University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 42 2 2026 06 22 Identification of hydrocarbon fluid types through integrated drilling mud gas geochemistry and petrophysical analysis in the Dalan Reservoir Identification of hydrocarbon fluid types through integrated drilling mud gas geochemistry and petrophysical analysis in the Dalan Reservoir 1 16 29947 10.22108/jssr.2025.146481.1318 FA Farzad Haghighi Graduate student of sedimentology, Department of Geology, University College of Science, University of Tehran, Tehran, Iran Seyyed Mohammad Zamanzadeh Associate professor, Department of Geology, University College of Science, University of Tehran, Tehran, Iran Ebrahim Sefidari Assistant Professor, Department of petroleum exploration, Research Institute for Applied Science, Tehran, Iran Hamze Mehrabi Associate professor, Department of Geology, University College of Science, University of Tehran, Tehran, Iran 0000-0002-2211-4899 Soroush Ghaderi Graduate student of sedimentology, Department of Geology, University College of Science, University of Tehran, Tehran, Iran Journal Article 2025 08 27 Abstract The Dalan Formation, a major carbonate-evaporite succession in the Persian Gulf Basin, is a significant gas-bearing reservoir in fields such as South Pars. This study integrates mud gas geochemistry and petrophysical evaluation to characterize hydrocarbon fluid types and assess reservoir quality in zones K3 and K4. Mud gas concentrations (C1–C8) were analyzed using Pixler diagrams and Haworth parameters (Wh, Bh, Ch), while petrophysical evaluation included gamma-ray, density, neutron, and resistivity logs to estimate effective porosity, water saturation, and hydrocarbon saturation. Results indicate that Zone K3 is a dry gas reservoir dominated by methane and ethane, with moderate to low porosity and hydrocarbon saturation, and limited economic potential. Zone K4 exhibits dual behavior, with a lower water-bearing interval of poor quality and an upper interval of moderate to good quality containing wet gas and light condensates. Integration of geochemical and petrophysical data provides accurate identification of hydrocarbon types, productive intervals, and fluid contacts, offering a robust scientific basis for reservoir development planning. This approach is particularly valuable in complex carbonate-evaporite reservoirs, reducing exploration risk and optimizing production strategies. Keywords: Dalan Formation, Mud gas analysis, Reservoir petrophysics, Haworth-Pixler gas ratios, South Pars Introduction The Dalan Formation, one of the most significant carbonate-evaporite sequences in the Persian Gulf Basin, plays a crucial role in the country’s gas supply. Despite extensive petrophysical studies, integrated analyses combining mud gas geochemistry and petrophysical data have received limited attention. Identifying hydrocarbon fluid types and evaluating reservoir quality, particularly in zones K3 and K4, is essential for optimizing exploration and production strategies. This study aims to address this gap by integrating real-time mud gas measurements with well log and petrophysical data from a well in the South Pars gas field. Materials & Methods Mud gas data, including hydrocarbon concentrations from methane (C1) to heavier hydrocarbons (up to C8), were recorded continuously during drilling using on-site mud logging systems equipped with gas chromatographs and flame ionization detectors. Quality control procedures were applied to remove background gas, drilling-induced artifacts, and unreliable measurements using the Gas QC index. Valid data were analyzed through Pixler diagrams and Haworth parameters (Wh, Bh, Ch) to identify hydrocarbon types and productive intervals. Petrophysical evaluation utilized gamma-ray, density, neutron, and resistivity logs, combined with lithological and mineralogical interpretation, to estimate effective porosity, water saturation, and hydrocarbon saturation. Probabilistic petrophysical models enabled correlation between fluid types and reservoir quality, providing a comprehensive understanding of the K3 and K4 intervals. Discussion of Results & Conclusions Zone K3 (3262–3379 m) is predominantly a dry gas reservoir, dominated by methane and ethane. Petrophysical analysis indicates moderate to low effective porosity (3–20%) and hydrocarbon saturation (40–60%), within the lower interval (3300–3382 m) displaying poor reservoir quality due to high anhydrite content and elevated water saturation. Mud gas ratios (C1/C2–C1/C5) and Haworth indices consistently indicate a dry gas system with limited heavier hydrocarbons. Zone K4 (3382–3547 m) exhibits dual behavior: the lower interval (3385–3450 m) shows low porosity, high water saturation, and poor reservoir quality, whereas the upper interval (3450–3500 m) demonstrates moderate to good quality, with effective porosity of 5–20% and water saturation of 40–70%. Mud gas geochemistry and Haworth analysis reveal the presence of heavier hydrocarbons and condensates (wet gas), corroborated by increased resistivity and cross-over effects in density–neutron logs. Pixler diagrams indicate that while the majority of K4 remains in the gas domain, certain depths approach the gas/oil boundary, confirming the existence of mixed fluids. The integration of mud gas geochemistry and petrophysical data highlights significant heterogeneity within the Dalan Formation. Zone K3 is a mature dry gas reservoir with limited economic potential, while the upper interval of K4 contains condensates that enhance production prospects. Conventional methods such as well logging and reservoir testing, although valuable, may be insufficient in complex carbonate-evaporite settings due to lithological heterogeneity and operational limitations. In contrast, mud gas analysis provides a rapid, cost-effective, and complementary approach to assess hydrocarbon types, identify productive zones, and reduce exploration risk. The combined use of Pixler and Haworth methods allows precise differentiation between dry gas, wet gas, and light condensates, enhancing the accuracy of reservoir characterization. Data were collected from a single well, limiting the ability to directly generalize results across the entire South Pars field. Instrumental errors, depth alignment uncertainties, and variations in drilling conditions may influence the accuracy of gas geochemistry measurements. Nevertheless, the study provides a reliable scientific basis for understanding fluid distribution in the upper Dalan Formation. Conclusions of this study follows: Zone K3 is a dry gas reservoir dominated by methane and ethane, with low to moderate reservoir quality and limited production potential. Zone K4 consists of a lower poor-quality, water-bearing interval and an upper interval with moderate to good quality containing wet gas and light condensates, contrary to prior assumptions