered example of depositional settings from the outer shelf to the upper slope.

Diversity indices and P/B ratios
Fisher's values are modest and fluctuate over the studied successions, ranging from 1 to 13, with higher numbers in the upper sections and lower numbers in the lower parts of Wadi El-Dakhl section, and ranging from 1 to 8 at El-Sheikh Fadl section (Figs. 6 and 7). Furthermore, the species richness found in the studied samples ranges from 23 to 46, with higher values recorded from samples 1-4 and 17-23 of the Wadi El-Dakhl section and samples 1-2 and 12-13 of the El-Sheikh Fadl section (Figs. 6 and 7). It also shows a significant decrease in the number of species for samples 7-8, 14 in the Wadi El-Dakhl section, and sample 4 in the El-Sheikh Fadl section (Figs. 6 and 7). Moreover, in our examined successions, the P/B ratios vary from 83 to 97% at Wadi El-Dakhl section and ranging from 86 to 97% at El-Sheikh Fadl section (Figs. 6 and 7), showing a rise in the planktic foraminifera and suggesting deep paleodepths.
Lime mudstone (FT1) microfacies have been documented in the middle and upper parts of the El-Sheikh Fadl section (samples 9 and 16-21) and the middle part of the Wadi El-Dakhl section (samples 9 and 18), and it is characterized by semi-hard yellowish-white laminated limestone with chert nodules (Figs. 7 and 8). This microfacies is composed chiefly of a cryptocrystalline calcite matrix (up to 92 percent of the rock) with a few bioclastic grains (about 8% of the rock) that are represented by benthic biserial foraminifera (e.g., Fursenkoina sp.), with their tests filled by calcite cement (Fig. 8a). The bulk of these bioclastic allochems is embedded in a micrite matrix partially recrystallized into spary-calcite cement, which is significant in poor preservation throughout diagenetic processes. Pore spaces in this microfacies account for 2-30% of the rock and are generally intergranular, intragranular, and vuggy types, with sparycalcite cement filling them locally (Fig. 8a). As a result, the sedimentological properties, including an enrichment of cryptocrystalline calcite matrix and a minor bioclastic fragments, point to a quiet inner to middle shelf depositional environment for this microfacies (Figs. 6 and 7).
Foraminiferal wackestone (FT2) microfacies type was identified in the field as grayish white semi-hard limestone and is well developed throughout the examined successions, including samples 7, 15-17, 19-23 at Wadi El-Dakhl section and samples 2, 7 at El-Sheikh Fadl section (Figs. 6 and 7). Texturally, it comprises allochems that make up to 20% of the rock and are represented mainly by various species of foraminifera (e.g., Acarinina sp. and Cibicidoides; Fig. 3f-l and ab-ag) filled with calcite cement and encrusted with isopachous crystalline calcite (Fig. 8b). This microfacies has a glauconitic matrix of highly minute pellets, with their allochems embedded in a cryptocrystalline calcite cement (Fig. 8b). Intergranular, intragranular, channel, and vuggy porosities are the most prevalent pore types in this microfacies, and account for up to 30% of the rock (Fig. 8b). As a result of the enrichment of the cryptocrystalline calcite matrix and the association of Acarinina sp. and Cibicidoides, this microfacies is regarded as typical deep-water deposits of an outer shelf environment according to Flügel (2010) and Giraldo-Gómez et al. (2018).
Foraminiferal wackestone/packstone (FT3) microfacies is mainly composed of yellowish-white massive hard limestone with thin chert nodules and marks numerous intervals within the Thebes Formation in the Wadi El-Dakhl section,  6). Bioclastic allochems are distinguished by few benthic and poorly sorted planktic foraminiferal tests, which account for 20-35% of the rock (Fig. 8c). Cibicidoides sp. and undifferentiated planktic forms, primarily Acarinina sp. (Fig. 3f-l), dominate the foraminiferal assemblage, which is filled with calcite cement and has encrusted dog teeth isopachous calcite cement on their walls (Fig. 8c). This microfacies also contains fragments of pelecypods (recrystallized into sparry-calcite cement), which is well developed in the middle part of Wadi El-Dakhl (sample 13; Figs. 6 and 8c). These allochemical components are held together by a micritic matrix (cryptocrystalline calcite matrix; Fig. 8c). Pore spaces account for up to 25% of the rock, are intergranular and vuggy in type, and were discovered in the middle of the Thebes Formation in the Wadi El-Dakhl section (sample 13; Fig. 8c). Furthermore, the micritic matrix and bioclasts of benthic foraminifera such as Cibicidoides sp., indicate deposition in a quiet open marine depositional environment on the outer shelf setting (Fig. 6).
