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The studied oilfield in the Zagros folded belt accommodates hydrocarbons in the Cretaceous carbonate rocks of the Ilam and Sarvak Formations. The compressional tectonic regime of the study area is reflected in the presence of the NW-SE trending active Ch.1 and Ch.2 thrust fault-related anticlines and associated NW-SE trending reverse faults. The NW-SE trending reverse faults have formed a pop-up structure in the crest of the Ch.2 anticline dividing it into 3 sectors (A, B, and C) with different geomechanical characteristics. Sector B as the high-strain central part of the pop-up structure despite Sectors A and B as the low-strain parts (in the footwall of the reverse faults forming the pop-up structure), is considered a high-risk region for performing acid fracturing stimulation of the reservoirs. In seismic time slices at the Gachsaran level, ChF.1 and ChF.2 faults with NE-SW trend display sinistral and dextral displacement respectively, representing tear faults related to the basal thrust of the Zagros Fore-dip Fault. Horizontal and vertical stress magnitudes for the study area were derived from well-log data. The horizontal stress orientations were established from the dominant directions of the induced fractures and breakouts observed from the FMI log data and are in ~ 030° (NE-SW) and ~ 120° (NW-SE), respectively. Based on these determined horizontal stress orientations, the expected induced fracture propagation direction is ~ 030° and their opening would likely occur in the direction ~ 120°. Based on this deduced stress information, the optimal azimuthal perforation would be from 000° to 060° for acid-fracture stimulation of Ilam and Sarvak reservoirs. [ABSTRACT FROM AUTHOR]

AbstractThe northwestern part of the Iranian sector of the Persian Gulf (NWPG) is a highly significant hydrocarbon offshore area, located within the southern Mesopotamian Foreland Basin. It lies in front of the Zagros Fold Belt along the subducting margin of the Arabian plate. Tectonic movements along the Hendijan–Bahregansar and Kharg–Mish palaeohighs influenced the Cretaceous stratigraphy of the NWPG. The reservoir significance of the Sarvak Formation makes data about microfacies, sedimentary environments, and sequence stratigraphy of the rock unit in areas adjacent to the palaeohighs (Hendijan, Bahregansar, Kharg, and Dorood oilfields) quite valuable. Rock samples and available palaeologs are used to define the microfacies and to interpret the depositional environments of the Sarvak Formation from the Dorood, Bahregansar, and Hendijan subsurface sections. Seven microfacies attributed to three sub-environments, namely, inner-, middle-, and outer ramp are designated. The absence of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area accumulated on a homoclinal ramp. Additionally, three third-order depositional sequences are identified. The first depositional sequence signifies that during late Albian–Cenomanian the area was structurally stable while in Cenomanian (the second and third sedimentary sequences) the Sarvak Formation comprises shallow facies as the result of the sea level fall. Finally, the late Cenomanian epirogenic movements and uplift along the Kharg–Mish and Hendijan–Bahregansar caused regression and erosion of the Cenomanian strata in adjacent areas.Keywords: Microfacies, Depositional environments, Sequence stratigraphy, Middle Cretaceous, Sarvak Formation, Persian Gulf. IntroductionThe Persian Gulf, 240000 km2 in the area, is bordered by the Abadan Plain and the southern part of the Dezful Embayment on the north, the Fars platform on the northeast, the Arabian Peninsula on the south, and Oman on the east (Ziegler 2001; Konyuhov and Maleki 2006; Ghazban 2007; Al-Qayim et al. 2018). Development of the Persian Gulf region occurred during the Late Cenozoic and after the formation of the Zagros Ranges (Ghazban 2007). The Hendijan–Bahregansar and Kharg–Mish are two major fault systems in northwestern the Persian Gulf. The Kharg–Mish palaeohigh was active from the Aptian whereas the former remained inactive till the latest Cenomanian when uplift along both resulted in favorable conditions for the formation of such oil traps as the Hendijan, Bahregansar, Kharg and Dorood oilfields. Of these, the Sarvak Formation is an important oil reservoir in the Zagros area (Motiei 1994; Van Buchem et al. 2011; Esrafili-Dizaji et al. 2015; Vincent et al. 2015). Tectonic movements along the Hendijan–Bahregansar and Kharg–Mish palaeohighs influenced the Cretaceous stratigraphy of the NWPG. The reservoir significance of the Sarvak Formation makes data about microfacies, sedimentary environments, and sequence stratigraphy of the rock unit in areas adjacent to the Hendijan–Bahregansar and Kharg–Mish palaeohighs in the Hendijan, Bahregansar, Kharg, and Dorood oilfields. Rock samples and the available palaeologs are used to define the microfacies and to interpret the depositional environments of the Sarvak Formation from the Dorood, Bahregansar, and Hendijan subsurface sections. Material & MethodsA total of 210 thin sections made of rock samples collected from the Sarvak Formation are examined regarding their petrographic, sedimentologic, and stratigraphic features. Additionally, available information, including palaeologs and unpublished internal reports documenting the Sarvak Formation in the Kharg Oilfield are consulted. The petrographic carbonate classification follows that of Dunham (Dunham 1962). Flügel (2010) facies belts and sedimentary models are also used. The sequence stratigraphic approach based on Sharland et al. (2001) model for the Arabian Plate and its subsequent updates (Bromhead et al. 2022; Van Buchem et al. 2011; Davies et al. 2002, 2019) are adopted herein. Discussion of Results & ConclusionsThe Sarvak Formation in the northwestern part of the Persian Gulf comprises 0–700 m carbonates with silty shales, dolomitized limestones, and dolostone intercalations. It succeeds the Kazhdumi Formation and precedes the Ilam or Gurpi formations. The stratigraphic distribution of foraminifera therein led to the introduction of some biozones/biofacies (Wynd 1965). These include Trocholina-Orbitolina assemblage biozone, Rudist debris biofacies, Nezazzata-alveolinids assemblage biozone and Oligostegina biofacies. According to the fossil content and also the lack of index fossils, the Sarvak Formation at the subsurface sections studied is dated as late Albian–Cenomanian.Seven microfacies attributed to three sub-environments, namely, inner-, middle-, and outer ramp are identified from the Hendijan, Bahregansar, and Dorood oilfields. Poorly fossiliferous dolomitized lime mudstone microfacies recognized in the Bahregansar and Hendijan oilfields are comparable with the tidal facies previously reported from the Kuh-e-Mond, Ahwaz oilfield, and the Sarvak Formation around Shiraz. The presence of peloidal miliolid grainstone, miliolid lime mudstone, and miliolid foraminifera grainstone microfacies shows that, for the most part, deposition occurred in such a low-energy environment as an inner ramp. These facies occurring in all wells studied are previously reported from the Masjed Soleiman, Kuh-e-Siah, Kuh-e-Mond, Kuh-e-Bangestan, Siri, and Ahwaz oilfields. Middle ramp sediments comprising rudist bioclast packstone-grainstone microfacies occur in the Dorood and Bahregansar oilfields and exhibit characteristics of high-energy environments. These facies have previously been reported from the Kuh-e-Siah, Kuh-e-Mond, Shivand stratigraphic section and Kuh-e-Bangestan. The outer ramp facies include benthic-planktonic foraminifera wackestone and planktonic foraminifera mudstone-wackestone. The main components of this microfacies are planktonic foraminifera accompanied by oligosteginids. The absence of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area accumulated on a homoclinal ramp.Additionally, three third-order depositional sequences are identified. The first is incomplete as its lower boundary occurs within the Kazhdumi Formation. The Transgressive System Tracts (TST) starts with a reduction in the arriving of fine-grained siliciclastic input (shale) from the Arabian Plate was associated with inner to outer ramp biofacies. The Maximum flooding surface (MFS) was associated with the microfacies 6 and 7 in the Bahregansar and Hendijan oilfields. In contrast, the MFS in other wells is located in the inner ramp (microfacies 3 and 4). The Highstand system tract (HST) is represented by a shallowing upward sequence in the middle to inner ramp facies with diverse benthic foraminiferal assemblages accumulated in a progradational pattern (microfacies 3). Comparison of the data from these wells with those from the Ahwaz oilfield (Bromhead et al. 2022), and well F in Kuwait shows that the MFS probably can be correlated with the K110 of Sharland et al. (2001) in other parts of the Arabian plate. Depositional sequences 2 and 3 are identified in the middle of the Sarvak Formation. The depositional sequence 2 represents the last one identified in the Bahregansar and Dorood oilfields due to tectonic activities along the Hendijan–Bahregansar and Kharg–Mish faults. The TST of this sequence starts with inner and outer ramp biofacies and the MFS was associated with microfacies 5 and 6. The sedimentary package representing stagnation (early HST) and the onset of sea level regression (late HST) includes inner ramp facies rich in benthic foraminifera with such porcelaneous and agglutinated representatives as Nezazzata, alveolinids, miliolids, and textulariids. The third depositional sequence (incomplete) is identified in the Kharg Oilfield. The TST starts with inner ramp biofacies and the MFS was associated with the microfacies 5. The HST is represented by inner ramp facies. Comparison of the data from these wells with those from the Arabian plate sequence stratigraphic model indicates that depositional sequences 2 and 3 are similar to the k120 and K130 identified from the Arabian plate.The first depositional sequence signifies that during late Albian–Cenomanian the area was structurally stable while in Cenomanian (the second and third depositional sequences) the Sarvak Formation comprises shallow facies resulting from the sea level fall. Finally, the late Cenomanian epirogenic movements and uplift along the Hendijan–Bahregansar and Kharg–Mish faults caused regression and erosion of the Cenomanian strata in adjacent areas.

AbstractThe Sarvak Formation, as one of Iran's most important hydrocarbon reservoirs, has been deposited and evolved under active tectonic conditions in various regions of the Zagros, including the Persian Gulf. The present study aims to investigate the effects of tectonic activities on the facies characteristics and diagenetic history of the Sarvak Formation in the eastern and western regions of the Persian Gulf. Facies analysis results indicate the deposition of the Sarvak Formation on a carbonate platform of ramp type. In the eastern part of the Persian Gulf, sedimentary facies of the Mishrif Member (Upper Sarvak) are predominantly deposited in inner platform areas (lagoon, shoal, reef deposits), while facies of the Khatiyah Member (Lower Sarvak) are mainly found in deep and outer platform areas (outer ramp). In the western part of the Persian Gulf, the Sarvak Formation encompasses a spectrum of shallow to deep-water facies, with open marine facies (middle and outer ramp) being notably more abundant. Two lower-order sequences (fourth-order) and one higher-order sequence (third-order) have been identified in this formation, indicating a high correlation with other areas of the Zagros and the Arabian Plate. Diagenetic effects related to a palaeoexposure surface have been observed in the uppermost part of the Sarvak Formation in both the eastern and western regions of the Persian Gulf. Dissolution porosity, brecciation, and iron oxide staining are among the most important diagenetic processes associated with these exposure surfaces. The thickness of the Sarvak Formation shows considerable variations in different parts of the Persian Gulf. Additionally, sedimentary facies of this formation exhibit significant lateral variations. The changes in thickness and facies variability of the Sarvak Formation were directly related to basement structures, indicating their activity during the deposition of this formation in the Persian Gulf. These tectonic events have had a significant impact on the burial history of the Sarvak Formation. Ultimately, the culmination of these facies and diagenetic characteristics has influenced the reservoir quality of the Sarvak Formation in the study areas.Keywords: Sarvak Formation, Persian Gulf, Basement structure, Facies, Diagenesis, Palaeoclimate IntroductionFacies characteristics and sedimentary environment, diagenetic processes, and fractures are the most important factors affecting the quality of hydrocarbon reservoirs (Ahr 2008). Therefore, considering tectonic features as one of the most important influencing factors on all the mentioned characteristics is a necessity in any integrated reservoir study (Nelson 2001; Hollis 2011; Lai et al. 2021). During the Late Cretaceous, especially in the Cenomanian-Turonian interval, tectonics was one of the main factors controlling the sedimentary basin of the Zagros region (Alavi 2007; Farahpour and Hessami 2012). These changes in the tectonic regime caused the reactivation of faults and salt domes in the region, creating a period of highly dynamic tectonic activity (van Buchem et al. 1996, 2002, 2011; Immenhauser et al. 2000, 2001; Sharp et al. 2010; Hollis 2011; Vincent et al. 2015). The Sarvak Formation, the second most important oil reservoir in Iran, has been significantly influenced by the aforementioned tectonic events, affecting its deposition and diagenetic history in various parts of the Zagros region. This is evidenced by the considerable variation in thickness and facies changes of the Sarvak Formation across different areas of the Zagros (Farahpour and Hessami 2012; Mehrabi et al. 2015; Bagherpour et al. 2021; Sadeghi et al. 2023).This study aims to integrate sedimentological data, including facies studies, diagenetic processes, and sequence stratigraphy, with existing information about the tectonic setting of the Persian Gulf during the Late Cretaceous, to examine the impacts of tectonic factors on the facies characteristics and diagenetic evolution of the Sarvak Formation in the eastern and western regions of the Persian Gulf. Material & MethodsIn this research, core data, thin-section microscopy, and X-ray diffraction (XRD) analysis results from two wells in two different fields in the eastern and western parts of the Persian Gulf, referred to as wells A and B, are utilized. The studied core length in Field-B is 23 meters, and in Field-A, it is 61 meters. The number of thin sections is 67 for Field-B and 265 for Field-A. The number of XRD analyses for well A is 28, and for well B, it is 12. The Embry and Klovan (1971) classification is used for the textural nomenclature of carbonate rocks. For facies analysis and reconstruction of the depositional environment, the standard model by Flügel (2013) is applied. The sequence stratigraphic division is based on the identification of certain significant surfaces (i.e., sequence boundaries and maximum flooding surfaces) that act as timelines in creating the chronostratigraphic framework of the target formation (Vail et al. 1977). In this study, the Transgressive-Regressive (T-R) sequence stratigraphy method is adopted for the depositional sequences of the Sarvak Formation. This method was mainly introduced by Curray (1964) and later developed by Embry and Johannessen (1993) and Embry (1993; 1995). Discussion of Results & ConclusionsThe Sarvak Formation was deposited in a carbonate ramp setting, which includes facies belts of the inner ramp, middle ramp, and outer ramp. Transitional facies between the inner and middle ramp areas are separated by the fair-weather wave base (FWWB). High-energy shoal complexes in the inner ramp are recorded in the Sarvak Formation (MF-5 and MF-6). These complexes are represented by grainstones and packstones containing benthic foraminifera, peloids, and skeletal fragments. The allochems of the shoal facies are mainly rudist debris and other bivalves, with less frequent fragments containing echinoderms, foraminifera, and peloids.The middle ramp facies assemblage encompasses a wide range of shallow to relatively deep marine facies, including mud to grain-supported (mudstone/wackestone to packstone) facies with planktic and benthic fossil assemblages (MF-1 and MF-7). Large benthic foraminifera and skeletal fragments (mainly bivalves and echinoderms) characterize packstones to wackestones attributed to the proximal middle ramp, while foraminifera (benthic and planktic) and skeletal fragments (mainly echinoderms, bryozoans, and red algae) characterize mudstones to wackestones in the distal middle ramp. Peloids and intraclasts are significant non-skeletal grains in these facies.The deep marine facies assemblage, including the outer ramp and basinal facies, forms a substantial part of the Upper Cretaceous carbonate sequences in the studied wells. They are recorded as low-energy, mud-dominated facies (mudstone to wackestone) where planktic organisms (such as planktic foraminifera and oligosteginids) are the dominant components (MF-2, MF-3, and MF-8). The outer ramp facies of the Sarvak Formation are primarily recorded as oligosteginid-rich and planktic foraminifera-rich (such as Hedbergella sp., Whitinella sp., Globigerinelloides sp., and Rotalipora sp.) mudstones to wackestones associated with middle ramp facies (distal parts).Diagenetic studies have shown that the Sarvak Formation has undergone marine diagenetic environments, meteoric diagenesis, followed by shallow and deep burial diagenesis. Based on the textural relationships of the diagenetic features and with the assistance of previous studies on the Sarvak Formation, the paragenetic sequence of the diagenetic characteristics of this formation is constructed.Due to local and regional sea level fluctuations during the Cenomanian-Turonian, the Sarvak Formation has experienced a complex diagenetic history in various parts of the Zagros region (Mehrabi and Rahimpour-Bonab 2014). Palaeoexposure surfaces had a major control on the diagenetic evolution of the Sarvak Formation (Rahimpour-Bonab et al. 2013; Hajikazemi et al. 2017; Mehrabi et al. 2022a, b). Depending on the presence or absence of palaeohighs (such as salt domes and horst structures), the number of these palaeoexposure surfaces and the intensity of the associated diagenetic alterations vary across different parts of the Zagros region.The four main stages of diagenetic evolution of the Khatiyah and Mishrif members of the Sarvak Formation are as follows:Stage 1: Marine transgression, sedimentation, and marine diagenesis of the Khatiyah Member in the early to mid-Cenomanian.Stage 2: Highstand of sea level, sedimentation, and marine diagenesis of the Mishrif Member in the mid- to late-Cenomanian.Stage 3: Relative sea level fall, emergence of the platform, exposure and meteoric diagenesis of the Mishrif Member at the Cenomanian-Turonian boundary.Stage 4: Deposition of shale facies in a lacustrine environment (Laffan Shale) during this stage.The diagenetic processes of the Sarvak Formation can be divided into two categories: 1) Diagenetic processes related to discontinuities, and 2) Diagenetic processes unrelated to discontinuities. Processes related to discontinuities represent a meteoric diagenetic environment and indicate exposure surfaces, such as extensive dissolution termed karstification, meteoric cementation, formation of paleosols, formation of pisoid horizons, dissolution-related brecciation and collapse, iron oxide staining, silicification, and neomorphism. Diagenetic processes unrelated to discontinuities include micritization, bioturbation, burial dolomitization, mechanical and chemical compaction, pyritization, and burial or marine cementation.Significant variations in the thickness of the Sarvak Formation from different parts of the Zagros sedimentary basin have previously been measured and reported by researchers (Mehrabi et al. 2015b). Additionally, remarkable changes in the nature of the facies and depositional sub-environments of the Sarvak Formation have also been reported from various regions of the Zagros (Esrafili-Dizaji et al. 2015). Similarly, such changes are clearly observable in terms of the thickness and facies of the Sarvak Formation in the eastern and western parts of the Persian Gulf. In the eastern Persian Gulf, salt domes have created the most significant tectonic structures, often associated with the formation of hydrocarbon fields in this area.In contrast, in the western part of the Persian Gulf, the pre-existing basement faults and uplifted structures (such as the Kharg and Mish anticlines, and the Hendijan–Bahregansar structures) are the most significant structural features and have formed the main traps for the accumulation of oil and gas in fields like Hendijan, Bahregansar, etc. (Kazem Shiroodi et al. 2015; Mohammadrezaei et al. 2020). On these pre-existing highs, the thinnest thickness of the Sarvak Formation has been recorded, and shallow-water facies dominate over them, indicating a shallower depositional environment on these highs. The changes of sedimentary facies in relation to these structures reflect their activity during the deposition of the Sarvak Formation in the eastern and western parts of the Persian Gulf.