of exclusively dry gas. Integration of mud gas geochemistry and probabilistic petrophysics enables accurate identification of hydrocarbon types, productive intervals, and fluid contacts. The upper K4 interval represents a valuable target for condensate production and should be prioritized in reservoir development planning. This integrated approach provides critical insights for the South Pars gas field and similar complex carbonate-evaporite reservoirs, offering a scientific basis for optimized well placement, production strategy, and risk mitigation. Abstract The Dalan Formation, a major carbonate-evaporite succession in the Persian Gulf Basin, is a significant gas-bearing reservoir in fields such as South Pars. This study integrates mud gas geochemistry and petrophysical evaluation to characterize hydrocarbon fluid types and assess reservoir quality in zones K3 and K4. Mud gas concentrations (C1–C8) were analyzed using Pixler diagrams and Haworth parameters (Wh, Bh, Ch), while petrophysical evaluation included gamma-ray, density, neutron, and resistivity logs to estimate effective porosity, water saturation, and hydrocarbon saturation. Results indicate that Zone K3 is a dry gas reservoir dominated by methane and ethane, with moderate to low porosity and hydrocarbon saturation, and limited economic potential. Zone K4 exhibits dual behavior, with a lower water-bearing interval of poor quality and an upper interval of moderate to good quality containing wet gas and light condensates. Integration of geochemical and petrophysical data provides accurate identification of hydrocarbon types, productive intervals, and fluid contacts, offering a robust scientific basis for reservoir development planning. This approach is particularly valuable in complex carbonate-evaporite reservoirs, reducing exploration risk and optimizing production strategies. Keywords: Dalan Formation, Mud gas analysis, Reservoir petrophysics, Haworth-Pixler gas ratios, South Pars Introduction The Dalan Formation, one of the most significant carbonate-evaporite sequences in the Persian Gulf Basin, plays a crucial role in the country’s gas supply. Despite extensive petrophysical studies, integrated analyses combining mud gas geochemistry and petrophysical data have received limited attention. Identifying hydrocarbon fluid types and evaluating reservoir quality, particularly in zones K3 and K4, is essential for optimizing exploration and production strategies. This study aims to address this gap by integrating real-time mud gas measurements with well log and petrophysical data from a well in the South Pars gas field. Materials & Methods Mud gas data, including hydrocarbon concentrations from methane (C1) to heavier hydrocarbons (up to C8), were recorded continuously during drilling using on-site mud logging systems equipped with gas chromatographs and flame ionization detectors. Quality control procedures were applied to remove background gas, drilling-induced artifacts, and unreliable measurements using the Gas QC index. Valid data were analyzed through Pixler diagrams and Haworth parameters (Wh, Bh, Ch) to identify hydrocarbon types and productive intervals. Petrophysical evaluation utilized gamma-ray, density, neutron, and resistivity logs, combined with lithological and mineralogical interpretation, to estimate effective porosity, water saturation, and hydrocarbon saturation. Probabilistic petrophysical models enabled correlation between fluid types and reservoir quality, providing a comprehensive understanding of the K3 and K4 intervals. Discussion of Results & Conclusions Zone K3 (3262–3379 m) is predominantly a dry gas reservoir, dominated by methane and ethane. Petrophysical analysis indicates moderate to low effective porosity (3–20%) and hydrocarbon saturation (40–60%), within the lower interval (3300–3382 m) displaying poor reservoir quality due to high anhydrite content and elevated water saturation. Mud gas ratios (C1/C2–C1/C5) and Haworth indices consistently indicate a dry gas system with limited heavier hydrocarbons. Zone K4 (3382–3547 m) exhibits dual behavior: the lower interval (3385–3450 m) shows low porosity, high water saturation, and poor reservoir quality, whereas the upper interval (3450–3500 m) demonstrates moderate to good quality, with effective porosity of 5–20% and water saturation of 40–70%. Mud gas geochemistry and Haworth analysis reveal the presence of heavier hydrocarbons and condensates (wet gas), corroborated by increased resistivity and cross-over effects in density–neutron logs. Pixler diagrams indicate that while the majority of K4 remains in the gas domain, certain depths approach the gas/oil boundary, confirming the existence of mixed fluids. The integration of mud gas geochemistry and petrophysical data highlights significant heterogeneity within the Dalan Formation. Zone K3 is a mature dry gas reservoir with limited economic potential, while the upper interval of K4 contains condensates that enhance production prospects. Conventional methods such as well logging and reservoir testing, although valuable, may be insufficient in complex carbonate-evaporite settings due to lithological heterogeneity and operational limitations. In contrast, mud gas analysis provides a rapid, cost-effective, and complementary approach to assess hydrocarbon types, identify productive zones, and reduce exploration risk. The combined use of Pixler and Haworth methods allows precise differentiation between dry gas, wet gas, and light condensates, enhancing the accuracy of reservoir characterization. Data were collected from a single well, limiting the ability to directly generalize results across the entire South Pars field. Instrumental errors, depth alignment uncertainties, and variations in drilling conditions may influence the accuracy of gas geochemistry measurements. Nevertheless, the study provides a reliable scientific basis for understanding fluid distribution in the upper Dalan Formation. Conclusions of this study follows: Zone K3 is a dry gas reservoir dominated by methane and ethane, with low to moderate reservoir quality and limited production potential. Zone K4 consists of a lower poor-quality, water-bearing interval and an upper interval with moderate to good quality containing wet gas and light condensates, contrary to prior assumptions of exclusively dry gas. Integration of mud gas geochemistry and probabilistic petrophysics enables accurate identification of hydrocarbon types, productive intervals, and fluid contacts. The upper K4 interval represents a valuable target for condensate production and should be prioritized in reservoir development planning. This integrated approach provides critical insights for the South Pars gas field and similar complex carbonate-evaporite reservoirs, offering a scientific basis for optimized well placement, production strategy, and risk mitigation.