Foraminiferal packstone (FT4) microfacies type is found in the field as yellowish-white hard limestone with occasional chert nodules. It is observed in the middle part of Wadi El-Dakhl (sample 11), as well as the middle and upper parts of the Thebes Formation in El-Sheikh Fadl (samples 3, 5-6, 10-11 and 14; Figs. 6 and 7). The bioclastic allochems in this microfacies account for up to 45% of the rock and are represented by moderately preserved Acarinina sp., numerous benthic forams such as Uvigerina cf. farinosa (Fig. 3o) and Fursenkoina sp., and a few nummulitic clasts (less than 2%; Fig. 8d). These bioclastic allochems are filled by sparite cement and encrusted by isopachous calcite cement. Furthermore, extremely tiny quartz grains of 3% can be found in this microfacies (Fig. 8d). The predominant pore types in this microfacies are intergranular and vuggy porosity, which account for up to 40% of the rock. Channel porosity is also found, but in lesser amounts (Fig. 8d). The vuggy porosity is more prevalent in the El-Sheikh Fadl section samples than in the Wadi El-Dakhl samples. Accordingly, the enrichment of Uvigerina cf. farinosa and small nummulitic fragments Molluscan bioclastic packstone (FT5) has grayish white bioturbated hard limestone lithology and is found in the upper part of the Thebes Formation in the El-Sheikh Fadl section, including sample 15 (Fig. 7). The microscopic examination indicated an enrichment of molluscan and foraminiferal tests in this microfacies, specifically gastropod, pelecypod, Acarinina sp., and Uvigerina cf. farinosa (Figs. 3o and 8E). The recognized gastropods and other extremely minute bioclastic grains are filled with micrite matrix. These gastropods are occasionally packed with iron oxide. The walls of these molluscan fragments are typically recrystallized into macrocrystalline calcite (Fig. 8e). Furthermore, the calcite overgrowth cement of the echinoid segments is a unique property of this microfacies (Fig. 8e). All these bioclastic allochems are well preserved and cemented by cryptocrystalline calcite cement. This microfacies's porosity accounts for 30% of the rock's volume and is represented by intergranular and intragranular types (Fig. 8e). The prevalence of molluscan shell fragments and cryptocrystalline calcite matrix indicates that inner shelf conditions was prevailing during the deposition of this microfacies (Fig. 7).
Sandy foraminiferal packstone (FT6) microfacies has a dominating yellowish-white massive hard limestone lithology and is infrequently seen in the examined successions. It occurred in the lower part of the Thebes Formation in the Wadi El-Dakhl section (sample 6; Fig. 6). Foraminiferal tests, particularly Cibicidoides sp. and Acarinina sp., are the primary components of this microfacies, accounting for up to 50% of the rock. Isopachous calcite covers the walls of these foraminiferal tests, which are packed with crystalline calcite (Fig. 8f). In addition, finely sorted subspherical quartz grains may be found in this microfacies, which account for around 10% of the rock (Fig. 8f). Most pores and certain foraminiferal chambers are filled with iron oxide cement. Minor porosity content of less than 15% may be observed in this microfacies, represented by the intergranular porosity type. The predominance of benthic Cibicidoides foraminiferal tests (Fig. 3ab-ag), cryptocrystalline calcite matrix, glauconitic pellets, and  Intraformational conglomerates (FT7) facies type is found in the upper part of the Thebes Formation in the Wadi El-Dakhl section, including sample no. 24. (Fig. 6). It has a thickness of about 40 cm and is made of rounded to sub-rounded conglomerate formed mainly of carbonate clasts embedded within a carbonate matrix (Fig. 8g). This lithofacies is generated by breaking up fragments of recently formed or partially consolidated limestone during a brief interruption in strata deposition.