This study investigates the diagenetic evolution and geochemical variations within the Sarvak Formation by integrating the core, thin-section, scanning electron microscopy, X-ray diffraction, trace elements, carbon-oxygen, and strontium isotopic data from three wells in the Abadan Plain. The formation displays diverse diagenetic processes, influenced by multiple exposure surface condition including the Cenomanian-Turonian boundary (CT-ES) and middle Turonian (mT-ES). Meteoric diagenesis, shaped by tectonic activities and eustatic sea level changes, played a pivotal role, leading to dissolution (karstification), brecciation, cementation, and palaeosol formation. Geochemical analysis revealed significant elemental variations across the studied wells. Magnesium (Mg) exhibited peaks at sequence boundaries, responding to facies changes, while iron (Fe) and manganese (Mn) concentrations increased below disconformities, reflecting changes in redox conditions and karstification. Sodium (Na) concentrations varied with facies changes, showing peaks at sequence boundaries, indicative of palaeoexposure events. Strontium (Sr) isotopes aided in delineating stratigraphic positions of palaeoexposure surfaces and absolute dating of sequences. Increases in87Sr/86Sr ratios below disconformities suggested subaerial exposure events, with shorter durations at CT-ES and longer durations at mT-ES, contributing to dissolution features development. The δ13Ccarb and δ18Ocarb values indicated severe meteoric diagenesis below sequence boundaries, characterized by dissolution and reprecipitation of calcium carbonate as low-magnesium calcite (LMC) cements. Cross plots of δ13Ccarb vs. δ18Ocarb displayed inverted-J patterns, indicative of meteoric diagenesis effects. Comparison with previous studies highlighted similar trends in geochemical variations, with notable differences in element concentrations between wells. This study enhances our understanding of diagenetic history of the Sarvak Formation in the Abadan Plain and, in a larger scale, provides insights into diagenetic evolution of palaeoexposure-related carbonate sequences. [ABSTRACT FROM AUTHOR]

Calculating the volume of hydrocarbons in place in carbonate reservoirs is associated with a high uncertainty due to the sedimentary texture and diagenetic characteristics. The Archie’s equation is one of the most common methods to measure the volume of reservoir fluids. Petrophysical parameters and geological attributes control the exponents of this equation. If Archie’s exponents (m and a) are considered constant, it can have a high uncertainty in the volume of reservoir fluids. This study investigates the effect of petrophysical parameters (pore type, pore size distribution, and pore throat radius) and geological attributes (sedimentary facies and diagenesis) on the Archie’s exponents. Based on petrography results, seven sedimentary facies related to a carbonate ramp were identified, and the diagenesis processes have caused a great heterogeneity of the studied carbonate reservoir. Archie’s exponents were calculated for four models of reservoir rock types. Finally, a total of five rock types were defined based on the study of thin sections, well logs, scanning electron microscopy images, special/conventional core analysis, and mercury injection testing. Combining pore types is influenced by diagenetic processes and sedimentary texture, so the diversity in the composition of pore types reduce the measurement accuracy of Archie’s exponents and resulted water saturation. The results show that pore type, pore size distribution, and fractures significantly affect the cementation exponent and the tortuosity factor. Accordingly, the determination of the rock types in heterogeneous reservoirs based on the pore size distribution, pore type, pore throat radius and fractures has the most significant impact on the accuracy of the Archie exponents. [ABSTRACT FROM AUTHOR]

AbstractThe Sarvak Formation, a crucial reservoir rock in the Abadan Plain, is extensively studied due to its sedimentary attributes and diagenetic evolution, heavily influenced by tectonic activities and palaeoclimatic conditions. This research focuses on analyzing palaeoclimatic indicators within the Sarvak Formation in a selected oil field in the Abadan Plain. Through a comprehensive approach integrating core data, thin section analyses, and electron microscopy, the study characterizes various facies, diagenetic processes, and sequence stratigraphy of the formation. Five distinct microfacies representing different depositional environments, such as lagoon, shoal, reef, reef-talus, and open marine belts, are identified, suggesting a ramp-type depositional setting. The investigation also reveals a paragenetic sequence of diagenetic processes, including transitions from marine to meteoric diagenesis and from shallow to deep burial environments. Notably, two palaeoexposure surfaces are identified, characterized by meteoric dissolution, brecciation, iron oxide staining, and silicification. Scanning electron microscopy analysis indicates prevailing kaolinite and montmorillonite clay mineral assemblages, indicative of warm and humid palaeoclimatic conditions. These findings provide insights into the palaeoclimatic conditions and paleaogeographical positioning of the Arabian Plate during the Cenomanian–Turonian period, suggesting a close proximity to low latitudes near the Equator.Keywords: Sarvak Formation, Palaeoclimate, Meteoric diagenesis, Abadan Plain IntroductionThe Middle East, particularly the Arabian Plate, is renowned for its vast oil and gas reserves. Among these, the Sarvak limestone formation in Iran's Zagros Basin stands out as a crucial reservoir rock. Its sedimentary characteristics and diagenetic processes are intricately linked to past climatic and geographical conditions. Climatic factors, along with fluctuations in sea levels and tectonic activity, have shaped highly productive reservoir units within the Sarvak Formation. Understanding these influences is pivotal for comprehensive reservoir studies, as they determine sedimentary facies and diagenetic alterations, thus impacting reservoir distribution within the formation.This study aims to analyze facies and diagenetic processes in the Sarvak Formation, particularly during the Cenomanian–Turonian period. By examining the effects of long-term climate on diagenesis, it seeks to enhance understanding of reservoir characteristics and improve modeling accuracy. Ultimately, this research aims to optimize hydrocarbon recovery from the Sarvak Formation's reserves, contributing to economic development in the region. Materials & MethodsIn pursuit of our study objectives, we examined 726 thin sections derived from the Sarvak Formation in two wells (K-01 and K-02) within the study area. The cored thickness of this formation in wells one and two amounted to 242 meters and 64 meters, respectively. We employed thin-section staining using a combination of alizarin red solution and potassium ferrocyanide to distinguish between dolomite and calcite. Generally, plug samples were extracted from the cored sections of the wells, with an average interval of one meter, focusing on capturing diagenetic processes during sampling. Additionally, we analyzed 65 SEM images using the MIRA3 TESCAN device at the Research Center of Razi Metallurgical Research Center in Tehran, Iran.For limestone examination, we utilized Dunham's (1962) and Embry and Klovan's (1971) classification methods. Furthermore, we applied Flügel's (2010) classification for facies description, interpretation, and establishing a conceptual sedimentary model. Our sequence stratigraphy studies aimed at identifying third and fourth-order sequences through the T-R method (separation based on transgressive systems tract and regressive systems tract). We also conducted petrographic studies to identify deepening and shallowing-upward facies to determine the maximum flooding surface (Embry 2002). Discussion of results & ConclusionsMicrofaciesThe study of skeletal, non-skeletal, and textural components related to thin sections of the Sarvak Formation in Wells 1 and 2 of the targeted oil field was conducted using the classification methods of Flügel (2010) and Dunham (1962) to identify five microfacies (Table 1). Benthic and planktonic foraminifera, rudists, corals, and echinoderms, along with other bio-clastic components, constitute the main constituents of these microfacies. Additionally, peloids and intraclasts have been identified as the most important non-skeletal components present in the studied microfacies. Sequence stratigraphyThe sequence stratigraphic study of the Sarvak Formation has led to the identification of two third-order depositional sequences named the Cenomanian sequence and the Turonian sequence, along with six fourth-order sequences. In the following, we will focus on a more detailed examination of the third-order sequences: Cenomanian Sequence (DSS-1)The thickness of the Cenomanian sequence in Well 1 is approximately 245 meters. The transgressive systems tract (TST) of this sequence consists of restricted lagoon facies (MF-1), reef and reef talus facies (MF-2 and MF-3), and open marine lagoon facies (MF-5), which are successively positioned upwards toward the maximum flooding surface (MFS-1). The regressive systems tract (RST) in the Cenomanian sequence comprises rudist and shoal facies (MF-4), indicating a shallowing-upward trend towards the Cenomanian-Turonian sequence boundary.The upper boundary of DSS-1, known as the Cenomanian–Turonian discontinuity (CT-ES), is exposed to meteoric diagenetic processes, displaying features such as meteoric dissolution, karstification, brecciation, and the formation of paleosols (Figure 3). The lower boundary of DSS-1 has not been identified in Wells 1 and 2 of the studied oil field. However, previous studies suggest its confinement at the base by the middle Cenomanian unconformity (Alsharhan and Nairn 1997; Aqrawi et al. 2010; Sharland et al. 2001; Hollis 2011). Turonian Sequence (DSS-2)The Turonian sequence in the studied wells has a thickness ranging from 15 to 25 meters. This sequence is predominantly composed of open marine lagoon facies (MF-5) in the lower half (TST) and restricted lagoon facies (MF-1) in the upper half (RST). The upper boundary of DSS-2 is characterized by a diagenetic boundary, exhibiting features such as silicification, brecciation, meteoric dissolution, and iron oxide staining. This discontinuity, known as the middle Turonian discontinuity (mT-ES), extends throughout the Tethyan basin (Sharland et al. 2001; Mehrabi et al. 2022). In the upper part of this sequence boundary, the shale of the Laffan Formation, dated to the Coniacian, limits the upper boundary of the Turonian sequence. DiagenesisThe types of diagenetic processes that have influenced the Sarvak sedimentary sequence in the studied section are as follows: micritization, cementation, recrystallization (neomorphism), dissolution, mechanical and chemical compaction, dolomitization associated with stylolitization, fracturing, silicification, pyritization, and formation of paleosol horizons. DiscussionDuring the Late Cretaceous era, the prevailing warm and humid climate in the Zagros region had a notable impact on the composition and distribution of carbonate organisms within the shallow-water carbonate platforms (Keller, 2008). Under such climatic conditions, the saturation level of seawater with respect to CaCO3 was often below the threshold, resulting in infrequent occurrences of evaporitic facies, primary dolomites, and ooids in the Sarvak platform. Instead, the platform predominantly consisted of bioclasts (such as rudists, algae, benthic and planktonic foraminifera, and molluscs), intraclasts, and peloids.Microfacies associated with shoal facies belts offer approximate insights into the climatic context of the Sarvak Formation. As previously noted, bioclastic-peloidal shoals indicate deposition in warm and humid climates (typical of the prevailing climatic conditions over the Sarvak carbonate platform during the Cretaceous). Conversely, algal-dominated shoals, often accompanied by evaporites indicative of warm and arid climates, are not observed in the studied oil field (Tucker and Wright 1990).Furthermore, the prevalence of organisms like rudists, corals, and green algae as the primary constituents of Sarvak Formation reefs, representing a coral-algal or coral-zoan skeletal association (Tucker and Wright 1990), underscores the dominance of warm climatic conditions in low latitudes (Flügel 2004).In warm and moist climates such as those during the Late Cretaceous era, like the conditions influencing the Sarvak Formation's deposition, the intensity and development of meteoric diagenetic processes heighten. Meteoric dissolution, karstification, low-magnesium calcite cementation, silicification, fracturing, and the development of paleosol horizons are all exacerbated. Conversely, in warm and arid climates, the meteoric diagenesis intensity diminishes due to limited meteoric waters, potentially leading to minimal sediment alterations over extended periods (Mehrabi et al. 2023).For instance, prolonged meteoric diagenesis of carbonate sediments under warm and humid climates can lead to the maturation and aging of karstified intervals, the principal product of meteoric diagenesis and sedimentation under such conditions. With increased dissolution and weathering, collapse events occur, filling dissolution cavities and caves with sedimentary debris, known as dissolution collapse breccias, compromising reservoir quality (Mazzullo and Chilingarian 1992; Keller 2008).Analysis of paleosol horizons from the Sarvak Formation reveals their rich content of montmorillonite and kaolinite minerals, with elevated iron and aluminum oxides. Upon closer examination away from discontinuities, the presence of Illite and Chlorite minerals becomes apparent. The occurrence of meteoric leaching and chemical weathering during the transition from the Cenomanian–Turonian and middle Turonian boundaries led to the formation of these paleosol horizons.