https://jssr.ui.ac.ir/article_29947_6b50bda08390573a966898da8a41e34a.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 42 2 2026 06 22 Depositional environment evolution and biogeochemical changes across the Permian–Triassic transition in the central Persian Gulf Depositional environment evolution and biogeochemical changes across the Permian–Triassic transition in the central Persian Gulf 17 50 29992 10.22108/jssr.2025.146573.1320 FA Nooshafarin Haghighat Ph.D., Department of Geology, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran Hossein Hashemi Associate Professor, Department of Geology, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran ٰVahid Tavakoli Professor, School of Geology, College of Science, University of Tehran, Tehran, Iran Journal Article 2025 09 05 Abstract In this study, the stratigraphic distribution of foraminifera and the environmental evolution of the Permian–Triassic boundary (PTB) successions in the central Persian Gulf (upper Dalan and the basal part of the Kangan formations) were investigated through the integration of micropaleontological, microfacies, and isotopic analyses (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr). The results reveal three local composite biozones within the upper Dalan and lower Kangan formations, along with the abundance of the index taxon Paradagmarita, which shows biogeographic affinity with approximately coeval strata in Turkey, the Caucasus, Saudi Arabia, and Oman. Microfacies analysis identified three facies belts including lagoonal, subtidal shoal, and tidal flat, indicating a general shallowing trend from lagoonal toward peritidal settings. A simultaneous decrease in δ¹³C and δ¹⁸O values was observed near the boundary, coinciding with the biotic crisis and the development of oxygen-depleted conditions. The increase in ⁸⁷Sr/⁸⁶Sr ratios across the boundary suggests enhanced influx of continental materials and intensified chemical weathering. The results also indicate a moderate increase in ooid size and significant changes in foraminiferal assemblages, reflecting a relatively shallow, warm, stressed, and oxygen-deficient environment. The novelty of this study lies in the combined application of three independent datasets, including sedimentological, geochemical, and paleontological, from multiple subsurface sections, enabling a more precise reconstruction of the environmental evolution and biotic events at the onset of the Triassic. Keywords: Foraminifera, Microfacies, Ooids, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Permian–Triassic boundary, Persian Gulf Introduction The Permian–Triassic transition marks the most profound biotic crisis in Earth’s history, with over 90% of marine species becoming extinct. This event significantly transformed carbonate platforms and their sedimentary environments. The Dalan and Kangan formations, representing the Late Permian to Early Triassic succession in the Zagros and Persian Gulf regions, record these changes in detail. Previous works have discussed the sedimentological and geochemical evolution of this interval (Abdolmaleki and Tavakoli 2016; Rafiei et al. 2016; Tavakoli et al. 2018; Haghighat et al. 2020). However, few studies have combined micropaleontological, isotopic, and microfacies evidence from multiple subsurface sections to elucidate the precise environmental evolution during this critical interval. The present study provides a comprehensive reconstruction of the Permian–Triassic boundary (PTB) based on integrated data from four wells in the central Persian Gulf. Materials & Methods The study is based on petrographic and geochemical analyses of 2,500 thin sections from the Dalan and Kangan formations in four wells (A, B, E, and F). Samples were taken at 30 cm intervals to ensure high stratigraphic resolution. Foraminiferal assemblages were identified under transmitted and polarized light microscopes. Microfacies were described and classified following Folk (1959), Dunham (1962), and Embry & Klovan (1971). Stable isotopes (δ¹³C and δ¹⁸O) were analyzed on micritic matrix samples to minimize diagenetic alteration. Samples with micritic fabrics (wackestones and fine packstones) were preferred due to their low permeability and better preservation of primary isotopic composition. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were determined from wells B, E, and F. Petrographic and geochemical screening confirmed the absence of secondary dolomitization or recrystallization features in the selected samples, following the procedures described in Tavakoli (2015) and Abdolmaleki and Tavakoli (2016). Discussion of Results & Conclusions Foraminiferal Assemblages: Micropaleontological analysis revealed 51 species (41 genera) of benthic foraminifera dominated by Globivalvulina, Dagmarita, and Paradagmarita. Two major biozones were defined: the Glomomidiellopsis–Paradagmarita Assemblage Zone (Changhsingian) and the Microconchus–Ammodiscus kalhori Assemblage Zone (Griesbachian). These assemblages correlate well with equivalent zones in Saudi Arabia, Oman, Turkey, and the Caucasus (Haghighat et al. 2020). The disappearance of Late Permian taxa and the dominance of opportunistic forms (Ammodiscus kalhori, Microconchus phlyctaena) above the boundary indicate severe environmental stress and low-oxygen conditions. Microfacies Evolution: Nine microfacies types were recognized, ranging from ooid and bioclastic grainstones to thrombolitic boundstones and dolomitic mudstones. The upward transition from subtidal grainstones to peritidal thrombolitic facies indicates progressive shallowing. Ooid grainstones in the uppermost Dalan suggest deposition in high-energy shoal settings. In contrast, microbial boundstones in the lowermost Kangan represent early recovery of carbonate production under oxygen-depleted, restricted conditions. This pattern matches earlier models for the PTB carbonates in the Persian Gulf (Tavakoli et al. 2018; Davoodi et al. 2024). Isotopic Trends: The isotopic curves show a pronounced negative excursion in δ¹³C (from +4‰ to −1‰) and δ¹⁸O, synchronous with a rise in ⁸⁷Sr/⁸⁶Sr ratios across the PTB. The δ¹³C drop corresponds to global disturbances in the carbon cycle, potentially related to methane release, enhanced continental weathering, and volcanogenic CO₂ emissions (Tavakoli and Rahimpour-Bonab 2012). The gradual depletion of δ¹⁸O toward the boundary likely reflects rising seawater temperature and meteoric diagenesis under greenhouse conditions (Abdolmaleki and Tavakoli 2016; Naderi-Khujin et al. 2016). Elevated ⁸⁷Sr/⁸⁶Sr ratios across the boundary further support intensified continental weathering and influx of radiogenic strontium during the end-Permian climatic crisis. Integrated Interpretation: Integration of faunal, sedimentological, and isotopic data indicates a shift from a well-oxygenated, open-marine platform during the Late Permian to a restricted, shallow, and stressed lagoonal system in the Early Triassic. The decline in biodiversity, increase in microbial facies, and negative isotopic excursions reflect the combined effects of eustatic fall, climatic warming, and