Diagenesis
Mineralogically, carbonates are more unstable, more susceptible to post-depositional diagenetic activities, and typically more easily broken down by physical processes that affect reservoir quality (Ahr 2008;Boggs Sam 2009). Because of the essential significance of diagenetic processes in the distribution of reservoir characteristics that determine reservoir quality, samples from the studied successions were submitted to diagenetic analyses by petrography of thin sections. The post-depositional environment controls diagenesis's influence on carbonate rocks, fluid flow, primary mineralogy, and period of exposure to the environment (Purdy 1968). The primary diagenetic processes impacting the reservoir quality of the investigated Thebes Formation include cementation, dissolution, fracturing, and recrystallization. The following subsections briefly describe the main identified diagenetic processes in the studied rocks.
One of the most prominent diagenetic processes impacting FT1 and FT4 in the thin sections investigated is cementation by sparry calcite, which fills the chambers of the bioclastic grains during the early phases of diagenesis ( Fig. 8A,D,H). Additionally, many fractures partially filled with crystalline calcite cement may be seen as the principal diagenetic process affecting the FT2 microfacies type (Fig. 8b). As a result, it was revealed that these two types of cement caused the blockage of pore spaces and permeability, decreasing the reservoir quality. The cementation by a micritic matrix, on the other hand, is the most visible diagenetic process impacting the FT5 microfacies type (Fig. 8e). This micritic component may increase porosity by preventing pore space closure during compaction (El Husseiny and Vanorio 2017).
Mechanical compaction causes sedimentary grain dewatering and deformation because of an abnormal rise in precementing pressure, faulting, or fluid pressure. The effect of mechanical compaction on the examined microfacies was apparent in the form of fracture patterns and bioclast deformation during the diagenetic processes' middle stages. These fractures are the most prevalent diagenetic events affecting FT3 microfacies (Fig. 8c). During the later phases of the diagenetic processes, they are represented in diverse directions and occasionally filled with calcite cement (Fig. 8I, J, K, L). Fractures, in general, have a crucial role in increasing the permeability of the investigated Thebes Formation, particularly those caused by tectonic processes. However, some of them are blocked with cement, reducing porosity and degrading reservoir quality.
Dissolution is more frequent in carbonate rocks, and it is the most important diagenetic event in the studied region, resulting in the leaching of unstable minerals and the formation of secondary pores. The dissolution of the samples investigated resulted in various secondary porosity types, including vuggy and channel porosity (Fig. 8h). Several stages of dissolution have influenced the investigated Thebes Formation, owing to the dissolving of the micrite matrix during the later stage of the diagenesis process, which forms irregular longitudinal channel pores. As a result of this process, medium to high porosity is developed in the studied successions (Figs. 6 and 7). The effect of dissolution on the limestone of the Thebes Formation was found to be greater in the El-Sheikh Fadl section than in the Wadi El-Dakhl section, according to the petrographical examination. This result might indicate that the limestone in the El-Sheikh Fadl section has been raised and exposed to a meteoric regime more than the limestone in the Wadi El-Dakhl section.
The replacement procedure generally includes the precipitation or growth of mineral crystals that replace dissolved minerals in rock fragments or previously formed cement (Ulmer-Scholle et al. 2015). This diagenetic process has been observed in several thin sections analyzed, where the interior of the foraminiferal tests has been dissolved and replaced by crystalline silica during the later stages of diagenesis (Fig. 8h).