The Upper Cretaceous carbonate successions of the Sarvak Formation host giant oil reservoirs in the Persian Gulf. In this research, a total of 28 oil samples from nine oilfields located in the western, central and eastern parts of the Persian Gulf region were studied to determine the genetic relationships of oils, depositional setting of possible source rocks, thermal maturity, and source-rock ages in the Persian Gulf basin. According to the measured geochemical data, the source rocks facies vary from marine carbonates and marl/carbonates in the central and eastern oilfields to shale/carbonates in the western oilfields. The Pr/Ph ratio, steranes and terpanes suggest anoxic to dysoxic conditions of the depositional environments. The depositional environments experienced both low water stratification/low salinity and normal salinity/unstratified conditions. Evaluation of the saturated and aromatic biomarkers shows that all oil samples are mature and most of the source rocks lie within the beginning of the oil-generation window. The thermal maturity of the central oilfields is higher than that of the other samples, and has gone beyond the oil-generation stage. The C28/C29 steranes ratio suggest that the central oilfields of the Persian Gulf have Paleozoic and Jurassic source rocks, whereas the Sarvak reservoir in other parts of this region is sourced from Cretaceous carbonate rocks.

The Late Albian- Early Turonian Formation is the second-largest carbonate reservoir in the South and Southwest of Iran hosting significant accumulations of hydrocarbons deposited on the edge of the Neotethys. The purpose of this study is to analyze the sequence stratigraphy and Milankovitch cycles of the Sarvak Formation in three wells of the Anaran exploration block by using the Maximum Entropy Spectral Analysis (MESA) of gamma ray logs and integration with core studies to achieve a comprehensive understanding of the formation. The study of 353 microscopic samples led to the identification of 8 microfacies including (a) mudstone to oligostigenid wackestone with benthic and pelagic foraminifera, (b) packstone-rudstone, (c) rudistic with benthic foraminifers, (d) bioclast intraclast, (e) grainstone with benthic foraminifers, (f) peloid Wackestone to peloid wackestone with low benthic foraminifera diversity, (g) Peloid packstone with diverse benthic Foraminifers and (h) Bioclast Wackestone, which are distributed in three facies belts ranging from lagoon to shoal and open marine settings. In addition to four third-order sedimentary sequences were also identified based on facies changes combined with the Prediction Error Filter Analysis (PEFA), Integrated Prediction Error Filter Analysis (INPEFA) diagrams. Examining the Milankovitch cycles based on the analysis of the distances between gamma peaks and MESA revealed 100 thousand year cycles. Orbital dating according to the counting of the Milankovitch cycles indicates the equivalent age of 11 to 12 million years for the Sarvak Formation. [ABSTRACT FROM AUTHOR]

Carbonate reservoirs are characterized by their complex depositional–diagenetic history and, consequently, intricate pore networks. In this study, pore types and reservoir zones of the Sarvak Formation, which is the second most important oil reservoir in Iran, are investigated by integrating core, thin-section, porosity–permeability, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) data. Depositional, diagenetic, and fracture pores are identified in macroscopic to microscopic scales. Frequency analysis of pore types revealed that diagenetic pores, including vuggy, moldic, intercrystalline, and fracture pores, formed the majority of pore spaces of the Sarvak Formation. They are dominantly recorded in the regressive systems tracts of both the Cenomanian and Turonian sequences, just below the paleoexposure surfaces. Away from the disconformities, there are two general trends regarding the pore-types distribution: (a) grain-supported facies of shoal and talus settings with dominant primary (depositional) pores including interparticle, intra-skeletal, and growth-framework pores, which are concentrated in the middle part of the formation, and (b) microporous mud-dominated facies with various types of primary and secondary pore types in the TSTs of third-order sequences, especially in the lower part of the Sarvak Formation. Stratigraphic modified Lorenz plot differentiated reservoir, speed, barrier, and baffle zones in this formation. Depositional–diagenetic characters, petrophysical properties, and dominant pore types are defined for each zone in a sequence stratigraphic framework. [ABSTRACT FROM AUTHOR]

AbstractThe Sarvak Formation is one of the most important oil reservoirs in southwestern Iran. The Hendijan–Bahregansar–Nowrooz strike-slip Fault (HBNF) is known as a major fault system in the northwestern of the Persian Gulf, extending for approximately 700 kilometers in a north-northeast to south-southwest direction. This fault line passes through the northwestern region of the Persian Gulf, exerting a significant influence on the geological evolution of the area. This study emphasizes on importance of the microfacies, sedimentary environments, and sequence stratigraphy of the Sarvak Formation in the western part of the Hendijan–Bahregansar–Nowrooz palaeohigh, specifically those of the Hendijan, Bahregansar, and Mahshahr oilfields in southwestern Iran. A total of 186 thin sections of rock samples was examined in terms of their petrographic, sedimentological, and stratigraphic aspects. This led to the identification of seven microfacies distributed in four facies belts of tidal flat, lagoon, shoal, and open sea. The lack of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area formed on a homoclinal ramp. Additionally, five third-order depositional sequences were identified in the strata studied northwest of the HBNF. It can be concluded that the studied area was structurally stable during the Early Cenomanian. However, starting from the Late Cenomanian, significant tectonic phases resulted in the uplift of the area along an old ridge. Furthermore, the data indicate that the uplift of the Arabian Plate during the Early Turonian had significant effects on sedimentary processes in the region. This resulted in the retreat of the sea and the occurrence of a subsequent notable erosion phase at the Cenomanian–Turonian boundary in many areas including the Bahregansar and Hendijan oilfields. The interpretation of sedimentary characteristics and depositional environments in the upper part of the Sarvak Formation in the Mahshahr Oilfield relies on seismic sections, petrophysical logs, and microfacies analysis. Based on the available information the sediments apparently accumulated in a north-northwest to south-southeast trend, forming an onlap over both sides of a palaeohigh.Keywords: Microfacies, Depositional environments, Sarvak Formation, Persian Gulf, Sequence stratigraphy. IntroductionThe Persian Gulf is widely recognized as one of the most economically significant hydrocarbon basins in the world. The development of the Persian Gulf region occurred during the Late Cenozoic, at the northeastern edge of the Arabian Plate, where the Zagros Mountains are located in the north and northeast and the Arabian Plate (Ghazban 2007). The HBNF is a major fault system in the northwestern of the Persian Gulf with an NNE-SSW direction. This fault has resulted in the uplift of the Hendijan–Nowrooz palaeohigh and created favorable conditions leading to the formation of such oil traps as the Hendijan and Bahregansar.The Sarvak Formation represents one of the most important oil reservoirs in southwestern Iran. Tectonic movements along the Hendijan–Nowrooz palaeohigh influenced the sedimentatary history of the Sarvak Formation in this area. The type section of the Sarvak Formation in Tang-e-Sarvak comprises 821.5m of limestones with intercalations of shales and claystones. The rock unit transgressively overlies the Kazhdumi Formation and unconformably underlies the Gurpi Formation.The Sarvak Formation consists of limestones, shales, dolostones, and dolomitized limestones in the northwest region of the Persian Gulf. The stratigraphic distribution of foraminifera led to the introduction of some biozones/biofacies (Wynd 1965). These include Trocholina-Orbitolina assemblage biozone, Oligostegina facies, Nezzazata-alveolinids assemblage biozone, and Nezzazatinella-Dicyclina assemblage biozone (Sadeghi et al. 2021). This study emphasizes on importance of the microfacies, sedimentary environments, and sequence stratigraphy of the Sarvak Formation in the western part of the Hendijan–Bahregansar–Nowrooz palaeohigh, specifically those of the Hendijan, Bahregansar, and Mahshahr oilfields of southwestern Iran. Material & MethodsA total of 186 thin sections of rock samples from the study area were examined in terms of their petrographic, sedimentological, and stratigraphic features. The petrographic classification for carbonates is based on the Dunham classification (Dunham 1962). Wilson (1975) and Flügel (2010) facies belts and sedimentary models are also used. The schemes allow for the identification and differentiation of microfacies types based on their unique sedimentological characteristics. The sequence stratigraphic approach based on Sharland et al. (2001)'s proposed model for the Arabian Plate and its subsequent updates (Bromhead et al. 2022; Van Buchem et al. 2011; Davies et al. 2002, 2019) is followed herein. Based on the system tracts and using GIS software, changes in sedimentary environments in the study area are determined. Discussion of Results & ConclusionsSeven microfacies distributed in four facies belts of tidal flat, lagoon, shoal, and open sea are identified from the Hendijan (HD-Y), Bahregansar (BS-X), and Mahshahr (MR-1) oilfields. Poorly fossiliferous (dolo)mudstone microfacies recognized in the Bahregansar Oilfield are comparable with the tidal facies previously reported from the Kuh-e Mond, Ahwaz Oilfield, and Sarvak Formation in the Shiraz area (Qomi Aveili 2016; Kazem Zadeh and Lotfpour 2016; Mirzaee 2020).The presence of mud matrix in bioclast mudstone and miliolid-foraminifera mudstone-wackestone microfacies shows that, for the most part, deposition occurred in a low to moderate energy environment such as a lagoon and benthic foraminifera are the main skeletal grains. These facies occur in the Mahshahr and Bahregansar wells and were previously reported from the Kuh-e Siah, Kuh-e-Mond, and Ahwaz Oilfield (Gholami Zadeh et al. 2019; Kazem Zadeh and Lotfpour 2016).Shoal sediments are composed of bioclast-peloid packstone-grainstone and echinoid-rudist debris grainstone microfacies. These facies occur in the Mahshahr and Bahregansar wells and exhibit characteristics of high-energy sub-environments. They were also recognized in the Kuh-e-Siah and Kuh-e-Mond, as well as in the Ahwaz, and Azadegan oilfields. The palynostratigraphic analysis of the Wara and Ahmadi formations in Kuwait (well F) revealed facies characteristics indicative of tidal and lagoon sub-environments. In the study area, however, sediments of the Wara and Ahmadi formations exhibit characteristics of lagoon-shoal sub-environments.The open marine facies include benthic-planktonic foraminifera wackestone and planktonic foraminifera mudstone-wackestone. The main components of this facies are planktonic foraminifera accompanied by oligosteginids.The lack of turbidites and continuous reefs indicates that carbonates of the Sarvak Formation in the studied area formed on a homoclinal ramp. Additionally, five third-order depositional sequences were identified in the strata studied northwest of the HBNF.Depositional sequence 1 is incomplete because the lower boundary occurs within the Kazhdami Formation. There are type II sequence boundaries in the Mahshahr and Bahregansar oilfields while a type I sequence boundary is identified in the Hendijan well, due to tectonic activities of the HBNF in the Cenomanian. Comparison of the data from these wells with those from the Ahwaz Oilfield, and well F in Kuwait and Nahr-Umar in Iraq shows that the maximum flooding level (MFS) probably can be correlated with the K110 of Sharland et al. (2001) in other parts of the Arabian Plate.Depositional sequences 2 and 3 are identified in the middle of the Sarvak Formation. The depositional sequence 2 represents the last depositional sequence identified in the Bahregansar due to the tectonic activities of the HBNF. Comparison of the data from the wells studied with those from the Ahwaz Oilfield, wells F, and Nahr-Umar signifies that the MFS of depositinal sequences 2 and 3 are comparable with K120 and K130, respectively, of Sharland et al. (2001) in other parts of the Arabian Plate.Depositional sequence 4 is the last depositional sequence identified in wells F and Nahr-Umar. Based on the similar facies changes observed in the Mahshahr well with those from Ahwaz Oilfield, Nahr-Umar, and Well F, the MFS is probably comparable to the K140 of Sharland et al. (2001) in other parts of the Arabian Plate.Depositional sequence 5 is identified in the upper of the Sarvak Formation in the Mahshar Oilfield. The upper boundary of this depositional sequence was coincident with the Middle Turonian disconformity. The MFS appears to be comparable with the KTu1 of Bromhead et al. (2022) in other parts of the Arabian Plate.It can be concluded that the studied area was structurally stable during the Early Cenomanian. However, starting from the Late Cenomanian, significant tectonic phases resulted in the uplift of the area along an old ridge. Furthermore, the data indicate that the uplift of the Arabian Plate during the Early Turonian had significant effects on sedimentary processes in the region. This resulted in the retreat of the sea and subsequently, a notable erosion phase occurred at the Cenomanian–Turonian boundary in many areas including the Bahregansar and Hendijan oilfields. The interpretation of sedimentary characteristics and depositional environments in the upper part of the Sarvak Formation in the Mahshahr Oilfield relies on seismic sections, petrophysical logs, and microfacies analysis. Based on the available information, sedimentation occurred in a north-northwest to south-southeast trend forming an onlap over both sides of a palaeohigh.