oceanic anoxia. These changes represent the regional expression of global end-Permian perturbations. The integrated multiproxy approach employed here refines previous reconstructions (Haghighat et al. 2020; Nazemi et al. 2021; Davoodi et al. 2024; Rezvannia et al. 2025; Shahkaram et al. 2025) and provides a more complete picture of the environmental transition in the central Persian Gulf. Abstract In this study, the stratigraphic distribution of foraminifera and the environmental evolution of the Permian–Triassic boundary (PTB) successions in the central Persian Gulf (upper Dalan and the basal part of the Kangan formations) were investigated through the integration of micropaleontological, microfacies, and isotopic analyses (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr). The results reveal three local composite biozones within the upper Dalan and lower Kangan formations, along with the abundance of the index taxon Paradagmarita, which shows biogeographic affinity with approximately coeval strata in Turkey, the Caucasus, Saudi Arabia, and Oman. Microfacies analysis identified three facies belts including lagoonal, subtidal shoal, and tidal flat, indicating a general shallowing trend from lagoonal toward peritidal settings. A simultaneous decrease in δ¹³C and δ¹⁸O values was observed near the boundary, coinciding with the biotic crisis and the development of oxygen-depleted conditions. The increase in ⁸⁷Sr/⁸⁶Sr ratios across the boundary suggests enhanced influx of continental materials and intensified chemical weathering. The results also indicate a moderate increase in ooid size and significant changes in foraminiferal assemblages, reflecting a relatively shallow, warm, stressed, and oxygen-deficient environment. The novelty of this study lies in the combined application of three independent datasets, including sedimentological, geochemical, and paleontological, from multiple subsurface sections, enabling a more precise reconstruction of the environmental evolution and biotic events at the onset of the Triassic. Keywords: Foraminifera, Microfacies, Ooids, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Permian–Triassic boundary, Persian Gulf Introduction The Permian–Triassic transition marks the most profound biotic crisis in Earth’s history, with over 90% of marine species becoming extinct. This event significantly transformed carbonate platforms and their sedimentary environments. The Dalan and Kangan formations, representing the Late Permian to Early Triassic succession in the Zagros and Persian Gulf regions, record these changes in detail. Previous works have discussed the sedimentological and geochemical evolution of this interval (Abdolmaleki and Tavakoli 2016; Rafiei et al. 2016; Tavakoli et al. 2018; Haghighat et al. 2020). However, few studies have combined micropaleontological, isotopic, and microfacies evidence from multiple subsurface sections to elucidate the precise environmental evolution during this critical interval. The present study provides a comprehensive reconstruction of the Permian–Triassic boundary (PTB) based on integrated data from four wells in the central Persian Gulf. Materials & Methods The study is based on petrographic and geochemical analyses of 2,500 thin sections from the Dalan and Kangan formations in four wells (A, B, E, and F). Samples were taken at 30 cm intervals to ensure high stratigraphic resolution. Foraminiferal assemblages were identified under transmitted and polarized light microscopes. Microfacies were described and classified following Folk (1959), Dunham (1962), and Embry & Klovan (1971). Stable isotopes (δ¹³C and δ¹⁸O) were analyzed on micritic matrix samples to minimize diagenetic alteration. Samples with micritic fabrics (wackestones and fine packstones) were preferred due to their low permeability and better preservation of primary isotopic composition. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were determined from wells B, E, and F. Petrographic and geochemical screening confirmed the absence of secondary dolomitization or recrystallization features in the selected samples, following the procedures described in Tavakoli (2015) and Abdolmaleki and Tavakoli (2016). Discussion of Results & Conclusions Foraminiferal Assemblages: Micropaleontological analysis revealed 51 species (41 genera) of benthic foraminifera dominated by Globivalvulina, Dagmarita, and Paradagmarita. Two major biozones were defined: the Glomomidiellopsis–Paradagmarita Assemblage Zone (Changhsingian) and the Microconchus–Ammodiscus kalhori Assemblage Zone (Griesbachian). These assemblages correlate well with equivalent zones in Saudi Arabia, Oman, Turkey, and the Caucasus (Haghighat et al. 2020). The disappearance of Late Permian taxa and the dominance of opportunistic forms (Ammodiscus kalhori, Microconchus phlyctaena) above the boundary indicate severe environmental stress and low-oxygen conditions. Microfacies Evolution: Nine microfacies types were recognized, ranging from ooid and bioclastic grainstones to thrombolitic boundstones and dolomitic mudstones. The upward transition from subtidal grainstones to peritidal thrombolitic facies indicates progressive shallowing. Ooid grainstones in the uppermost Dalan suggest deposition in high-energy shoal settings. In contrast, microbial boundstones in the lowermost Kangan represent early recovery of carbonate production under oxygen-depleted, restricted conditions. This pattern matches earlier models for the PTB carbonates in the Persian Gulf (Tavakoli et al. 2018; Davoodi et al. 2024). Isotopic Trends: The isotopic curves show a pronounced negative excursion in δ¹³C (from +4‰ to −1‰) and δ¹⁸O, synchronous with a rise in ⁸⁷Sr/⁸⁶Sr ratios across the PTB. The δ¹³C drop corresponds to global disturbances in the carbon cycle, potentially related to methane release, enhanced continental weathering, and volcanogenic CO₂ emissions (Tavakoli and Rahimpour-Bonab 2012). The gradual depletion of δ¹⁸O toward the boundary likely reflects rising seawater temperature and meteoric diagenesis under greenhouse conditions (Abdolmaleki and Tavakoli 2016; Naderi-Khujin et al. 2016). Elevated ⁸⁷Sr/⁸⁶Sr ratios across the boundary further support intensified continental weathering and influx of radiogenic strontium during the end-Permian climatic crisis. Integrated Interpretation: Integration of faunal, sedimentological, and isotopic data indicates a shift from a well-oxygenated, open-marine platform during the Late Permian to a restricted, shallow, and stressed lagoonal system in the Early Triassic. The decline in biodiversity, increase in microbial facies, and negative isotopic excursions reflect the combined effects of eustatic fall, climatic warming, and oceanic anoxia. These changes represent the regional expression of global end-Permian perturbations. The integrated multiproxy approach employed here refines previous reconstructions (Haghighat et al. 2020; Nazemi et al. 2021; Davoodi et al. 2024; Rezvannia et al. 2025; Shahkaram et al. 2025) and provides a more complete picture of the environmental transition in the central Persian Gulf.