Petrophysical investigations
Petrophysical studies such as porosity (%), bulk density (g/ cm 3 ), permeability (mD), reservoir quality index (RQI), and flow zone indicator (FZI) were performed to assess the reservoir characterization of the examined successions. These petrophysical studies were carried out to see how the controlling impact of diagenetic events and mineral composition affects the petrophysical behavior of the Thebes Formation in the study area and determine its economic potential for the petroleum system. Due to variances in rock types, clay content and distribution, heterogeneity of mineralogical composition and fossil content, and the complexity of pore space distribution, the obtained petrophysical data are remarkably heterogeneous. The obtained porosity ranges from 4 to 41%, and the permeability ranges from 1 to 650 (mD) (Figs. 6 and 7).
The foraminiferal wackestone/packstone microfacies had the lowest average porosity and permeability values, whereas the foraminiferal wackestone microfacies had the most incredible average porosity (Table 1). On the other hand, the foraminiferal packstone microfacies had the most significant average permeability value (Table 1). The observed petrophysical parameters and their standard deviations for all Thebes Formation microfacies are summarized in Table 1. In addition, a set of petrophysical relationships were performed to analyze the interrelationships between the various petrophysical parameters and investigate diagenetic processes' influence on the obtained data.

Porosity (Φ)-permeability (k) relationship
The effective pore volume is one of the most critical elements affecting permeability, which rises with increasing porosity. Plotting permeability values as a function of effective porosity values shows a high degree of consistency between porosity and permeability for all facies in the El-Sheikh Fadl section (Fig. 9a). The graph, on the other hand, shows a weak relationship for the facies of the Wadi El-Dakhl section (Fig. 9a), which might be related to the heterogeneity of the pore space distribution. Considering the variance in porosity for all facies, it is possible to conclude that the samples from the El-Sheikh Fadl section are more permeable than those from the Wadi El-Dakhl section. The calculated equation describes the relationship between porosity and permeability for the foraminiferal wackestone/ packstone microfacies is k = (10) (11.64Φ−2.28) , with the coefficient of determinations (R 2 = 0.83).

Porosity (Φ)-reservoir quality index (RQI) relationship
The porosity is directly related to the reservoir quality index values (RQI) of the examined Eocene carbonate samples (Fig. 9b). The RQI values for the El-Sheikh Fadl section samples are defined by RQI ≥ 1 µm, indicating a fair reservoir quality (Fig. 9b). However, this relationship is weak for all facies in the Wad El-Dakhl section, indicating that the reservoir is impervious to poor quality (Fig. 9a). This relationship indicates that the carbonate of the Thebes Formation in the El-Sheikh Fadl section is more homogeneous than the carbonate of the Wadi El-Dakhl section. This relationship has a coefficient of determination of (R 2 = 0.76), and the following equation represents it for foraminiferal wackestone/packstone facies: RQI = [10] (4.64 Φ−2.04) .
We get the following equations to represent this relationship for the lime-mudstone facies: for foraminiferal wackestone for foraminiferal wackestone/packstone for foraminiferal packstone As a result of the preceding relationships, the reservoir quality index (RQI) is controlled mainly by permeability, and the exponents of (k) are near 0.5, as suggested by the equation of Amaefule et al. (1993).

Flow zone indicator (FZI)-porosity/permeability relationship
For the examined limestone samples, the relationship between flow zone indicator (FZI) and porosity/permeability is directly proportional (Fig. 9D,E). The porosity was inversely related to the FZI, whereas the permeability was shown to be directly related (Fig. 9D,E). Furthermore, we discovered that the samples from the El-Sheikh Fadl section have greater FZI values than the samples from Wadi El-Dakhl, yielding the following equation for foraminiferal wackestone/packstone facies: with a coefficient of determination (R 2 = 0.31). The relation between FZI and permeability are described by the following equations for lime mudstone: for foraminiferal wackestone   Figure 4 illustrates a schematic diagram of the examined successions' depositional history and sequence stratigraphic framework. The microfacies and benthic foraminiferal bathymetric ranges provide a specific dataset for reconstructing the reservoir depositional model of the studied formation. Accordingly, the sedimentological properties of the FT1, FT4, FT5, and FT7 imply an inner to middle shelf depositional environment. These microfacies are restricted to the middle and topmost intervals of the El-Sheikh Fadl succession (Fig. 7). They are distinguished by micritic matrix enrichment, moderately sorted planktic foraminiferal tests, and small nummulitic fragments that imply a quiet inner to middle shelf depositional environment (Fig. 8A, D, E). These intervals also contain Uvigerina cf. farinosa (14-45%), Valvulineria scorbiculata (10-17%), Fursenkoina sp. (6-21%), and Siphogeneroides eleganta (5%; Fig. 10), which represents inner to middle shelf paleodepths (Holbourn et al. 2013;Giraldo-Gómez et al. 2018).