The Sarvak Formation (Late Albian–Middle Turonian) is the most important carbonate reservoir in interior Fars in southwest Iran. The carbonate rocks of this formation have different and complex reservoir properties due to their tolerance to different diagenetic conditions and sedimentary environments. This formation in the Sarvestan oil field mainly consists of white to cream limestone with thin interbedded cream-colored argillaceous limestone and shale. This study aims to identify facies, interpret the sedimentary environment, and identify diagenetic processes and their effect on the quality of the Sarvak reservoir in the Sarvestan oil field. Four wells from the Sarvestan oil field were scrutinized to study petrography and sedimentary environment and to explore the equivalence of diagenetic processes of Sarvak formation from the Upper Cretaceous-aged (Bangestan Group) with the system tracts in the framework of sequence stratigraphy architecture. Based on the study, over 600 thin sections provided from cutting and cores, 14 carbonate microfacies were recognized to be deposited in 4 facies belt zone tidal flat, lagoon, shoal/bar, and open marine in a homoclinal carbonate ramp. Eight types of hybrid porosity derived from deposition, diagenetic and tectonic processes were determined by the petrographic investigations. Six depositional sequences were recognized according to lithological variation, log gamma changes, and vertical changes of microfacies. Facies’ associations strongly control the reservoir quality and dissolution near sequence boundaries (Cenomanian–Turonian and Middle Turonian). Some diagenetic features such as karstification, dissolution, cementation, and dolomitization are controlled by relative sea level change, especially in Lowstand Systems Tract (LST). Shoal/bar reservoirs near sequence boundaries at the top of HST have higher porosity than those in other facies belts and sequence positions. In general, in the TST and HST systems tract, the primary marine diagenetic processes, such as micritization and cementation, expands. In the LST systems tract and sequence boundaries, the meteoric diagenetic processes, such as dissolution, develop. The best reservoir quality occurs in the upper parts of the highstand systems tracts, which contain primary grain-supported intervals or dissolution porosity. The studies show that reservoir potential is controlled in a sequence stratigraphy framework (A combination of depositional environment and diagenesis) in the carbonate sequence of the Sarvak Formation in sequence stratigraphy framework. [ABSTRACT FROM AUTHOR]

Abstract The subsurface section of the Sarvak Formation with a thickness of 455 m is located in the North-Dezful Embayment. The Sarvak Formation is underlain by the Ilam Formation with erosional discontinuity and is overlain probably by the Shale of the Kazhdumi Formation (due to the lack of drilling to the underlying boundary). The probable age of this formation is Albian–Turonian. Based on petrographic studies of 2111 thin sections from cutting, the dominant lithologies are limestones and dolomitic limestones. Facies analysis has led to the identification of nine microfacies in three facies belts of the lagoon, bioclastic shoal and shallow open marine. The gradual changes in microfacies, lack of calciturbidite, slump structure and absence of large reefs structure, confirm the ramp type-carbonate platform. Diagenetic processes consist of micritization, neomorphism, physical and chemical compactions and dissolution. These processes have changed the reservoir quality of the Sarvak Formation. The quality of the reservoir is carried out in the framework of sequence stratigraphy and the Sarvak Formation is divided into third depositional sequences and six system tracts, and the reservoir potential of each system tracts was calculated and the result have shown 6 reservoir zones, which transgressive systems tract (TST)from the second depositional sequence has the highest reservoir potential in this well. Keywords: Sedimentary environment, Petrophysical evaluation, Sequence stratigraphy, Sarvak Formation Introduction The Albian–Turonian Sarvak Formation is the reservoir rock of many reservoirs in Zagros Basin and the Persian Gulf that holds significant parts of hydrocarbon reservoirs. Carbonate rocks are one of the most common reservoirs in the world, often in the form of limestones and dolostones (Motiei 1993). Iran's oil and gas reserves, such as the Asmari, Ilam and Sarvak formations located in the south and southwest fields of Iran, are accumulated in carbonate reservoirs. Dissolution of limestone and also brittleness have caused joints and fractures in them, resulting in increased porosity and permeability, which has provided suitable conditions for the formation of reservoir rock (Motiei 1993; Ghabeishavi et al. 2010; Asadzade et al. 2017). Studies conducted in central and eastern Zagros show that with the relative transgressive of sea level in the Middle Cretaceous, the sediments of the Sarvak Formation have been deposited on a homoclinal carbonate ramp with rudist patch reefs (Mehrabi et al. 2022). The position of studied oil field is located in the southwest of Iran, north of the Dezful Embayment. The structure has a northwest-southeast trend. The thickness of the drilling part of this formation in this well is about 455 meters, and its upper boundary with the Ilam Formation is erosional and the lower boundary is probably the Kazhdomi Formation. The study aims to understand the sedimentary environment, microfacies, diagenetic processes and reservoir quality of the Sarvak Formation. Material & Methods In order to determine sedimentary microfacies and diagenetic processes affecting carbonates of the Sarvak Formation, petrographic studies have been carried out on 2111 thin sections prepared from cuttings in the studied field. Classification for carbonate rocks used in this study is the terminology introduced by Dunham (1962) and its modified classification by Embry and Klovan (1971). The facies analysis and interpretation of the depositional environments were conducted using the standard microfacies of Flügel (2016). The quality of the reservoir has been evaluated by Geolog software and the multimin method. The sedimentary sequences are based on changes in microfacies, gamma ray logs peaks and biozones. The data in the palaeologs were used to identify the age of the Sarvak Formation. Discussion of Results & Conclusions The drilled thickness of the Sarvak Formation in this well is 455 meters. It consists of limestones, cherty limestones, dolomitic limestones and some shales. Generally, in order to determine the depositional environment of carbonates, it is necessary to study the constituent components and sedimentary facies. Therefore, petrographic analysis of the constituent components and identification of the facies have been conducted to determine the depositional environment of the Sarvak Formation in the studied field. The probable age of this formation is Albin–Turonian and it is unconformably overlain on the Ilam Formation and possibly resets on the Kazhdumi Formation. The petrographic studies led to the determination of nine facies associations deposited on three facies belts: the lagoon, shoal and the shallow open marine. The lagoonal microfacies are more abundant than the other facies belts. Considering the gradual changes of facies, the absence of large reefal structures, slump structures, and calciturbidite, the sedimentary environment of the Sarvak Formation in this study is a carbonate ramp setting. One of the most important diagenetic processes that led to a reduction in porosity in this formation is cementation. Processes such as stylolitization, dissolution, dolomitization and fracturing have resulted in increased porosity. The expansion of diagenetic processes including fracture porosity and vuggy porosity has improved the reservoir quality of the fine-grained sedimentary microfacies. Intergranular and fracture porosity predominantly occur in the grainstone facies, while vuggy porosity is mainly associated with the wackestone facies. Sequence stratigraphic studies have led to the identification of three 3rd-order depositional sequences and based on net-to-gross ratio, shale volume, porosity and water saturation, the TST of the second depositional sequence shows the best reservoir potential. The presence of detailed facies in conjunction with the depositional sequences and zones with reservoir potential indicates a direct relationship between the fine-grained grainstone microfacies associated with bioclastic units and vuggy microfacies associated with the lagoon environment with the zones exhibiting high reservoir potential.

The present study has focused on the lithology and mineralogy of the Bangestan reservoir shale unit, 5 meters thick at the 45 m below the upper boundary of Sarvak formation, located in the Lali oil field, to clear the existing lithostratigraphic ambiguities which refer it as a tongue of the Surgah Formation. After examining the petrophysical logs to determine the exact location of these deposits and core descriptions, the petrography of the collected samples was performed by light, reflective, cathodoluminescence and electron microscopy, and XRD analyses to recognize the mineralogy of the total sample as well as clay minerals. Core studying indicates that the unit is pyrite bearing gray shales. Moreover, based on petrography, the studied samples are argillaceous and fossil-free mudstone. The results of semiquantitative X-ray diffraction (XRD) analysis show that the mineralogy of these deposits is mainly clay minerals, calcite, quartz, and pyrite. The samples' average abundance of clay minerals is 56%, and the most frequent ones are the mixed-layered clays (illite/smectite), illite, and kaolinite, respectively. The electron microscopy (SEM) images often exhibit clay minerals of detrital origin. Due to the lack of fossils, the age of this unit was determined based on its stratigraphic position, which is surrounded by the Sarvak Formation deposits of the Cenomanian age and is distinguished from the Sorgah Formation (Turonian- Santonian age). Due to the absence of deep-marine organisms in this unit and the identification of lagoon and shoal carbonate microfacies in its surrounding layers, a calm and reducing lagoon environment is attributed to these sediments, which is against the deep marine environment of the Surgah Formation. Most common diagenetic processes (cementation, replacement, chemical compaction, and fracturing) indicate that the shale unit has been influenced mainly by shallow to deep burial diagenesis. There fore this shale unit with a shallow sedimentary environment, low thickness, relatively high content of detrital clay minerals, and their location at 45 meters below the boundary of Sarvak and Ilam formations belong to the Sarvak and is not correlatable with deep-marine shales of Surgah Formation. [ABSTRACT FROM AUTHOR]

AbstractThe Sarvak Formation, with a total thickness of 1566 m and major limestone lithology in the Ahvaz and Mansouri oil fields, is studied to determine its palaeoenvironment, diagenetic overprint and geochemistry. In the studied wells, the formation conformably overlies the Kazhdumi Formation and is overlain by the Ilam Formation. The petrographical studies led to identifying twelve facies precipitated in four major sub-environments, including inner-, middle-, and outer ramp and an intra-shelf basin on a homoclinal ramp-type setting. Dissolution, compaction, and cementation are the main diagenetic alterations that changed the primary chemical composition and the reservoir property of the Sarvak Formation. These diagenetic processes occurred in phreatic marine, meteoric, and burial realms. Sedimentary geochemistry of major and trace elements, including Ca, Mg, Fe, Mn, and Sr, along with O and C stable isotopes, reveal aragonite as the original carbonate mineralogy of the Sarvak Formation. The depositional environment in the lower Sarvak with the predominant shallow open marine and stratigraphic succession with no evidence of exposure, change to stratigraphic succession with more shallow sub-environments (lagoon and bioclastic-shoal), and the evidence of exposure such as cementation and extensive dissolution, which led to depletion in Sr, δ18O, and δ13C and enrichment in Fe and Mn during sea level fall in the inner ramp microfacies. The Fe and Mn cross-plot with a positive trend shows the effects of diagenetic phases in minor phreatic marine and mainly meteoric realm, all confirmed by petrography. The Sr/Ca ratio has the most correlation to the modern tropical warm-water shallow marine that could confirm the primary aragonite mineralogy; which is supported by the predominance of carbonate mud in the identified facies, primary isopachous fibrous marine cements, and extensive dissolution evidence. The cross-plot of Fe versus Sr and Mn versus Sr/Mn ratio suggests deposition in mainly an oxygenation state that experienced low to high water/rock interaction during different depositional sequences. The δ18O and δ13C co-variation and their comparison with the other fields of the Sarvak conducted in different regions show precipitation in an isotopic equilibrium in the carbonates with the Upper Cretaceous seawater, since calculating salinity (Z=130) and temperature (T= 26°C) of the Sarvak Formation confirm the above statement.Keywords: Diagenesis, Sedimentary geochemistry, Water/rock interaction, Upper Cretaceous, Sarvak Formation. Introduction:The Albian–Turonian Sarvak Formation (Motiei 1993), which forms a part of the Bangestan Group the second most important reservoir succession in the Zagros Basin and Persian Gulf (Esrafili-Dizaji et al. 2015; Assadi et al. 2016). This succession is equivalent to Mishrif, Ahmadi and Rumaila in Saudi Arabia, Natih in Oman, Derder in Turkey, and Mishrif in Iraq, which was deposited on a shallow carbonate platform along with interashelf basins on the passive margin of the Arabian Plate (Ziegler 2001; Piryaei et al. 2011; Alsharhan 2014). Various studies conducted on the geochemistry of carbonates reveal its adequacy in determining palaeoclimate and original mineralogy, defining the rate and type of diagenetic alteration, distinguishing diagenetic realms and trends as well as the stratigraphic position of depositional sequence stratal surfaces and boundaries (Adabi and Asadi Mehmandosti 2008; Crowe et al. 2013; Fallah-Bagtash et al. 2020; Omidpour et al. 2021). In this study, regarding the oil industry needs for exploration and production and based on petrographical and geochemical analysis, the facies, paloeoenvironment, diagenetic characteristics and geochemistry of the Sarvak Formation are studied in the northern Dezful Embayment, southern Iran. Material & MethodsThe facies analysis and diagenetic characteristics of the Sarvak Formation in two wells in the Ahvaz and Mansouri oil fields are studied using 900 thin sections obtained mainly from core samples. The nomenclature for carbonate rocks used in this study is the terminology introduced by Dunham (1962) and its modified classification by Embry and Klovan (1971). The facies analysis and interpretation of the depositional environments were conducted using the standard microfacies of Burchette and Wright (1992) and Flügel (2010). All thin sections were stained with potassium ferricyanide and Alizarin Red-S to distinguish carbonate minerals (Dickson 1965). The geochemical studies are done using analyzing trace and major elements including Ca, Mg, Fe, Mn, and Sr along with O and C stable isotopes of carbonate rocks both in the Exter University of England on 57 samples from the Ahvaz Oil Field and 43 samples from the Mansouri Oil Field. The trace and major elements are analyzed using Agilent 5110 VDV Inductively Coupled Optical Emission Spectrometer (ICP-OES) with 0.02 mmol/mol precision for Mg/Ca ratio, 0.4 µmol/mol for Sr/Ca ratio, 0.02 mmol/mol for Fe/Ca ratio and 0.006 mmol/mol for Mn/Ca ratio. For stable isotopes, the analysis is done using SerCon 20-22 Gas Source Isotope Ratio Mass Spectrometer (GS-IRMS) based on the Copenhagen University Standard (LEO, Carrara Marble) and with the precision of ±0.08‰ for δ13C and 0.28‰ for δ18O. Discussion of Results & ConclusionThe Sarvak Formation, with dominant limestone lithology and 831 and 735 m thickness, respectively in Ahvaz and Mansuri Oil Fields, conformably overlies the Kazhdumi Formation and is unconformably overlain by the Ilam Formation. The petrographical studies led to determinate five facies association deposited on a low angle ramp type setting along with an intrashelf basin. Diagenetic studies reveal the impact of marine, unconformity-related meteoric, and shallow and deep burial diagenesis that mainly caused intense compaction, dissolution and cementation. These main diagenetic features changed the nature of the associated facies regarding the reservoir property of the Sarvak Formation. The impact of main diagenetic features along the Sarvak succession improved the understanding of the elemental, and O and C stable isotope analyses. The geochemical analysis as well as petrographical evidences such as diversity of fauna and flora and primary marine diagenetic features confirm the original aragonite mineralogy for the Sarvak carbonate, which was formed in a warm and humid condition on a shallow marine platform. The analyzed δ18O and δ13C, along with the variable concentration of Sr, Fe and Mn suggest maintenance of the primary composition along with alteration in burial realm and mainly meteoric diagenesis. The cross-plot of trace element and O and C stable isotopes indicate that the Sarvak carbonate rocks altered in the open to close diagentic system during different depositional stratigraphic sequences depending on the exposure of the carbonate platform and its scale and duration

This study focuses on the intricate process of discerning productive layers within reservoir formations, taking the Sarvak Formation as a primary case. The employed methodology combines geology, comprehensive log interpretations, and petrophysical analyses to facilitate the evaluation of these productive layers. Initially, routine well logs are interpreted to determine key petrophysical parameters such as shale content, porosity, and water saturation. Subsequently, core measurement results are utilized for calibrating these log interpretations. The study further determines cut-off values through a calculated method of 5% cumulative hydrocarbon volume against porosity, shale content, and water saturation. These cut-off values are then applied to the petrophysical results to enhance their reliability. To resolve any inconsistencies or uncertainties in petrophysical evaluation, petrographical analyses, including scanning electron microscope imaging and thin section studies, are employed. The Sarvak Formation is categorized into seven distinct subzones, each thoroughly investigated to ascertain their respective productivity potential. The final results illustrate a substantial heterogeneity within the Sarvak Formation, revealing a range of diagenetic processes including compaction, dissolution, and cementation. Despite this complexity, three subzones are identified as the most productive layers with the maximum net pay, demonstrating the efficacy of the integrated approach.