https://jssr.ui.ac.ir/article_29992_4650b140c77c71beede580144da021fa.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 42 2 2026 06 22 Petrography, geochemistry, and provenance of the Aghajari Formation (North of Hoseynieh, Andimeshk, Khuzestan Province) Petrography, geochemistry, and provenance of the Aghajari Formation (North of Hoseynieh, Andimeshk, Khuzestan Province) 51 80 30258 10.22108/jssr.2026.147717.1324 FA Mohamad Mallah Ph.D. Student in Sedimentology and Sedimentary Rocks, Bu-Ali Sina University, Hamedan, Iran Behrouz Rafiei Associate professor, Department of Geology, Bu-Ali Sina University, Hamedan, Iran Hassan Mohseni Professor, Department of Geology, Bu-Ali Sina University, Hamedan, Iran Journal Article 2025 12 07 Abstract The purpose of this study was to investigate the mineralogy, geochemistry, and provenance of clastic strata of the Aghajari Formation (Upper Miocene–Pliocene) in the north of Hoseynieh and Andimeshk. Petrographic studies and modal analyses of sandstones indicate that quartz, feldspar of igneous origin, and lithic fragments (igneous, metamorphic, and sedimentary) are the main constituents of the sandstones. It was also revealed that the sandstone intervals fall within the recycled or transitional recycled orogenic provenance field, which accumulated during the late Miocene–Pliocene under warm and semi-arid climates. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest andesitic source rocks for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentation cycle. The orogenic events during Miocene–Pliocene (the Savian and Strian tectonic phases) in the Folded Zagros led to the erosion of a mixture of igneous–ophiolitic rocks from the Neotethyan oceanic crust and metamorphic rocks exhumed in northern Lorestan and Kermanshah regions, along with the sedimentary successions of the folded Zagros (the Amiran, Talehzang, Kashkan, Shahbazan and Asmari formations). These formations supplied the clastic sediments of the Aghajari Formation within the Hoseynieh and Andimeshk syncline. Keywords: Petrography, Geochemistry, Aghajari Formation, Provenance, Zagros Basin Introduction Sedimentary rocks are the main source of information about past conditions. Based on the chemical composition of sediments and siliciclastic sedimentary rocks, the provenance and other depositional processes, such as weathering, transportation, and diagenesis, can be evaluated (Dickinson and Suczek 1979; Sharafi et al. 2018; Asiedu et al. 2019; Salehi et al., 2018; Zamanian et al. 2019; Peng et al. 2020). Among the various controlling factors, the tectonic setting and lithology of the source area are the most important parameters governing the formation of siliciclastic deposits (Yan et al. 2012; Salehi et al., 2014; Khazaei et al. 2018). The tectonic setting of siliciclastic rocks can be interpreted using petrographic and geochemical data through discrimination diagrams, as these reflect sediment-generation processes, source-area morphology, and paleoclimatic conditions (Sabbagh et al. 2018; Pourdivanbeigi Moghaddam et al., 2020). The Aghajari Formation, the youngest unit of the Fars Group, was formally introduced by James and Wynd (1965) and consists of a thick succession of syn-orogenic red molasse deposits widely distributed throughout the Zagros region. Its age ranges from Middle Miocene to Pliocene and varies spatially across the basin (Motiee 2003). In this study, mineralogical and geochemical evidence are used to investigate the provenance of the Aghajari siliciclastic strata. Since sedimentary rocks represent the main remnants of eroded ancient crust (Condie et al. 2001; Basu 2003), provenance analysis provides key insights into the Miocene–Pliocene tectonic evolution of the Zagros Fold-Thrust Belt. Material & Methods In this study, three stratigraphic sections of the Aghajari Formation, including Paalam (600 m), Khoshab (550 m), and Piravali (520 m), were investigated in the Hosseinieh area of the Andimeshk region. Sampling in each section was carried out based on facies variations and, in some cases, systematically. A total of 180 rock samples (60 hand specimens from each section) were collected from different facies. To investigate the provenance of the Aghajari siliciclastic strata, 60 sandstone samples (mostly coarse-grained) were selected for thin-section preparation. From these, 17 thin sections (7 from the Aghajari Formation and 10 from the Lahbari Member) were selected for modal analysis. The percentages of cement and matrix in each sample were calculated relative to the total rock framework. To determine major and trace elements, identify lithological characteristics, and evaluate the tectonic setting, 20 mudstone samples were analyzed using ICP-MS at the Zarazma Laboratory (Tehran). These samples were selected to represent the entire stratigraphic succession under study. Additionally, 10 mudstone samples were analyzed by X-ray diffraction (XRD) at the Central Laboratory of Lorestan University to identify the mineralogical composition of fine-grained deposits. Modal analysis of sandstones was performed by counting 300–500 points per thin section following the Gazzi–Dickinson method (Dickinson 1970; Ingersoll et al. 1984), and sandstone classification was carried out according to Folk (1980). Petrographic and geochemical data, together with established discrimination diagrams and the Chemical Index of Alteration (CIA) (Nesbitt & Young, 1984), were used to infer the tectonic setting, palaeoclimate, and paleocurrent patterns. Discussion of Results & Conclusions The study area is located at the beginning of the Zagros folded zone. The Aghajari Formation is the main outcropping rock unit in the study area and consists mainly of sandstone, siltstone, claystone, mudstone, and shale. The investigated sandstones are mainly composed of sedimentary rock fragments, such as chert and carbonate fragments. The studied sandstones are generally poorly sorted, as seen in the thin sections. Petrographic studies revealed that sandstones of the Aghajari Formation are mainly lithic-arenite (sed-arenite), which is mostly composed of chertarenite and calclithite with calcite cement. The origin of the cement in these facies can be attributed to the dissolution of unstable carbonate grains, with an average composition of (Q38R60F2) in the upper part (Lehbari Member) and (Q33R66F1) in the lower part of the Aghajari Formation. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin.The grain size and particle geometry, as well as the degree of sorting of the studied sandstones, indicate that they are texturally immature in terms of textural maturity. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest mixed acidic–basic and andesitic source signatures in the studied samples for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentary cycle. The petrographic and geochemical studies of sandstones in the Aghajari Formation indicate that the investigated sediments have a transitional recycled origin, with recycling in an orogenic zone. Moreover, the results of the modal analysis indicate that the climatic conditions were semi-arid during the deposition of this formation. According to geochemical analyses, the investigated sandstones originated from intermediate or andesitic rocks. Furthermore, geochemical diagrams indicate that the studied sandstones formed in an oceanic arc-island field setting. The geochemical data in the A-CN-K triangular diagram indicate moderately weathered conditions in the source area of the Aghajari Formation. Abstract The purpose of this study was to investigate the mineralogy, geochemistry, and provenance of clastic strata of the Aghajari Formation (Upper Miocene–Pliocene) in the north of Hoseynieh and Andimeshk. Petrographic studies and modal analyses of sandstones indicate that quartz, feldspar of igneous origin, and lithic fragments (igneous, metamorphic, and sedimentary) are the main constituents of the sandstones. It was also revealed that the sandstone intervals fall within the recycled or transitional recycled orogenic provenance field, which accumulated during the late Miocene–Pliocene under warm and semi-arid climates. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest andesitic source rocks for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentation cycle. The orogenic events during Miocene–Pliocene (the Savian and Strian tectonic phases) in the Folded Zagros led to the erosion of a mixture of igneous–ophiolitic rocks from the Neotethyan oceanic crust and metamorphic rocks exhumed in northern Lorestan and Kermanshah regions, along with the sedimentary successions of the folded Zagros (the Amiran, Talehzang, Kashkan, Shahbazan and Asmari formations). These formations supplied the clastic sediments of the Aghajari Formation within the Hoseynieh and Andimeshk syncline. Keywords: Petrography, Geochemistry, Aghajari Formation, Provenance, Zagros Basin Introduction Sedimentary rocks are the main source of information about past conditions. Based on the chemical composition of sediments and siliciclastic sedimentary rocks, the provenance and other depositional processes, such as weathering, transportation, and diagenesis, can be evaluated (Dickinson and Suczek 1979; Sharafi et al. 2018; Asiedu et al. 2019; Salehi et al., 2018; Zamanian et al. 2019; Peng et al. 2020). Among the various controlling factors, the tectonic setting and lithology of the source area are the most important parameters governing the formation of siliciclastic deposits (Yan et al. 2012; Salehi et al., 2014; Khazaei et al. 2018). The tectonic setting of siliciclastic rocks can be interpreted using petrographic and geochemical data through discrimination diagrams, as these reflect sediment-generation processes, source-area morphology, and paleoclimatic conditions (Sabbagh et al. 2018; Pourdivanbeigi Moghaddam et al., 2020). The Aghajari Formation, the youngest unit of the Fars Group, was formally introduced by James and Wynd (1965) and consists of a thick succession of syn-orogenic red molasse deposits widely distributed throughout the Zagros region. Its age ranges from Middle Miocene to Pliocene and varies spatially across the basin (Motiee 2003). In this study, mineralogical and geochemical evidence are used to investigate the provenance of the Aghajari siliciclastic strata. Since sedimentary rocks represent the main remnants of eroded ancient crust (Condie et al. 2001; Basu 2003), provenance analysis provides key insights into the Miocene–Pliocene tectonic evolution of the Zagros Fold-Thrust Belt. Material & Methods In this study, three stratigraphic sections of the Aghajari Formation, including Paalam (600 m), Khoshab (550 m), and Piravali (520 m), were investigated in the Hosseinieh area of the Andimeshk region. Sampling in each section was carried out based on facies variations and, in some cases, systematically. A total of 180 rock samples (60 hand specimens from each section) were collected from different facies. To investigate the provenance of the Aghajari siliciclastic strata, 60 sandstone samples (mostly coarse-grained) were selected for thin-section preparation. From these, 17 thin sections (7 from the Aghajari Formation and 10 from the Lahbari Member) were selected for modal analysis. The percentages of cement and matrix in each sample were calculated relative to the total rock framework. To determine major and trace elements, identify lithological characteristics, and evaluate the tectonic setting, 20 mudstone samples were analyzed using ICP-MS at the Zarazma Laboratory (Tehran). These samples were selected to represent the entire stratigraphic succession under study. Additionally, 10 mudstone samples were analyzed by X-ray diffraction (XRD) at the Central Laboratory of Lorestan University to identify the mineralogical composition of fine-grained deposits. Modal analysis of sandstones was performed by counting 300–500 points per thin section following the Gazzi–Dickinson method (Dickinson 1970; Ingersoll et al. 1984), and sandstone classification was carried out according to Folk (1980). Petrographic and geochemical data, together with established discrimination diagrams and the Chemical Index of Alteration (CIA) (Nesbitt & Young, 1984), were used to infer the tectonic setting, palaeoclimate, and paleocurrent patterns. Discussion of Results & Conclusions The study area is located at the beginning of the Zagros folded zone. The Aghajari Formation is the main outcropping rock unit in the study area and consists mainly of sandstone, siltstone, claystone, mudstone, and shale. The investigated sandstones are mainly composed of sedimentary rock fragments, such as chert and carbonate fragments. The studied sandstones are generally poorly sorted, as seen in the thin sections. Petrographic studies revealed that sandstones of the Aghajari Formation are mainly lithic-arenite (sed-arenite), which is mostly composed of chertarenite and calclithite with calcite cement. The origin of the cement in these facies can be attributed to the dissolution of unstable carbonate grains, with an average composition of (Q38R60F2) in the upper part (Lehbari Member) and (Q33R66F1) in the lower part of the Aghajari Formation. The clay mineral content of these sediments is mainly chlorite and illite, of detrital origin.The grain size and particle geometry, as well as the degree of sorting of the studied sandstones, indicate that they are texturally immature in terms of textural maturity. Th/Co versus La/Sc cross-plots indicated a siliceous source rock, and Ti/Zr versus La/Sc and La/Th versus Hf suggest mixed acidic–basic and andesitic source signatures in the studied samples for the Aghajari Formation. Furthermore, Th/Sc versus Zr/Sc showed a first-order sedimentary cycle. The petrographic and geochemical studies of sandstones in the Aghajari Formation indicate that the investigated sediments have a transitional recycled origin, with recycling in an orogenic zone. Moreover, the results of the modal analysis indicate that the climatic conditions were semi-arid during the deposition of this formation. According to geochemical analyses, the investigated sandstones originated from intermediate or andesitic rocks. Furthermore, geochemical diagrams indicate that the studied sandstones formed in an oceanic arc-island field setting. The geochemical data in the A-CN-K triangular diagram indicate moderately weathered conditions in the source area of the Aghajari Formation.