According to our findings, the Wadi El-Dakhl succession is distinguished by well-defined facies belts with an outer shelf to upper slope benthic foraminiferal assemblages. In contrast, the El-Sheikh Fadl succession is dominated by inner to middle shelf assemblages (Figs. 6,7,10,and 11). This paleodepth is demonstrated by characteristics such as The stratigraphic sequence framework for the Thebes Formation was constructed in this study using facies analyses, vertical relationships between lithofacies, biostratigraphic data, and cluster analysis (Figs. 4, 6 and 7). The depositional sequences are identified and divided into system tracts based on changes in depositional environment trends and variations in the relative sea-level interpreted from paleobathymetric ranges of foraminifera and microfacies investigations. Accordingly, the studied Thebes Formation holds two incomplete depositional sequences (SQ1 and SQ2). The SQ1 comprises shallowing-up facies assemblages that constitute an incomplete highstand systems tract (HST), whereas the SQ2 is composed of an incomplete transgressive systems tract (TST). The essential features of each depositional sequence, their location in the established sedimentological log, their microfacies, and their biostratigraphic parameters are illustrated in Figs. 4, 6, and 7.
The first depositional sequence SQ1 extends from the Wadi El-Dakhl section's base to the level from where the intraformational conglomeratic bed at sample no. 24 was obtained (Fig. 6) and encompasses the whole Thebes Formation in the El-Sheikh Fadl section (Fig. 7). SQ1 was deposited over 1.30 myr from the middle Ypresian maximum flooding surface (Yp7) at 51.35 Ma to the upper Ypresian sequence boundary (PaYp9) at 50.05 Ma (Fig. 4). The base of this sequence and its lower component (TST) is not reached in the present study. This depositional sequence (SQ1) is represented by highstand systems tracts (HST), which are petrographically distinguished by cryptocrystalline calcite matrix, spary, and isopachous calcite cement, and iron oxide that fills most intergranular pores and some foraminiferal chambers. Additionally, these intervals are characterized by an upward decrease in P/B ratios (from 93 to 86%), the species richness (from 37 to 34), and the relative abundance of the outer shelf to upper slope clusters such as cluster A (from 85 to 2%) and cluster C (from 60 to 10) with a general decrease in a Fisher's α Fig. 10 The relative abundances of the most frequent benthic foraminiferal species in each cluster from the Wadi El-Dakhl section values (11-6; Figs. 6 and 7). All these findings point to a decline in paleodepths, supporting the prevalence of a regressive trend.
Furthermore, the second depositional sequence SQ2 is represented by just a portion of the transgressive system tracts (TST). It comprises the upper part of the Thebes Formation in the Wadi El-Dakhl section (Fig. 6). SQ2 has a duration of about 0.32 myr, spanning from the upper Ypresian sequence boundary (PaYp9) at 50.05 Ma to the upper Ypresian maximum flooding surface (Yp9) at 49.73 Ma (Fig. 4). The sequence boundary SB 1, which defines the lower border of the second depositional sequence, is located at the base of the intraformational conglomerates, indicating a distinct shift from the outer shelf to the inner shelf environment. Petrographically, the interval of the SQ2 is distinguished by a high abundance of bioclasts such as Acarinina sp. and Cibicidoides sp., which are all found in transgressive system tracts. Moreover, the relative abundance of cluster C increases upward (from 54 to 86%) through these intervals, supporting the presence of transgressive systems tracts (TST; Fig. 6).