Abstract This study focuses on reservoir heterogeneities of the Sarvak Formation in the Abadan Plain. Coefficient of variation (CV) and Dykstra-Parsons (VDP) approaches are used for quantification of reservoir heterogeneities. To define the hydraulic flow units, rock types, and reservoir zonation, flow zone indicator (FZI), Winland R35, and Lorenz (SMLP) methods are adopted. Heterogeneities of porosity and permeability data are quantified in each rock type, HFU and reservoir zone. Then, results of sedimentological studies are integrated with petrophysical data to analyze their scale, origins, and predictability in sequence stratigraphic framework. Depositional and diagenetic heterogeneities are differentiated. Facies variations and changes in textural characteristics and sedimentary structures provided small scale heterogeneities. Meteoric dissolution beneath the Cenomanian–Turonian palaeoexposure surface formed the best reservoir zone of the formation. Cementation and compaction, mostly related to burial diagenesis, reduced the reservoir quality. Results of this study indicate that large scale heterogeneities of the Sarvak Formation are predictable in the framework of third-order sequences. Meteorically dissolved rudist dominated facies provided the best productive zones in the regressive systems tract (RST) of the Cenomanian and Turonian sequences. However, small scale heterogeneities are not easily predictable in third-order depositional sequences and systems tracts. Keywords: Sarvak Formation, Reservoir heterogeneity, Coefficient of variation, Dykstra-Parsons, Sequence stratigraphy, Abadan Plain Introduction Reservoir heterogeneity refers to the changes in petrophysical properties of rocks (i.e., porosity, permeability, water saturation, capillary pressure). They have variable origins and scales (Nurmi et al. 1990; Tiab and Donaldson 2015). Carbonate rocks host considerable amounts of hydrocarbon resources in the Middle East and around the World. Carbonate reservoirs are strongly heterogeneous because of their complex depositional and diagenetic processes (Ahr 2008; Wei et al. 2015; Tavakoli 2020). Major parts of these heterogeneities are predictable in sequence stratigraphic framework (Lucia, 2007; Rahimpour-Bonab et al. 2012; Enayati-Bidgoli and Rahimpour-Bonab 2016; Mehrabi et al. 2019). Quantification of geological reservoir heterogeneities was the subject of some carbonate reservoirs in Iran (Tavoosi Iraj et al. 2021) and other countries (Nurmi et al. 1990; Dutilleul 1993; Fitch et al. 2015). This study presents the results of quantitative analysis of reservoir heterogeneities in the Sarvak Formation in an oilfield from the Abadan Plain. The main targets of this study are; A review of depositional setting and diagenetic history of the Sarvak Formation in the Abadan Plain. Sequence stratigraphic analysis of the Sarvak Formation. Reservoir rock typing and zonation of the Sarvak Formation. Quantification of reservoir heterogeneities based on the CV and VDP approaches, and Delineation of origins, scale of occurrence, and predictability of reservoir heterogeneities in sequence stratigraphic framework. Material & Methods Dataset of this study includes 258 m of core samples, 550 thin sections, porosity–permeability measurements (532 plug samples), and petrophysical logs (GR, LLD, LLS, RHOB, and NPHI) of the Sarvak Formation in one well from an oilfield in the Abadan Plain. Facies analysis was adopted using the standard facies nomenclature schemes (Embry and Klovan 1971; Dunham 1962) and facies models (Flügel 2010). Transgressive–Regressive (T-R) method is used for sequence stratigraphic analysis (Embry 2002; Catuneanu et al. 2011). Reservoir rock types and zones are identified based on the following approaches; Winland method This method uses R35 values by using the following equation: Log R35 = 0.732 + 0.588 Log (K) – 0.864 Log (φ) Hydraulic flow units (HFU) definition using the flow zone indicator (FZI) This method was firstly presented by Amaefule et al. (1993) that uses porosity and permeability data to calculate the reservoir quality index (RQI), normalized porosity (PhiZ), and flow zone indicator (FZI): RQI = 0.0314√(K/∅e ) ∅_z = ∅e/(1-∅e ) FZI = RQI/∅z Lorenz reservoir zonation Gunter et al. (1997) proposed a method of reservoir zonation that incorporates porosity and permeability data to calculate the storage capacity (PhiH) and flow capacity (KH): KHcum= K1 (h1-h0)/Khtotal + K2 (h2-h1)/Khtotal +…. + Kn (hn-hn-1)/Khtotal PhiHcum= Ф1 (h1-h0)/Фhtotal + Ф2 (h2-h1)/Фhtotal +…. + Фn (hn-hn-1)/Фhtotal Quantitative analysis of reservoir heterogeneities For quantification of reservoir heterogeneities, two methods are uses; Coefficient of variation (CV) method: CV = Standard Deviation (STDEVA) / Mean Dykstra-Parsons (VDP) method: = = Discussion of Results & Conclusions Facies analysis indicates that the Sarvak Formation has composed of eight microfacies deposited in a ramp type carbonate platform. They include lagoon (MF8), rudistid reef talus (MF7), shoal (MFs 4 to 6), and open marine (middle- to outer ramp) facies (MFs 1 to 3). Diagenetic processes include intense meteoric dissolution and cementation, dolomitization, mechanical and chemical compaction, neomorphism, burial cementation, and fracturing. Two third-order depositional sequences are defined with two major palaeoexposure surfaces as the sequence boundaries. They are attributed to the Cenomanian–Turonian boundary (CT-ES) and middle Turonian (mT-ES) palaeoexposure events (Navidtalab et al. 2016; Mehrabi et al. 2020,2022; Bagherpour et al. 2021). Nine hydraulic flow units (HFUs), six Winland’s classes, and ten Lorenz zones have been differentiated within the Sarvak Formation. Detailed sedimentological and petrophysical properties of HFUs, WRTs, and Lorenz zones are discussed. Statistical analysis of reservoir heterogeneities is adopted by using the CV and VDP approaches for all rock types and zones. Results of these studies and measurements have revealed that reservoir heterogeneities of the Sarvak Formation are originated from the both depositional (facies) characteristics and diagenetic alterations (especially meteoric diagenesis below the palaeoexposure surfaces). The best example for the control of facies on reservoir heterogeneities is the development of rudist-dominated facies with high reservoir potential within the RSTs of third-order depositional sequences (especially Cenomanian sequence). Consequently, a major part of reservoir heterogeneity in the Sarvak Fmormation depends on the distribution of such rudist-dominated intervals. On the other hand, palaeoexposure-related diagenetic processes had major control on reservoir properties of the Sarvak Fmormation, below the CT-ES and mT-ES. In this regard, meteoric dissolution, dolomitization, and fracturing have improved reservoir properties, and, in contrast, compaction and cementation largely decreased the reservoir quality in this formation. Results of this study have revealed that large-scale reservoir heterogeneities of the Sarvak Formation are traceable in the framework of third-order depositional sequences. In this regard, high-quality (reservoir) units of this formation are concentrated withing the RSTs of third-order depositional sequences. They have composed of dissolved, grain-supported facies of reef-talus and shoal complexes. In contrast, compacted and cemented mud-dominated facies provided non-reservoir units of the Sarvak Formation in the studied well.

An integrated geological‐petrophysical analysis of the rudist‐bearing sequence of the Cretaceous Sarvak Formation is given one giant oilfield, and provides an improved understanding of this main reservoir in the Abadan Plain, in the Zagros Basin, SW Iran. The main objective of this study is to evaluate reservoir potential of the Sarvak Formation, and then to utilize the calibrated well log signature to correlate reservoir potential in un‐cored wells. Eight main facies are recognized and categorized in five facies groups: lagoon, shoal, rudist‐biostrome, slope, and shallow open marine, deposited on a shelf carbonate platform. Given the distribution of diagenetic products and their effects on pore systems, three diagenetic facies namely, (DF‐1) low dissolution and cementation; (DF‐2); high dissolution; and (DF‐3) high cementation are differentiated. The initial sedimentary characteristics in combination with distribution of diagenetic products play an important role in reservoir quality heterogeneity. The effect of diagenetic processes related to disconformities mainly depends on the facies nature below these surfaces. Grain‐dominated facies of shoal and rudist debris, observed below the Cenomanian–Turonian disconformity, are mostly characterized by high dissolution and interconnected pore systems. Finally, depositional and diagenetic facies in the studied wells are correlated by petrophysical well log data, leading to distribution of the reservoir zones. Data obtained can be utilized for efficient reservoir characterization of the Sarvak Formation and its equivalent units in the Arabian Plate. [ABSTRACT FROM AUTHOR]

The cementation factor is one of the basic parameters for calculating water saturation and then hydrocarbon saturation of reservoirs. The best way to determine the cementation factor is through laboratory measurements. To generalize this coefficient for samples without laboratory measurements, experimental relationships versus petrophysical properties by researchers can be somewhat helpful. The method of artificial neural networks, with the help of training, validation, and data analysis, has given better results in determining the cementation factor of carbonate samples. It is one of the best methods to use petrophysical data as training data and make acceptable predictions with analytical methods. Therefore, laboratory measurement of the cementation factor has been performed for 159 carbonate cores from the Sarvak formation in southwest Iran. For the studied samples, the cementation factor in porosity was determined as a quadratic equation with the highest correlation coefficient. In this study, the compatibility of the experimental relationship shows better conformity by considering the permeability of each sample. Improvement of empirical relationships by the authors, correlation coefficients between the laboratory data, and the experimental relationships have been increased. Therefore, it is better to use improved experimental relationships for the studied carbonate samples. Artificial neural network methods have been used to process the data, best adapt the laboratory data, and present a suitable model. The Bayesian Regularization algorithm with five hidden layers has the least error in the test, validation, and testing stages.

Carbonate reservoirs are characterized by their complex depositional–diagenetic history and, consequently, intricate pore networks. In this study, pore types and reservoir zones of the Sarvak Formation, which is the second most important oil reservoir in Iran, are investigated by integrating core, thin-section, porosity–permeability, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) data. Depositional, diagenetic, and fracture pores are identified in macroscopic to microscopic scales. Frequency analysis of pore types revealed that diagenetic pores, including vuggy, moldic, intercrystalline, and fracture pores, formed the majority of pore spaces of the Sarvak Formation. They are dominantly recorded in the regressive systems tracts of both the Cenomanian and Turonian sequences, just below the paleoexposure surfaces. Away from the disconformities, there are two general trends regarding the pore-types distribution: (a) grain-supported facies of shoal and talus settings with dominant primary (depositional) pores including interparticle, intra-skeletal, and growth-framework pores, which are concentrated in the middle part of the formation, and (b) microporous mud-dominated facies with various types of primary and secondary pore types in the TSTs of third-order sequences, especially in the lower part of the Sarvak Formation. Stratigraphic modified Lorenz plot differentiated reservoir, speed, barrier, and baffle zones in this formation. Depositional–diagenetic characters, petrophysical properties, and dominant pore types are defined for each zone in a sequence stratigraphic framework.

In this study, the Sarvak Formation in the Kuh-e Zana stratigraphic section has been investigated in terms of Cenomanian biostratigraphy, microfacies analysis, and depositional environment interpretation. The section is located in the Interior Fars zone in the Zagros Basin. The selected section of the Sarvak Formation consists of 211 m of limestone and overlies the Kazhdumi Formation conformably whereas the upper limit of the Sarvak Formation is not obvious. Based on paleontological data three biozones in the study area were established: (1) Nezzazata simplex interval zone (Early Cenomanian), (2) Chrysalidina gradata interval zone (Middle Cenomanian), (3) Cisalveolina fraasi (fallax) & Cisalveolina lehneri—Praetaberina bingistani assemblage zone (Late Cenomanian). Based on petrographic studies, eleven microfacies were identified as follows: MF1: Oligosteginid bioclast wackestone to packstone, MF2: Bioclastic wackestone–packstone, MF3: Bioclastic peloidal wackestone–packstone, MF4: Rudist floatstone, MF5: Rudist rudstone, MF6: Foraminifera intraclastic grainstone, MF7: Benthic foraminifera rudist debris packstone–grainstone, MF8: Bioclastic grainstone–packstone, MF9: Bioclastic benthic foraminiferal wackestone to packstone, MF10: Bioclastic peloidal packstone, MF11: Algal-foraminiferal wackestone–packstone. Microfacies analysis shows that the Sarvak Formation comprises three paleoenvironments as the outer ramp, middle ramp, and inner ramp (shoal and lagoon). [ABSTRACT FROM AUTHOR]

This study examines the influence of tectonics and paleoclimate on the diagenetic evolution of Upper Cretaceous carbonate reservoirs (Sarvak Formation) in the Abadan Plain, Zagros Basin, Iran. Through petrographic analysis and scanning electron microscopy, the sedimentary facies, diagenetic processes, and pore system within the Sarvak Formation were analyzed. Facies analysis identified eight microfacies, which were deposited in shallow to deep settings of a carbonate ramp. Among these, the grain-dominated facies (packstone and grainstone) of shoal complexes and reef-talus settings were identified as the most important reservoir facies. The formation's complex diagenetic history includes marine, meteoric, and burial diagenetic processes, with meteoric diagenesis playing a significant role beneath paleoexposure surfaces. The Sarvak Formation was categorized into eight flow units, seven pore size classes, eleven electrofacies, eight velocity deviation zones, and thirteen Lorenz zones. The study's findings indicate that the most favorable reservoir units are located beneath the Cenomanian–Turonian exposure surface, characterized by significant dissolution of rudist-dominated facies due to meteoric processes and karstification. Tectonic uplift, along with weathering and erosion within a warm and humid (tropical) paleoclimate, significantly impacts the diagenetic evolution, pore types, and petrophysical properties of the Sarvak Formation. This research underscores the complex interplay between tectonics, paleoclimate, and petrophysical properties in the Abadan Plain, enhancing our understanding of carbonate systems in tectonically active and paleoclimatically dynamic regions. • Depositional facies and diagenetic features of the Sarvak Formation are investigated. • Pore types of the reservoir are determined by petrographic–petrophysical approach. • Reservoir zonation is adopted using the velocity deviation log and Lorenz plot. • Combined effects of tectonics ad paleoclimate governed reservoir quality of this formation. [ABSTRACT FROM AUTHOR]