https://jssr.ui.ac.ir/article_30258_c37e88f19e2938c77502a8e705d8e533.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 42 2 2026 06 22 Facies analysis, depositional environments, and sequence stratigraphy of the Asmari Formation in the Chahar Bishe Oil Field, Zagros Basin, Iran Facies analysis, depositional environments, and sequence stratigraphy of the Asmari Formation in the Chahar Bishe Oil Field, Zagros Basin, Iran 81 98 30351 10.22108/jssr.2025.147537.1322 FA Akbar Heidari Assistant Professor, Department of Petroleum and Sedimentary Basin Geology, Faculty of Earth Sciences and GIS, Shahid Chamran University of Ahvaz, Ahvaz, Iran 0000-0002-4839-3902 Mohadeseh Hosseini-Lagha MSC Graduated from Department of Petroleum and Sedimentary Basin Geology, Faculty of Earth Sciences and GIS, Shahid Chamran University of Ahvaz, Ahvaz, Iran Narges Shokri Assistant Professor, Department of Petroleum and Sedimentary Basin Geology, Faculty of Earth Sciences and GIS, Shahid Chamran University of Ahvaz, Ahvaz, Iran Armin Omidpour PhD of Geology, National Iranian South Oil Company, Ahvaz, Iran Journal Article 2025 11 25 Abstract The Asmari Formation, one of the most important carbonate reservoirs in the Zagros Basin, was studied in the Chahar Bishe Oil Field to identify its facies, depositional environments, and sequence stratigraphic framework. Examination of 316 meters of core and cutting samples of a well from Chahar Bishe Oil Field led to the recognition of fourteen carbonate facies that were deposited within four main facies belts: tidal flat, semi-restricted lagoon, reefal barrier, and open marine. The vertical succession of facies indicates a general shallowing-upward trend from open-marine to nearshore environments, suggesting deposition on a low-angle homoclinal carbonate ramp. Sequence stratigraphic analysis resulted in the identification of three third-order depositional sequences, each composed of a Transgressive Systems Tract (TST) and a Highstand Systems Tract (HST). Type-2 sequence boundaries and Maximum Flooding Surfaces (MFS) were clearly recognized, reflecting relative sea-level fluctuations during the Miocene. The results of this study define the Asmari Formation as a homoclinal carbonate ramp in which facies distribution was mainly controlled by sea-level oscillations and variations in depositional energy. Keywords: Asmari Formation, Carbonate facies, Depositional environment, Sequence stratigraphy, Chahar Bishe Oil Field Introduction The Asmari Formation represents one of the most significant carbonate reservoirs within the Zagros Basin and plays a fundamental role in hydrocarbon production across southwestern Iran. Deposited during the Late Oligocene to Early Miocene, this formation records the evolution of a shallow-marine carbonate platform that developed widely across the Dezful Embayment and adjacent sub-basins. Previous studies have consistently interpreted the Asmari depositional system as a homoclinal carbonate ramp, characterized by a gradual transition from tidal flat to open-marine environments. However, despite extensive investigations in major oilfields such as Ahvaz and Marun, relatively limited data are available from smaller fields such as the Chahar Bishe Oil Field. This lack of detailed facies and sequence stratigraphic frameworks introduces uncertainties in reservoir correlation and geological modeling at local scales. The present study aims to (1) identify carbonate facies and depositional environments, (2) reconstruct the depositional model, and (3) establish a sequence stratigraphic framework for the Asmari Formation in the Chahar Bishe Oil Field. The results provide new insights into local depositional controls and improve the understanding of reservoir heterogeneity within the Zagros Basin. Material & Methods This study is based on the analysis of 316 meters of core and cutting samples obtained from well No. 2 in the Chahar Bishe Oil Field. A total of 105 thin sections were prepared and examined using a polarizing microscope for microfacies analysis. Facies classification was conducted based on standard carbonate microfacies schemes, integrating textural and compositional attributes. Identified components include skeletal grains (e.g., echinoids, bryozoans, red algae, larger benthic foraminifera such as miliolids and nummulitids) and non-skeletal grains (e.g., peloids, ooids, intraclasts). Sequence stratigraphic interpretation was carried out using vertical facies stacking patterns and key stratigraphic surfaces, following established models. Key surfaces such as sequence boundaries (SB) and MFS were identified, and depositional sequences were subdivided into TST and HST. Discussion of Results & Conclusions Facies analysis and depositional environments: In this study, fourteen carbonate facies were identified and grouped into four main depositional belts as follows: Tidal flat (Facies A): Characterized by dolomudstone and mudstone with fenestral fabrics, bird’s-eye structures, and evaporitic minerals, indicating supratidal to intertidal conditions with restricted circulation. Semi-restricted lagoon (Facies B): Dominated by wackestone to packstone textures with miliolids, peloids, and limited faunal diversity, reflecting low-energy, restricted marine conditions. Reefal barrier (Facies C): Composed of coral framestones and floatstones, representing high-energy environments within the euphotic zone. Open marine (Facies D): Characterized by floatstone facies rich in red algae, bryozoans, and large hyaline foraminifera, indicating deposition in deeper, low-energy settings with normal marine salinity. The vertical facies succession exhibits a general shallowing-upward trend, transitioning from open marine to tidal flat environments, consistent with progradational stacking patterns. Depositional model: The absence of turbiditic deposits, lack of abrupt facies changes, and gradual lateral transitions indicate deposition on a low-gradient homoclinal carbonate ramp. The platform consists of inner, middle, and outer ramp settings, reflecting a continuum of depositional energy from low-energy open marine environments to high-energy shoal and tidal flat settings. Sequence stratigraphy: Three third-order depositional sequences (DS1–DS3) were identified within the studied interval.DS1: Initiated with open-marine facies and culminated in tidal flat deposits. The MFS is marked by the deepest marine facies. DS2: Defined by a Type-2 sequence boundary (SB2), with a transgressive trend from tidal flat to open marine followed by regression. DS3: Characterized by a transgressive phase from tidal flat to reefal environments, followed by a highstand dominated by shallow lagoonal and tidal deposits. Each sequence consists of TST and HST systems tracts, with clearly identifiable MFS surfaces. The stacking patterns reflect relative sea-level fluctuations during the Miocene. Facies distribution within the Asmari Formation is primarily controlled by relative sea-level changes and depositional energy gradients along the carbonate ramp. The development of restricted lagoonal facies and tidal flat deposits during highstand conditions suggests reduced accommodation space and increased progradation. In contrast, transgressive phases are marked by the expansion of open-marine and reefal facies, indicating increased accommodation and deeper depositional conditions. The dominance of homoclinal ramp geometry suggests minimal tectonic segmentation during deposition, although subtle variations in sequence thickness may reflect localized subsidence patterns. Comparison with regional studies indicates a strong correspondence between identified sequence boundaries and global sea-level curves, suggesting that eustatic controls played a major role, superimposed on regional tectonic influences. Fourteen carbonate facies were identified, reflecting a systematic variation in depositional energy and water depth. The vertical facies distribution indicates an overall shallowing-upward trend. The Asmari Formation in the Chahar Bishe Oil Field was deposited on a low-angle homoclinal carbonate ramp comprising