Moreover, we compared the relative sea-level changes identified within the studied lower Eocene successions to the sea-level events hypothesized on the global chart of Hardenbol et al. (1998) and the Arabian Plate sequence stratigraphy of Simmons et al. (2007) to evaluate whether eustasy or tectonics are the essential contributing factors in the evolution of the depositional sequences, because this issue is still being widely debated (Payton 1977;Zecchin et al. 2010). The biostratigraphic age calibration obtained from the observed successions' planktic foraminiferal zonation was utilized as a chronological outline to assist this comparison.
According to our findings, there are two incomplete relative sea-level cycles in the lower Eocene. Most of the studied successions were in the regressive phase. The transgressive phase reported in the highest portion of the investigated Thebes Formation at the Wadi El-Dakhl section is bounded by the sequence boundary SB 1 (Fig. 4). The base of the Wadi El-Dakhl section (E5 Zone) marks the Fig. 11 The relative abundances of the common benthic foraminiferal species in each cluster observed from the El-Sheikh Fadl section most deep-water deposits that ever recorded in the studied successions (Fig. 6). Accordingly, we attribute it as being linked to the middle Ypresian maximum flooding surface (Yp7) at 51.35 Ma, identified within the lower part of the M. aragonensis / M. subbotinae Zone ( Fig. 4; Hardenbol et al. 1998). In addition, the SB1, which is suggested by the existence of an intraformational conglomeratic bed (Fig. 8g), is of eustatic origin since it correlates well with PaYp9 of Hardenbol et al. (1998). Therefore, it represents the onset of an eustatically triggered transgression in the middle part of the Acarinina cuneicamerata Zone (Fig. 4). On the other hand, this sequence boundary SB1 roughly correlates to the Pg20, which indicates the maximum flooding surface of the Ypresian Arabian Plate (Simmons et al. 2007). Therefore, the culminating rise in sea-level at the Arabian plate's Pg20 maximum flooding surface is not identified in the examined successions (Fig. 4). The tectonics of the Syrian Arc system might be a factor contributing to such inconsistencies.
Accordingly, the stratigraphic sequence development of the Thebes Formation in the studied area was associated with an event where the sedimentation rate exceeds the rate of the accommodation space creation, followed by erosion and a rapid rise in relative sea level. Furthermore, the proposed sequences pattern was primarily influenced by Tethyan eustatic sea-level changes and tectonics related to the Syrian Arc system. Moreover, these inferred sequences are well-correlated with the proposed sequence stratigraphic framework for European basins and compatible with the global sea-level decline identified by Hardenbol et al. (1998;Fig. 4).

Reservoir characterization
The key elements affecting the reservoir characterization of the Thebes Formation are the pore system properties and the Tethyan sea-level fluctuation during the early Eocene. The reservoir quality of the studied successions was properly evaluated using a hydraulic flow unit, a requirement for reservoir units, and influenced by pore system properties and depositional and diagenetic features. The hydraulic flow units (HFU) were calculated using Amaefule et al. (1993)'s reservoir quality index (RQI) and flow zone indicator (FZI) to categorize reservoir quality and heterogeneity in the study area. The flow units divide the reservoir into distinct units, each with its own flow zone indicator, reservoir quality index, and normalized porosity index (NPI) value.
The porosity and permeability values determined from statistical characteristics of the reservoir facies in the Wadi El-Dakhl section indicated a low reservoir quality, as porosity varies from 4 to 41 with an average of 22.5%, and permeability ranges from 1 to 23 with an average of 12 mD (Fig. 6). Furthermore, the HST and TST of the Wadi El-Dakhl succession exhibited poor reservoir characteristics (i.e., impervious), as evidenced by the low reservoir quality index (RQI < 0.25 µm; Fig. 9f). This reservoir zone includes the middle and uppermost intervals of the succession, which is made up of microfacies often deposited in outer shelf to the upper slope settings (FT2 and FT3; Fig. 6). These impervious characteristics are due to authigenic pore lining, filling, and bridging by clay, as well as rich ferruginous materials, resulting in low FZI < 1 µm. (Fig. 9f).