Abstract Biostratigraphic studies of the Sarvak and Ilam formations in wells No. 1 and 14 of the Abteymour Oil Field led to the recognition of six species of foraminifers from the two mentioned formations for the first time. In this research, the species of Fischerina carinata and Spiroloculina cenomana in the Sarvak Formation and the species of Palaeosigmoilopsis apenninica and Sigmomassilina ottadunensis in the Sarvak and Ilam formations belonging to the suborder of MILIOLINA Delage and Herouard (1896) and the species Rotorbinella sp. nov. in the Sarvak and Ilam formation and Pararotalia boixae in the Sarvak and Ilam formations belonging to the suborder of ROTALIINA Delage and Herouard (1896) were identified and described. These species are not yet reported from the Sarvak and Ilam formations. Based on the identified fauna, the Sarvak Formation with a thickness of 812.98 and 238 meters in wells No. 1 and 14 respectively is Cenomanian–Turonian in age, and the Ilam Formation with a thickness of 151.7 and 136 meters in wells No. 1 and 14 respectively is Santonian–Campanian? in age. Keywords: Abeteymour Oil Field, Ilam Formation, Foraminifera, Rotaliidae, Sarvak Formation Introduction The Sarvak and Ilam formations, as one of the most important carbonate hydrocarbon reservoirs after the Asmari reservoir in the Zagros Basin, have special importance in petroleum studies. So far, several studies have been conducted on the two mentioned formations from different aspects especially biostratigraphic studies (James and Wynd 1965; Wynd 1965; Wood and Lacassagne 1956; Wells 1966, 1967, 1968; Stoneley et al. 1975; Speers 1967; Rahimpour-Bonab et al. 2012; Omidvar et al. 2014; Amiri Bakhtiar 1991; Khosroo Tehrani and Fonooni 1994; Allahbakhsh Ghiyasvand 2003; Teimourian 2004; Akbari Bas Kelayeh and Taheri 2005, 2006; Kamyabi Shadan 2005; Agh 2005; Sadeghi et al. 2006; Ghalavand 2009; Soleimani 2010; Ghobeyshavi et al. 2010; Sajjadi and Omidvar 2011; Omidvar 2011). However, these biostratigraphic studies of the Sarvak and Ilam formations in different areas of the Zagros Basin, introducing several biozones, some coexistence of foraminifera in these zones have been introduced . In the Dezful Embayment, due to the tectonic phases that occurred after Cenomanian and after Turonian (sub Hercynian tectonic phases), there is some absence of stratum or hiatus. The shallowing trend of the Late Albian to Cenomanian basin produced large amounts of shallow carbonate sediments of the Sarvak Formation in the Zagros Basin. However, during the Late Turonian, a regional uplift has taken place causing partial or complete erosion of the Turonian sediments, which create an erosional unconformitiy after Turonian, characterized by the development of iron-bearing sediments and sedimentary discontinuities in the Dezful Embayment and Fars areas. Omidi et al. 2018, in order to identify the fossil content with an emphasis on recognizing foraminifera of the Sarvak and Ilam formations in the south of Dezful embayment, studied two subsurface sections of wells 1 and 14 of the Abteymour oil field. They introduced four biozones based on the identification of 71 species belonging to 57 genera of foraminifera: one biozone in the Ilam Formation, called Biozone No. 1: Rotorbinella-Pararotalia Assemblage Zone, and three biozones in the Sarvak Formation called biozone No. 2: Nezzazatinella picardi-Dicyclina Schlumbergeri Assemblage Zone; biozone No. 3: Rotorbinella mesogeensis Total Range Zone and biozone No. 4: Nezzazata-alveolinid Assemblage Zone. Based on these identified biozones, the ages of the Sarvak and the Ilam formations are Cenomanian–Turonian and Santonian–Campanian, respectively. The identified benthic foraminifera in the Abteymour Oil Field is somehow similar to the other parts of the Zagros, but in addition to the benthic foraminifera reported in previous studies of these two formations; six other foraminifera species are identified and introduced from this oil field for the first time in this research. Since the Ilam and Sarvak formations face several high-pressure zones during drilling in the Abteymour oil field, identification of high-pressure oil and gas zones in strata is one of the issues that are of great importance. As a result, fossil studies, especially foraminifera can be helpful in finding the relative and sometimes exact location of high-pressure zones. The Ilam Formation in drilled wells in parts of the south of Dezful Embayment (Abteymour Oil Field) has low interest in terms of diversity of fossil genera and species (foraminifera); however, some foraminiferal members have been less studied, for example, the genus and species of the Rotaliidae family. In addition to the introduction of the Rotalia sp. 22-algae assemblage zone (due to the high diversity of Rotaliidae family members, sometimes up to 60 species, sp. 60) in the James and Wynd (1965) studies, and the high focus of systematic studies by Hottinger et al. (2014) on this family confirms the complexity and importance of the study in identifying and distinguishing the genus and species of this family; therefore, geologists on oil wells in Iran know the Ilam Formation as the "Rotalia Formation". Thus, the importance of the systematic study of the Rotaliidae family for separation of genera and species is more important. In this study, the foraminifera of the Ilam Formation have been systematically studied (to separate and distinguish several genera and species of Rotaliidae from each other), and this can be used to complete the chain of biostratigraphic studies in identifying high-pressure oil zones. In the Sarvak Formation, the study and knowledge of benthic foraminifera fossil assemblages could have great importance in the enrichment of fauna associations and recognizing biostratigraphic zones, determining the exact age of strata, knowing more local biozones, and identifying oil zones. Also, the rate of change in the diversity and abundance of foraminifera and their morphological characteristics increases in anoxic periods, for example, OAE1a (Leckie et al. 2002; Erba 2004). However, one of the biological characteristics of this incident is the presence of planktonic foraminifera with elongated chambers (Coccioni et al. 2006). However, here it is enough to describe the species that have been reported for the first time from the Sarvak and Ilam formations to identify more foraminifera and the fauna associations in the enrichment of the biozonations related to these two formation.. Material & Method In order to do biostratigraphy and identifying foraminifera in the Sarvak and Ilam formations in Abteymour Oil Field, the two wells of No.1 and 14, with the most suitable sample, were selected.A total of 1347 thin sections of the two wells (716 thin sections from well No. 1, mainly prepared from drilling cuttings and 631 thin sections from well No. 14, mostly prepared from drilling cores) were studied. These thin sections were studied microscopically to identify the microfossil contents, and after identification, they were photographed. Identification and description of the foraminifera were performed based on Loeblich and Tappan (1988), Boix (2007), Chiocchini et al. (2012), and Hottinger (2014). Discussion of Results & Conclusions The Sarvak and Ilam formations (Bangestan oil reservoir) are considered the second largest oil reservoir in Iran after the Asmari reservoir. Therefore, a careful study of the fossil content of these two oil formations is of particular importance in biostratigraphic studies. For this reason, in this study, a detailed study of the fossil content (especially foraminifera) of these two formations has been considered. The Ilam Formation in wells No. 1 and 14 has a thickness of 152 and 136 meters, respectively, and its lithology is mainly limestones and limestone with shale interbeds. The Sarvak Formation in well No. 1 is 813 meters thick, and in well No. 14, 238 meters of the Sarvak Formation has been drilled. The lithology of the Sarvak Formation is mainly limestones and dolomitic limestones. The biostratigraphy of the Sarvak and Ilam formations in wells No. 1 and 14 of Abteymour Oil Field, in addition to benthic foraminifera, which has been reported in previous studies of the Sarvak and Ilam formations, the following six benthic foraminifera are identified and introduced for the first time from these two formations:1- Fischerina carinata Peybernès, 1984 (Cenomanian, Sarvak Formation); 2- Spiroloculina cenomana Chiocchini, 2008 (Cenomanian, Sarvak Formation); 3- Sigmomassilina ottaduenesis Chiocchini, 2008 (Turonian, Sarvak Formation and Santonian–Campanian?, Ilam formations); 4- Paleosigmoilopsis apenninica Chiocchini, 2008 (Cenomanian–Turonian, Sarvak Formation); 5- Pararotalia boixae Piuz and Meister, 2013 (Cenomanian, Sarvak Formation and Santonian, Ilam Formation); 6- Rotorbinella sp. nov. (Turonian for Sarvak Formation and Santonian-Campanian? for Ilam Formation). These species have never been reported before from the Sarvak and Ilam formations.

The Late Albian-Early Turonian succession on the Arabian Platform stands out as one of the world's richest hydrocarbon provinces. This study delves into the Seismic Sequence Stratigraphy (SSS) of these successions to unravel reservoir characterization and characterized potential hydrocarbon plays. Utilizing data from five exploration wells, including well logs, selected core data, biostratigraphical information, and chemostratigraphy analysis, alongside a comprehensive 3D seismic volume, enabled the identification of four 3rd-order sequences and multiple 4th-order sequences within the studied successions. Sequence-1 (DS-I), dating back to the Late Albian age, features mudstone and bioclast wackestone in an open marine environment. DS-I is a key sequence, notably witnessing the disappearance of siliciclastic input from west to east in the study area. DS-II (Early Cenomanian) and DS-III (Mid-Cenomanian) were deposited on carbonate platforms with grain-supported facies in shoal and lagoonal depositional environments. A distinct regional unconformity at the Turonian base, evident on seismic sections, aligns with depleted δ18O values on the Cenomanian-Turonian Boundary. The maximum flooding surfaces of DS-I and DS-II are characterized by continuous reflectors with negative amplitude. Furthermore, the lateral grading of reservoir facies and their intercalation with non-reservoir facies within 4th-order sequences suggest that reservoir facies heterogeneity is strongly influenced by depositional processes at this scale. Potential reservoirs, particularly in the Highstand System Tract (HST) of each 3rd sequence, are highlighted, with DC-III's HST standing out, hosting patch reef facies resulting from local tectonic activities and carbonate build-up movements. • High resolution seismic sequence stratigraphy was applied to Reservoir characteristics. • New insight to distinguished petroleum play by using HRSS approaches. • Biostratigraphy and chemostratigraphy analysis to detect Sequence boundaries. • 3rd and 4th sequences were characterized within the Late Albian-Early Turonian successions. • Heterogeneity is strongly controlled by the depositional processes operated at the scale of 4th-order cycles. [ABSTRACT FROM AUTHOR]

To investigate biostratigraphy on the Sarvak Formation in coastal and sub-costal Fars (south of Iran), two subsurface stratigraphic section were selected. The selected sections are Aghar and Homa wells which are located in sub-coastal Fars and coastal Fars, respectively. Aghar well is located in the northwest of Ghir in Iran and the Sarvak Formation is observed from the depth of 787–1046 m and its thickness is 259 m, and Homa well is located in the Homa gas field which can be observed from the depth of 679–769 m and its thickness is 90 m. In terms of lithostratigraphy, the Sarvak Formation in these wells is composed of limestone in the lower part and limestone and shale in the upper part; furthermore, its lower lithostratigraphic limits in these wells with the Kazhdumi Formation are conformable, and the upper limit in the Homa well is covered unconformably by the Ilam Formation and the Aghar well is covered unconformably by the Laffan Formation. Biostratigraphy study in the Aghar and Homa wells revealed the occurrence of 18 genera and 20 species of both planktonic and benthic foraminifera. Besides, the following biozones in the Aghar well were identified in our biostratigraphical studies: Trocholina–Orbitolina assemblage zone (Late Albian-Early Cenomanian), Upper Kazhdumi–Lower Sarvak formations, Nezzazata–Alveolinids assemblage zone (Middle-to-Late Cenomanian), Upper Sarvak (Lower Ahmadi member (, Oligostegina biozone (Late Cenomanian), and Upper Sarvak (Upper Ahmadi member). Also in the Homa well, two biozones were identified which are Trocholina Orbitolina sp. assemblage zone (Late Albian-Early Cenomanian) and Nezzazata–Praealveolina cretacea assemblage zone (Middle-to-Late Cenomanian). [ABSTRACT FROM AUTHOR]

Appropriate drill bit selection can lead to enhance remarkable drilling operation efficiency and cost-saving. There are several factors affecting the bit selection process. Among these parameters, some factors such as specific energy (SE), cost per foot (CPF), formation drillability (FD), and rate of penetration (ROP) are considered the most important ones. One of the best approaches to select the optimum drill bit is to apply Multiple Criteria Decision-Making (MCDM) techniques. In this research work, Fuzzy Technique for Order-Preference by similarity to Ideal Solution (FTOPSIS) is used to choose the optimum bit for drilling operations in Sarvak and Asmari formations due to higher accuracy and validity of findings achieved from the FTOPSIS. With this respect, three types of bits (i.e. 517, 527, and 537) were considered as available candidates and were then ranked through the FTOPSIS, resulting in the best option (bit). In Asmari formation, similarity factors for bit types of 517, 527, and 537 bits were obtained as 0.479, 0.438, and 0.382, respectively indicating bit type 517 can be chosen as a proper option compared to other ones. Similarly, in Sarvak formation, results showed 0.5405, 0.5019, and 0.5622 values for 517, 527, and 537 bit types respectively, demonstrating the bit-type 537 is the most appropriate alternative in such formation. [ABSTRACT FROM AUTHOR]

In case of a decrease in reservoir pressure, injection projects are inevitable that the desired production rate is maintained. Moreover, one of the key issues in injection operations is optimum injection pressure determination to prevent fault reactivation . The aim of this study is to evaluate fault reactivation tenacity in one of SW Iranian oil fields at different injection stages. Two main steps in this study are geomechanical modeling and fault extraction. In addition, fullset data, image log, dipole sonic log, and MDT pressure points were used to construct the geomechanical model. Also, 3-D seismic data were used for fault extraction and characterization. Ultimately, according to the geomechanical model, the stress state is strike-slip normal, and the maximum horizontal stress direction is toward NE-SW. Furthermore, 18 faults have been extracted using seismic data, which their strikes are often NW-SE. In addition, the faults are inactive in the present-day stress state, but the first fault will be reactivated with a 33 MPa increase in pore pressure.