four main depositional environments: tidal flat, lagoon, reef, and open marine. Sequence stratigraphic analysis reveals three third-order depositional sequences, each composed of TST and HST systems tracts, bounded by Type-2 sequence boundaries and marked by well-developed MFS. These findings highlight the dominant role of relative sea-level fluctuations in controlling facies architecture and depositional cycles, providing a robust framework for reservoir characterization and stratigraphic correlation in the Zagros Basin. Abstract The Asmari Formation, one of the most important carbonate reservoirs in the Zagros Basin, was studied in the Chahar Bishe Oil Field to identify its facies, depositional environments, and sequence stratigraphic framework. Examination of 316 meters of core and cutting samples of a well from Chahar Bishe Oil Field led to the recognition of fourteen carbonate facies that were deposited within four main facies belts: tidal flat, semi-restricted lagoon, reefal barrier, and open marine. The vertical succession of facies indicates a general shallowing-upward trend from open-marine to nearshore environments, suggesting deposition on a low-angle homoclinal carbonate ramp. Sequence stratigraphic analysis resulted in the identification of three third-order depositional sequences, each composed of a Transgressive Systems Tract (TST) and a Highstand Systems Tract (HST). Type-2 sequence boundaries and Maximum Flooding Surfaces (MFS) were clearly recognized, reflecting relative sea-level fluctuations during the Miocene. The results of this study define the Asmari Formation as a homoclinal carbonate ramp in which facies distribution was mainly controlled by sea-level oscillations and variations in depositional energy. Keywords: Asmari Formation, Carbonate facies, Depositional environment, Sequence stratigraphy, Chahar Bishe Oil Field Introduction The Asmari Formation represents one of the most significant carbonate reservoirs within the Zagros Basin and plays a fundamental role in hydrocarbon production across southwestern Iran. Deposited during the Late Oligocene to Early Miocene, this formation records the evolution of a shallow-marine carbonate platform that developed widely across the Dezful Embayment and adjacent sub-basins. Previous studies have consistently interpreted the Asmari depositional system as a homoclinal carbonate ramp, characterized by a gradual transition from tidal flat to open-marine environments. However, despite extensive investigations in major oilfields such as Ahvaz and Marun, relatively limited data are available from smaller fields such as the Chahar Bishe Oil Field. This lack of detailed facies and sequence stratigraphic frameworks introduces uncertainties in reservoir correlation and geological modeling at local scales. The present study aims to (1) identify carbonate facies and depositional environments, (2) reconstruct the depositional model, and (3) establish a sequence stratigraphic framework for the Asmari Formation in the Chahar Bishe Oil Field. The results provide new insights into local depositional controls and improve the understanding of reservoir heterogeneity within the Zagros Basin. Material & Methods This study is based on the analysis of 316 meters of core and cutting samples obtained from well No. 2 in the Chahar Bishe Oil Field. A total of 105 thin sections were prepared and examined using a polarizing microscope for microfacies analysis. Facies classification was conducted based on standard carbonate microfacies schemes, integrating textural and compositional attributes. Identified components include skeletal grains (e.g., echinoids, bryozoans, red algae, larger benthic foraminifera such as miliolids and nummulitids) and non-skeletal grains (e.g., peloids, ooids, intraclasts). Sequence stratigraphic interpretation was carried out using vertical facies stacking patterns and key stratigraphic surfaces, following established models. Key surfaces such as sequence boundaries (SB) and MFS were identified, and depositional sequences were subdivided into TST and HST. Discussion of Results & Conclusions Facies analysis and depositional environments: In this study, fourteen carbonate facies were identified and grouped into four main depositional belts as follows: Tidal flat (Facies A): Characterized by dolomudstone and mudstone with fenestral fabrics, bird’s-eye structures, and evaporitic minerals, indicating supratidal to intertidal conditions with restricted circulation. Semi-restricted lagoon (Facies B): Dominated by wackestone to packstone textures with miliolids, peloids, and limited faunal diversity, reflecting low-energy, restricted marine conditions. Reefal barrier (Facies C): Composed of coral framestones and floatstones, representing high-energy environments within the euphotic zone. Open marine (Facies D): Characterized by floatstone facies rich in red algae, bryozoans, and large hyaline foraminifera, indicating deposition in deeper, low-energy settings with normal marine salinity. The vertical facies succession exhibits a general shallowing-upward trend, transitioning from open marine to tidal flat environments, consistent with progradational stacking patterns. Depositional model: The absence of turbiditic deposits, lack of abrupt facies changes, and gradual lateral transitions indicate deposition on a low-gradient homoclinal carbonate ramp. The platform consists of inner, middle, and outer ramp settings, reflecting a continuum of depositional energy from low-energy open marine environments to high-energy shoal and tidal flat settings. Sequence stratigraphy: Three third-order depositional sequences (DS1–DS3) were identified within the studied interval.DS1: Initiated with open-marine facies and culminated in tidal flat deposits. The MFS is marked by the deepest marine facies. DS2: Defined by a Type-2 sequence boundary (SB2), with a transgressive trend from tidal flat to open marine followed by regression. DS3: Characterized by a transgressive phase from tidal flat to reefal environments, followed by a highstand dominated by shallow lagoonal and tidal deposits. Each sequence consists of TST and HST systems tracts, with clearly identifiable MFS surfaces. The stacking patterns reflect relative sea-level fluctuations during the Miocene. Facies distribution within the Asmari Formation is primarily controlled by relative sea-level changes and depositional energy gradients along the carbonate ramp. The development of restricted lagoonal facies and tidal flat deposits during highstand conditions suggests reduced accommodation space and increased progradation. In contrast, transgressive phases are marked by the expansion of open-marine and reefal facies, indicating increased accommodation and deeper depositional conditions. The dominance of homoclinal ramp geometry suggests minimal tectonic segmentation during deposition, although subtle variations in sequence thickness may reflect localized subsidence patterns. Comparison with regional studies indicates a strong correspondence between identified sequence boundaries and global sea-level curves, suggesting that eustatic controls played a major role, superimposed on regional tectonic influences. Fourteen carbonate facies were identified, reflecting a systematic variation in depositional energy and water depth. The vertical facies distribution indicates an overall shallowing-upward trend. The Asmari Formation in the Chahar Bishe Oil Field was deposited on a low-angle homoclinal carbonate ramp comprising four main depositional environments: tidal flat, lagoon, reef, and open marine. Sequence stratigraphic analysis reveals three third-order depositional sequences, each composed of TST and HST systems tracts, bounded by Type-2 sequence boundaries and marked by well-developed MFS. These findings highlight the dominant role of relative sea-level fluctuations in controlling facies architecture and depositional cycles, providing a robust framework for reservoir characterization and stratigraphic correlation in the Zagros Basin.

https://jssr.ui.ac.ir/article_30351_a28b1e8f3e43f864dac87ad3c598377f.pdf