Otherwise, the porosity and permeability values in the El-Sheikh Fadl succession vary from 16 to 33 with an average of 24.5% and from 12 to 650 with an average of 331 mD, respectively, showing that the studied succession has the most significant reservoir characteristics (Fig. 7). Moreover, the reservoir quality index values along the HST of the El-Sheikh Fadl succession demonstrated fair to good reservoir quality (0.25 < RQI < 1.50 µm; Fig. 9f). This reservoir zone is in the middle and upper part of the studied succession and is composed of microfacies indicative for inner to the middle shelf (FT1 and FT4; Fig. 7). This good quality is attributed to diagenesis' effect of producing horizontal fractures, which may increase connectivity by establishing potential flow pathways. Accordingly, these intervals have a reduced surface area, a lower shape factor, and a higher FZI value (Fig. 9f).
Furthermore, the reservoir quality heterogeneity of the Wadi El-Dakhl succession is primarily governed by mineral grain compaction, cementation, and dissolution, with an RQI of 0.02-0.2 µm in comparison to the restricted range of normalized porosity index (NPI ranges from 0.10 to 0.95; Fig. 9g). As a result of our findings, the hydraulic flow units of the investigated successions are represented by three flow zone indicators (HFU1, HFU2, and HFU3; Fig. 9g).

Conclusions
Because lower Eocene carbonate rocks may contribute to Egypt's petroleum system, the current study aimed to examine reservoir zonation, interpret the stratigraphic sequence framework, and reconstruct the relative sea-level changes of these rocks along the Tethyan Ocean's southern margin. We investigated the microfacies, pore system, and foraminiferal assemblage of lower Eocene succession at the Wadi El-Dakhl and El-Sheikh Fadl sections of the Gulf of Suez's western side. Biostratigraphically, the planktic foraminiferal species identified three biozones (E5-E7) within the examined successions, encompassing the time interval 52.46-49.74 Ma. Dissolution, fracture, and extensive meteoric diagenesis are the primary diagenetic processes affected the examined rocks and probably improved their reservoir quality. The effect of diagenesis on the studied rocks was more noticeable in the El-Sheikh Fadhl section than in the Wadi El-Dakhl section. This effect could indicate that the limestone in the El-Sheikh Fadl section has been elevated and exposed to the meteoric regime to a greater extent than the limestone in the Wadi El-Dakhl section. Porosity and permeability results revealed two zones of best reservoir quality were recognized in the middle and upper parts of the El-Sheikh Fadl section, as represented by the lime mudstone (FT1) and the foraminiferal packstone (FT4) with reservoir quality indexes of 0.50 < RQI < 1.50 μm. In contrast, the foraminiferal wackestone/packstone (FT3) has a poor reservoir quality (RQI < 0.25 μm) due to its fractures and pores are filled with crystalline calcite.
The petrographic studies and quantitative analysis of benthic foraminifera show that foraminifera from the outer shelf to the upper slope (e.g., Osangularia plummerae, Anomalinoides acutus, and Cibicidoides dutemplei) predominate throughout the successions investigated. The middle and uppermost intervals of the El-Sheik Fadl section are made up of lime-mudstone microfacies inhabited by inner to middle shelf benthic foraminiferal species (e.g., Uvigerina cf. farinosa, Siphogeneroides eleganta, and Valvulineria scorbiculata). These results suggest that the studied rocks were deposited in various environments, ranging from upper slope to middle shelf paleodepths. Furthermore, we propose that the lower Eocene carbonate rocks of the Thebes Formation were deposited mainly in a regressive phase (HST). The inferred relative sea-level falls gradually, consistent with the global sea-level decline, to generate progradational parasequence sets. Consequently, depositional environments, textural properties, and diagenesis are predicted to be the factors controlling the reservoir quality of the Thebes Formation, according to the relationship between reservoir zonation and sequence stratigraphy. Finally, eustatic sea-level changes and tectonics were the primary factors regulating the deposition of the Thebes Formation on the Tethyan Ocean's southern margin.