The Sarvak Formation is a carbonate sequence of Late Albian–Early Turonian age in Zagros Basin in Iran. In this investigation, the Sarvak Formation at the Pyun Anticline section (Izeh Zone) has been studied. It consists of 797 m thick limestone which overlies the Kazhdumi Formation conformably and underlies the Gurpi Formation unconformably at the Pyun Anticline. A very rich fossil association (85 genera and 132 species) characterizes the Pyun section. Based on the stratigraphic distribution of diagnosed foraminifera, five biozones are established: 1, Muricohedbergella‐Globigerinelloides sp. assemblage zone (Late Albian); 2, Praealveolina iberica‐Chrysalidina gradata interval zone (Early Cenomanian); 3, Chrysalidina gradata‐Cisalveolina fraasi (fallax) and C. lehneri interval zone (Middle Cenomanian); 4, C. fraasi (fallax) and C. lehneri‐Praetaberina bingistani assemblage zone (Late Cenomanian); 5, Nezzazatinella picardi‐Mangashtia‐Dicyclina assemblage zone (Early Turonian). Based on the petrographic and sedimentological analyses, some 13 carbonate microfacies were identified. The investigated microfacies confirm a ramp‐type paleoenvironment. These microfacies, from distal to proximal environments, consist of: MF1: planktonic foraminifera wackestone‐packstone; MF2: Oligostegina planktonic foraminifera wackestone to packstone; MF3: planktonic‐benthic foraminifers sponge spicules wackestone‐packstone; MF4: rudist floatstone; MF5: rudist rudstone; MF6: bioclast intraclast grainstone; MF7: peloid intraclast grainstone; MF8: benthic foraminifers rudist grainstone‐packstone; MF9: peloid bioclast grainstone‐packstone; MF10: bioclast (benthic foraminifers) wackestone‐packstone; MF11: peloid bioclast packstone; MF12: dacycladacea benthic foraminifera packstone‐wackestone; MF13: miliolids wackestone‐packstone. [ABSTRACT FROM AUTHOR]

The Sarvak Formation is a thick carbonate sequence of Cenomanian age that has been deposited in a foreland basin across the Zagros Basin. Sarvak Formation is the main petroleum reservoir rock in the Zagros Basin. To study the foraminiferal paleoecology in the Sarvak Formation, the section was selected in the east of Shiraz (Khane Kat section) and 66 samples with a total thickness of 140 m were prepared. Then, these thin sections were studied precisely based on the lithostratigraphy, microfacies, geochemical stratigraphy (chemical XRF analysis), and paleoecology. Besides, the biozones were identified and geochemical changes of elements in each biozone were investigated. Totally, three biozones were identified in which the hyaline tests were abundant in biozone-1 and porcelaneous tests were accumulated in biozones-2 and -3. Based on the study of the foraminifera dispersion pattern, the age of the Sarvak Formation in Khane Kat section was determined as Cenomanian. The results indicated that the various environmental factors affected the test type, test size and the quantitative expansion of the test in the sedimentary basin. Geochemical, microfacies and paleoecology data indicated that the most appropriate and inappropriate condition of life was belonged to wackestone and mudstone facies, respectively. Increase in the ratio of benthic to pelagic (B/P ratios) throughout the section and the enhancement of microfossils with agglutinated tests represented the shallowness of the sedimentary basin and retrogradation of the sedimentary basin from the open marine to the Subtidal and intertidal. [ABSTRACT FROM AUTHOR]

Sarvak Formation as the oil reservoir with the most hydrocarbon potential in Yaran oil field is of special importance. The present research was conducted to study the reservoir parameters of this Formation. For this purpose, petro-physical crude data was first prepared and using the Geolog6.7 software, a petro physical plot was drawn. Afterward, petro-physical properties of reservoir were done by modeling and using cross plots and logs, including porosity, permeability, lithology type, shale volume, water saturation and fractures analysis were investigated. Finally, with the adaptation of petro-physical parameters and other geological assessments from drilling cores, reservoir zoning and reservoir quality was analyzed. The obtained results showed that in the studied well there is a depth of 2600 to 2693 m of hydrocarbon potential friends. Effective porosity of the Sarvak Formation varies from 10 to 15%. Moreover, all of the investigated methods present of the prevailing lithology Carbonate, which can be on some outcrops of the shale layers, which has led to a decrease in reservoir quality in other zones.

Abstract Middle Albian to Cenomanian strata of the Sarvak Formation indicate pelagic facies in Tang-e-Chenarbashi stratigraphic section. These strata deposited in five sequences of the third order and nine sequences of the fourth order. These sequences were determinated in a succession including six microfacies mudstone, sponge spicule packstone, peloidal packstone, radiolarian packstone, Oligosteginid, planktonic foraminifera wackestone - packstone, benthic foraminifera echinoidal wackestone - packstone. Study of the third order sequences show that in some of them exist paraconformity which related to sequence boundary of the fourth order sequences. Investigating these sequence boundaries and recognition the sequences were done based on planktonic foraminifera biozones. In this study, separating the fourth order sequences is generally based on fossil fauna. In the fourth order sequences, maximum flooding surfaces (mfs) were recognized based on maximum diversity and abundance of planktonic taxa and decreasing of benthic foraminifera. Introduction The Sarvak Formation, one of the four formations of the Bangestan Group in Zagros basin, generally includes two facies, deep facies in the Lurestan province and shallow facies in the Khuzestan and Fars provinces. The Sarvak Formation deposited during Middle Albian to Early Turonian which the two main orogenic phases, Austrian (Albian / Cenomanian) and Sub-Hercynian (Cenomanian / Turonian) occurred. The warmest climate and the highest sea level took place along the Earth history in Albian to Cenomanian, and this could be a good and unique objective to investigation the effects of many interesting phenomena due to the amazing and unrepeated events. In addition, depositions of this time span could be included interesting depositional sequences. The aim of this research is recognition of depositional sequences of the Sarvak sediments by using of fossils. Material and Methods One stratigraphic section with 712 meters thickness was selected for studying of deep facies of the Sarvak Formation in Lurestan province. Totally, 148 thin sections were studied for recognizing microfacies characteristics and paleoenvironment based on Flugel (2010) and Wilson (1975). Study of sequence stratigraphy of the Sarvak Formation in the examined section was carried out based on Hunt and Tucker (1992, 1995) by using microfacies with fossil content. Discussion and conclusion A critical and detail biostratigraphic study with relay on planktonic foraminifera and microfacies is used here is based on Daneshian et al.(2011, 2013). On the basis of the mentioned references, age determination clears to us a Middle Albian to Cenomanian age for the Sarvak Formation in the selected section. Also, 6 microfacies including mudstone, sponge spicule packstone, peloidal packstone, radiolarian packstone, Oligosteginid, planktonic foraminiferal wackestone - packstone, benthic foraminiferal echinoidal wackestone – packstone, were determinated. Study of depositional sequences in the pelagic and hemipelagic sediments is complicated and confusing. In fact, lack of exposures and also similarity and monotonous between the facies constructing the depositional sequences, make it difficult to differentiate systems tracts. Geological field studies do not play a critical role in sequence stratigraphic studies of the Sarvak deposits in Tang-e-chenarbashi section. thus, we focused on microfacies data to find the system tracts. The microfacies show that the strata deposited in five third-order sequences. The problem displayed when we faced to some paraconformity existed in the third order sequences. The paraconformities were recognized by biostratigraphic studies. Since these paraconformities are belong to the fourth order sequences, we persuade to elicit the fourth order sequences for the studied section, but when we know this fact the cause of fourth and lower order sequences is Milankovitch cycles, we should draw our attention to fossils than microfacies. The purpose of this paper is not Cyclostratigraphy but is to clear role of fossils in separating depositional sequences. In this research two functions of fossil data are considered: 1) dating the depositional sequences using global biozones, and 2) investigation of system tracts where microfacies are ineffective tools. In this study the abundance of four main fossil groups including planktonic foraminifera, benthic foraminifera, oligosteginids and echinoid fragments illustrated against lithostratigraphic column, sea level changes and investigated sequences. Many features such as amount of planktonic foraminifera, broken fossil fragments, dramatically the increase or decrease in r-selected or k-selected taxa and many other similar to these, coincide with ts surface or mfs which help us to demonstrate the forth-order depositional sequences. This study tries to establish relation between classic usage of fossils just for age determining and use it as main allochem that is able to reflect many small scale events which tracking them in the deposit is impossible. Generally, five third-order sequences in a succession including six microfacies were recognized. Biostratigraphic data helped us to know the age of depositional sequences to attribute them to correct order with more confidence. Also, on the basis of extracted biozones, some paraconformities have been identified.

The uppermost Albian–Turonian interval has been studied in the south of the Izeh Zone (SW Iran) to investigate the depositional history of the Sarvak Formation in this area. This interval in the Middle East is characterized by the presence of very large shallow water carbonate platforms, surrounding the intra-shelf basins. The expansion of up to 90% hemipelagic strata throughout the studied outcrop proposes that it has been deposited in an intra-shelf basin. Three Cenomanian and one Turonian depositional sequences were distinguished in the studied interval based on the alternation of three microfacies associations including oligosteginid-dominated, oligosteginid-free, and platform-top microfacies. The biostratigraphical calibration by means of the planktic foraminifera indicates that these sequences can be well correlated on a global scale. The simultaneous influence of eustatic, tectonic, and environmental factors on the depositional sequences in the Lar Anticline section makes it difficult to identify the dominance of one over the others. The ability to correlate the sequence boundaries in the studied section with the global events suggests the effect of eustasy on the deposition of this interval. However, the tectonic control by the creation of a paleo-depression, subsidence (during the TST of Sequence 3), and uplift (at the Cenomanian/Turonian boundary (SB4) and mid-Turonian (SB5)) overprinted the eustatic factor. The influx of clay and the black shale formation during OAE2 conditions are the environmental factors that cannot be ignored in the deposition of sequence 1 and sequence 3, respectively. [ABSTRACT FROM AUTHOR]

The objective of this research is to study the relationship between reservoir parameters and seismic attributes in order to determine the reservoir quality, estimate the porosity model, and plan for infill drilling in the oil field under study, based on seismic and well log data. Sarvak Formation in this oil field is characterised, using a combination of the seismic data, porosity logs, and seismic attributes. By estimating the porosity model through various methods including single-attribute regression, multi-attribute regression, and artificial neural network, the probabilistic neural network has shown to represent reliable results. Interpretation of data shows that the estimation of porosity model for the Sarvak Formation indicates high reservoir quality. Besides, comparing the porosity values, the well BS-06 has higher porosity than the well BS-01, which indicates that, the higher reservoir quality at the BS-06 well location. The obtained porosity model, showed that the highest porosity values are found around the seismic CDP location No. 30850. Therefore, this area can be considered for prospective infill drilling in the field development plans. [ABSTRACT FROM AUTHOR]

Introduction: The Sarvak Formation is mainly composed of limestone and is one of the most important rock reservoirs in the Zagros sedimentary basin. This formation has been studied extensively in many parts of the Zagros; however the information about various aspects of this formation in northern Lorestan is limited. Due to the discovery of a pycnodont fish tooth plate from Aleshtar area, and because of scarce data on Cretaceous deposits in this region, we conducted this research. Material &Methods: In order to study the age, sedimentary environment and sequence stratigraphy of the Sarvak Formation, field and microscopic studies of 100 thin sections from an outcrop near Aleshtar (north of Lorestan) has been carried out. Discussion of Results &Conclusions: The Sarvak Formation in the studied section is Cenomanian in age and is 835 meters thick. It is bounded by Garu and Surgah formations at the base and top respectively. It is mainly composed of thick, medium, and thin bedded limestone rich in chert nodules, bioturbation and rudists. The studied outcrops can be subdivided into four units litho-stratigraphically. The first unit is 200 meters of thick, grey limestone with benthic foraminifera and echinoids. Second unit is 320 meters of mainly grey, medium bedded limestone with reddish intercalation of shale, containing benthic foraminifera, rudist debris and bioturbation. 180 meters of thick bedded, grey limestone with rudist and other bivalve debris forms the third unit. The last litho-stratigraphic unit consists of 135 meters of medium to thin bedded, grey limestone with interlayers of dolomitic limestone and rudist/bivalve debris and bioturbation. Benthic foraminifera are less abundant here. Our study led to the identification of 12 microfacies and 3 facies-belts, which belong to tidal flat, lagoon, and open marine environments. These microfacies are differentiated based on abundance and diversity of skeletal and non skeletal remains and rock texture. These facies, ordered from shallower to deeper environments, include: bioclastic stromatolite wackestone to bindstone, and pisoid rudstone in tidal-flat facies belt; bioclast peloid intraclast grainstone, bioclast green algae wackestone to packstone, peloid ooid grainstone to packstone, bioclast echinoderm benthic foraminifera wackestone, rudist benthic foraminifera wackestone, orbitolina miliolid wackestone to packstone, bioclast miliolid wackestone, and benthic foraminifera rudist floatstone to rudstone in lagoon facies belt; and rudist floatstone to rudstone, and bioclast oligosteginid wackestone within open marine facies belt. According to type and gradual changes of microfacies in lower parts of the Sarvak Formation, and compared to abrupt changes of microfacies and presence of extensive rudists in the upper parts of this formation, lower and upper parts of the Sarvak Formation are deposited in two different sedimentary environments. Presence of micrite in most of microfacies, formation of sedimentary textures such as wackestone to packstone and absence of traceable reefs on the ground, shows that the carbonate sediments of the Sarvak Formation in the lower parts in the studied section were deposited in a ramp carbonate platform. Widespread distribution of rudists in the field, abundance of oligosteginids, and abrupt changes of microfacies along the section; like alteration of pisoid and stromatolite facies to oligosteginids in a short interval, indicates that the upper parts of the studied section were deposited in a carbonate shelf. Three third-order depositional sequences were determined based on the deepening and shallowing trend of facies. The transgressive systems tract consists mainly of microfacies belonging to the lagoon and open marine sedimentary environments, which contain the benthic foraminifera, echinoids and oligosteginids. The highstand systems tract consists mainly of microfacies belonging to the lagoon with green algae and the benthic foraminifera, and also to the microfacies belonging to the tidal flat, which contain stromatolite and pisoid. The maximum progression level in sediment sequences 1 and 2 was determined by microfacies rich in echinoid and rudist debris. In sedimentary sequence 3 it is identified with oligosteginid rich microfacies.

Bangestan group in SW Iran consists of Sarvak and Ilam formations. The Sarvak and Ilam formations comprise a thick succession of carbonate rocks in Zagros basin of Southern Iran. These formations form one of the main hydrocarbon reservoirs in central Fars, Shiraz area. This study presents a facies analysis, sedimentary environments and sequence stratigraphy. To study petrography, microfacies, sedimentary environments and sequence stratigraphy of Sarvak and Ilam formations one stratigraphic section has been chosen and studied (MajAbad section). Facies analysis led to recognition of 13 microfacies and 2 lithofacies. Study of microfacies components such as allochems and ortochems, relative sea level change and sequence stratigraphy evidences show these facies were deposited in carbonate ramp sedimentary environment in 4 facies belt zones: open marine (A), Shoal (B), Lagoon (C) and Tidal flat (D) during Middle Creataceous. Petrography, microfacies analysis and sedimentary environment of Sarvak and Ilam formations show these formations in study area consist of 5 3 order depositional sequences. Sequence 1 to sequence 4 related to Sarvak Formation and sequence 5 covered of Ilam formation. These depositional sequences have been seperated by both type-1 and type-2 sequence boundaries. Sequence stratigraphy evidences show that the Sarvak and Ilam Formations constitute five separate depositional sequences which are separated by a thin palaeosol, representing a type-one and type -two sequence boundary which can be correlated with global curves of relative sea-level. [ABSTRACT FROM AUTHOR]

Albian–Cenomanian successions (Kazhdumi and Sarvak formations) represent remarkable variations in thickness, facies, fauna, and environments throughout the Zagros area. In the Coastal Fars (Charmu section), sedimentological and paleontological data evidence an intrashelf, with depths of 10s–100s m, surrounded by a shallow carbonate platform. Due to its depth, deposition of sequences in this setting has been controlled by eustatic sea-level changes rather than eurybathic changes, and several condensation episodes occurred related to marine transgressions. These observations are different from those in the adjacent sections in the Coastal Fars which recorded subaerial exposures instead. Combined with previous studies, this study denotes several intrashelf basins enclosed by a shallow carbonate platform on the southeastern margin of the Neo-Tethys during the Albian–Cenomanian. Development of intrashelf basins corresponds to basement faults in the Fars Salient. Likely, an extensional tectonic regime associated with a rifting event created horst–graben architecture by exerting extension along the basement faults and reactivating salt structures. Deposition on these troughs and highs led to the facies and thickness variations of the concomitant sequences. Development of several intrashelf basins on the southeastern margin of the Neo-Tethys indicates that syn-depositional continental rifting event could occur during the Albian–Cenomanian, prior to the tectonic inversion around the earliest Turonian. [ABSTRACT FROM AUTHOR]

Cenomanian/Turonian boundary (upper Sarvak Formation) benthic foraminiferal assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ13C positive excursion (peak "a"), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (" Heterohelix shift") in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ13C peaks "b" and "c") coincides with harbinger of Neo-Tethys closure in the Arabian Plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera. Unlabelled Image • The study interval was deposited during OAE2 across the C/T boundary. • OAE2 starts with rotaliporid extinction and population of infaunal benthic foraminifera. • Benthic foraminifera became absent in the black shale strata of W. arch. Zone. • From the middle of OAE2 upward local tectonic inversion controls benthic assemblage. [ABSTRACT FROM AUTHOR]

In this study, an integrated geomechanical analysis was carried out for the Sarvak carbonate reservoir in three wells of one of the oil fields in Abadan Plain, SW Iran. The static Young's modulus (ES), unconfined compressive strength (UCS), cohesion (C), and angle of internal friction (Φ) were determined directly, using rock mechanics tests. Subsequently, some correlations were introduced based on tests-to-logs relationships. The 1D geomechanical models were constructed for three wells in the north, center, and south of the studied field. The results show that the stress state is not uniform for the studied wells, as it is strike-slip normal in northern well, strike-slip in central well, and strike-slip reverse in southern well. Likewise, the stability of the wells was evaluated using Mohr-Coulomb and Mogi-Coulomb criteria, and it was observed that the Mogi-Coulomb criterion could more accurately estimate the breakout pressure. According to the recognized borehole breakouts in image logs, the SHmax direction is to NE-SW in all three wells. Based on the wellbore stability analysis, it was concluded that the more stable directions for drilling are NW-SE in the northern part, E-W and N-S in the central part, and NE-SW in the southern part of the studied oil field. [ABSTRACT FROM AUTHOR]

This study is scoped on biostratigraphy, and palaeodepositional analysis of the Sarvak Formation, Fars Zone, and Zagros, Iran. The biostratigraphy and microfacies of this Formation form four field sections, are discussed. The Sarvak Formation in the study area is located on Kazhdumi Formation and below the Gurpi Formation and comprise of cream to grey medium bedded to massive limestone. Seven foraminiferal assemblages have been identified from the Sarvak Formation and these assemblages form the basis of the biostratigraphy. The lower and upper parts of Sarvak Formation are lacking diagnostic microfossils and based on their stratigraphic position, Late Albian-Late Cenomanian, ages was assigned respectively. Field and microscopic data led to the identification of five facies groups; Planktonic foraminifera-dominated facies, Benthic foraminifera-dominated facies, Rudist facies, Orbitolina facies and Mudstone facies, these facies were subdivided into twelve subfacies, which were grouped into three depositional environments: inner, middle and outer shelf and overall representing a shelf setting. Facies changes during the Late Albian-Late Cenomanian were generally associated with carbonate shelf models. These containing a number of separate aspect, namely: Orbitolina conica sp.-dominated shelf with minor Orbitolina sp. during the Late Albian, a Trocholina sp. and Conicorbitolina-dominated shelf during the Late Albian, a Rudist-to algae-dominated shelf during the most of the Early Cenomanian and a pelagic and benthic foraminifera-dominated shelf during Late Cenomanian. In general, from the Kuh-e-Gadvan section to the southwest of the Fars zone (Rashtanu section), the depth of the sedimentary basin of Sarvak Formation increased. [ABSTRACT FROM AUTHOR]

The Sarvak Formation was deposited in the Zagros foreland basin and which was formed during the Albian to Cenomanian. This formation is a part of the Bangestan group comprising of a thick series of carbonate rocks and is one of the main hydrocarbon reservoirs in the south-west of Iran. The subsurface sedimentary sequence of the Sarvak Formation has been studied using core s taken from Gachsaran, Mansuri, and Kupal (55 samples) oil fields. Based on major (Ca, Mg) and minor (Mn, Fe, Na, Sr) elements and the rate of change in the carbon and oxygen isotope, aragonite was main carbonate mineralogy. Sr/Na concentrations which range from 1.29 to 7, show that carbonate of the Sarvak formation situated in warm water aragonite limestone. The elemental composition in Zagros oil fields carbonates also demonstrates that they have stabilized in a non-marine diagenesis environment. Variation of Sr/Ca vs. Mn values in Zagros oil field suggests that diagenetic alteration occurred in a semi-closed to open geochemical system. Changes in Mn, Fe, and V, show regenerated environment (without sulfate with Eh) with medium to low pH condition. These chemical indicators suggest variable paleo redox conditions in the Sarvak Formation. [ABSTRACT FROM AUTHOR]

This study focuses on the facies associations and depositional sequences of a Cenomanian/Turonian (C/T) outcrop, spanning the upper part of the Sarvak Formation in the Izeh zone, Zagros basin, Iran. The significant variations in sedimentary facies of the studied succession suggest an evolution from a late Cenomanian open-shelf to an early Turonian ramp system. This change is believed to be response of transition in the tectonic regime of Arabian plate from a passive to an active margin during the late Cretaceous. Two partial and one complete third-order depositional sequences and also several fourth-order sequences and small-scale cycles were recognized in the studied section. The global correlation of the third-order sequences suggests that they were eustatically-driven. However, local tectonic activities overprint the eustatic signature from the latest Cenomanian. The association of transgressions with cooling and regressions with warming during the fourth-order sequences support the aquifer-eustasy model as a controlling factor for the sea-level fluctuations under C/T supergreenhouse conditions. [ABSTRACT FROM AUTHOR]

The "facies" is a frequently used term for describing sedimentary units. In the literature, this term has been used for different purposes, as depositional environment, rock type, lithofacies, etc. In subsurface geology, the core samples are essential for facies studies. While lacking cored intervals, the well logs provide precious subsurface information, but the complexity of well log responses leads most of the time to the complexity of interpretations. In this paper, a method is proposed to study the facies types through well logs. The case study is a carbonate platform system, deposited in the upper Cretaceous, named Sarvak Formation, in one of Iranian onshore oilfields, located in the Abadan Plain. For this purpose, parametric and non-parametric (k-nearest neighbor) classifiers were applied to the dataset. Detailed petrography, assisted by core descriptions, led to 37 microfacies, grouped into three main lithofacies, four carbonate rock types and six environmental facies. Classifiers could not identify the microfacies due to a limited number of observations and high variations. However, the environmental facies were truly classified. In addition, lithofacies classification and Dunham carbonate rock typing were carried out correctly. It is discussed that k-nearest neighbor is clearly the outperformed classifier, and the classical parametric models were inadequate due to the nature of the input well logs (dependency of well logs and may be their distribution), and the overlapping of the input feature space. [ABSTRACT FROM AUTHOR]

A high–resolution carbon isotope record of pelagic carbonates (δ13C carb) from the Zagros Mountains, Iran, documents a 1.8‰ positive carbon isotope excursion (CIE) in the southeastern Neotethys during the Cenomanian–Turonian transition, corresponding to Ocean Anoxic Event (OAE2). The succession is controlled by biostratigraphy that includes the Rotalipora cushmani , Whiteinella archaeocretacea , and Helvetoglobotruncana helvetica biozones. The CIE is characterized by a positive shift culminating in a first positive peak (at 3.3‰), followed by a 1‰ negative excursion, accompanied by the highest δ18O carb values that can be interpreted as the result of a short–lived global cooling, the Plenus Cold Event. A subsequent second positive peak and a plateau of higher δ18O carb values are followed by relatively low values in the earliest Turonian akin to pre–excursion values. The biostratigraphic positions of the three diagnostic maximum points, referred to as peaks A, B, and C, within the generally positive CIE indicate a synchronous δ13C carb variability worldwide. The average δ13C carb values in southeastern Neotethys are generally lower compared to the open–ocean records of the same period. We attribute this offset to a substantial regional regression combined with recycling of isotopically light organic carbon. The diachronous deposition of black shales/mudstones after the onset of positive CIE suggests that its formation strongly depends on local paleogeography and paleoenvironmental factors. • Peaks A, B, C and Plenus Cold Event of the OAE2 are recognized in the Zagros Basin. • C-isotope perturbations of the OAE2 are synchronous in Zagros and across Tethys. • C-isotope values of the OAE2 in the Zagros Basin is 2‰ lower than open-ocean records. • A regional regression caused the lower C-isotope values and thinner OAE2 interval. • Deposition of black shales is a function of local, rather than global, conditions. [ABSTRACT FROM AUTHOR]

Paleontological studies lead to recognition of 21 genera and 16 species of benthic foraminifera, 5 genera and 6 species of planktonic foraminifera and 3 genera and 3 species of oligosteginids. The vertical distribution of fauna lead to identification of 5 biozones including: Favusella washitensis Range Zone, Oligostegina Assemblage Zone, Rudist debris Zone, Nezzazata-Alveolinids Assemblage Zone, Nezzazatinella-Dicyclina Assemblage Zone. Based on the indicated biozones, the age of the Sarvak Formation is Late Albian to Early Turonian in the study area. Eleven carbonate facies belonging to four environments including tidal flat, restricted and semi-restricted lagoon, shoal and open marine are recognized. The identified facies are deposited on the homoclinal ramp setting. Based on the vertical changes of facies and recognized depositional environments, four third-order depositional sequences are represented. The transgressive systems tracts mainly comprises of open marine facies including sponge spicule, oligosteginid, echinoid and benthic foraminifera, while the highstand systems tracts mainly consists of shoal facies rich in bioclast, and restricted and semi-restricted lagoon facies rich in porcellaneous and hyaline benthic foraminifera and peloid. The maximum flooding surface represented by open marine facies including echinoid and planktonic foraminifera

Abstract In this paper, an integrated study of micro-fractures is carried out in Sarvak Formation. To achieve this goal the data from three wells (A, B and C) in one of the southwestern Iranian oil fields were used. In this study petrographic thin sections, porosity-permeability core data, image logs, Dipole Shear Sonic Imager (DSI) log, Velocity Deviation Log (VDL), Hydraulic Flow Units (HFU) concept, resistivity logs and drilling mud reports were used to identify and analyze micro-fractures in Sarvak Formation. The micro-fractures were first detected in thin sections and image logs directly, then the results of other methods in micro-fracture zones were investigated. The results show there is an abnormal trend of porosity-permeability in core data which is compatible with micro-fracture zones. Also, the relationship between micro-fractures and flow units was studied. Eight HFUs were recognized using porosity-permeability core data and FZI* (Flow Zone Indicator-Star) method. Later each of the HFUs was divided into two sub-HFUs; one of them is well-matched with the micro-fracture zones. The negative parts of VDL were in good agreement with the micro-fracture zones. The slowness anisotropy was mainly affected by the micro-fracture zones, but the energy and traveltime anisotropies were under the influence of the other phenomena such as solution seams, shale layers and borehole washouts. The fast shear wave azimuth was mainly aligned with the S Hmax (maximum horizontal stress) direction, however, in micro-fracture zones, the direction suddenly changes. It was also observed that the micro-fractures do not have a considerable effect on Stoneley wave reflection, while the borehole washouts and shale layers have a great impact on this. The mud loss and mud invasion were very low in micro-fractured zones, which is related to the lack of lateral continuity of micro-fractures in the formation. The measured fracture apertures using thin section and image logs are often less than 0.1 mm, but the calculated values using Pezard-Anderson equations are very high. Highlights • Part of the recognized Hydraulic Flow Units matches with the micro-fractures. • Micro-fractures do not affect the quality of Sarvak reservoir in the studied field. • The energy anisotropy is strongly affected by solution seams and stylolites density. • The slowness anisotropy affected by the micro-fracture zones. • The mud loss and mud invasion are minimum in micro-fracture zones. [ABSTRACT FROM AUTHOR]

Palaeo-exposure surfaces are common geologic features in the Cretaceous sedimentary record of the Middle East. Two important disconformity surfaces have influenced the Cenomanian-Turonian intervals of the Sarvak Formation in the Abadan Plain. These surfaces had substantial controls on reservoir quality of their underlying succession. In this study obtained data from macroscopic core descriptions, microscopic petrography, geochemical analysis, spectral gamma ray log signatures, and clay minerals frequency in three hydrocarbon fields are combined and used for interpretations. Dissolution, cementation and recrystallization are the main diagenetic features related to the disconformity surfaces. Intensity and development of these diagenetic alterations are highly dependent on depositional facies characteristics of the underlying sequences. In mud-dominated facies, the main diagenetic process is recrystallization, while the grain-dominated facies were highly subjected by dissolution and cementation. Based on the geochemical analysis, there is an increase in Al 2 O 3 , SiO 2 , Fe 2 O 3 , Al, Fe, Mn, and Rb content beneath the disconformities. In contrast, the Sr concentration displays a decreasing trend towards the disconformities. On spectral gamma ray logs, depletion of the uranium and high values of CGR/SGR and TH/U ratios are attributed to the meteoric diagenesis. In addition, obtained data from SEM and XRD analysis indicate that the montmorillonite is the most abundant clay mineral in uppermost part of the Sarvak Formation. It shows the highest concentration at the disconformity surfaces which can be quoted to the impact of meteoric diagenesis and development of palaeo-soil horizons. These results are used for reconstruction of diagenetic sequence of the upper Sarvak Formation in subsurface sections of the Abadan Plain. [ABSTRACT FROM AUTHOR]