University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 Prediction of thermal maturity by indirect methods using seismic attributes in the central part of the Persian Gulf Prediction of thermal maturity by indirect methods using seismic attributes in the central part of the Persian Gulf 1 22 27899 10.22108/jssr.2023.138211.1261 FA Elnaz Aliakbardoust PhD candidate. Department of Sedimentary Basins and Petroleum, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran Mohammadhossein Adabi Professor. Department of Sedimentary Basins and Petroleum, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran Ali Kadkhodaie Professor. Earth Sciences Department, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran. Ali Chehrazi Assistant Professor, Iranian Offshore Oil Company (IOOC), Tehran, Iran. Journal Article 2023 06 29 Abstract In this paper, a method is proposed for the prediction of thermal maturity in the source rock using indirect methods. The applied data are well logs (neutron, density, resistance, and acoustic) in 13 wells and seismic data in six oil and gas fields in the central part of the Persian Gulf. Well-logs and seismic data are much more abundant than geochemical data and cover an extensive area in the oil and gas fields. These properties compensate for the lack of geochemical data that are scattered and limited to a few wells. This study is carried out in two steps. First, the amount of thermal maturity in the Kazhdumi Formation is calculated from well logs and is presented as an index in each well. Data obtained from organic thermal evaluation analyses are used to validate the results of thermal maturity prediction. These data include Rock-Eval pyrolysis in two wells. Then, seismic data are processed and studied in two-dimensional sections at the location of the target fields. In this step, seismic attributes are extracted from the seismic data using the multi-attribute regression analysis method, and thermal maturity is calculated using these attributes. Prediction is performed by probabilistic neural network analysis, and a seismic section is extracted indicating variations in thermal maturity in the Kazhdumi Formation. Keywords: Source rock, Thermal maturity, Well log, Seismic attribute Introduction The Kazhdumi Formation is an important source rock in the Persian Gulf basin, in the south of Iran (Bordenave Burwood, 1990). Thermal maturity of the Kazhdumi Formation is low in the central Persian Gulf compared to the eastern and western sectors (Rabbani 2008; Ghasemi-Nejad et al. 2009; Rezaie Kavanrudi et al. 2015; Rabbani et al. 2014; Baniasad et al. 2019). This formation is over-mature in proximity to the Hormoz Strait. The TOC content of this formation depends on the variety of depositional environments across the basin, increasing to the northwest with a maximum of 6 wt% in the proximity of the Hormoz strait (Rezaie Kavanrudi et al. 2015; Noori et al. 2016). The kerogen type is mainly ⅡS and type Ⅲ in different areas (Ghasemi-Nejad et al. 2009). The stratigraphic equivalents of the Kazhdumi Formation are the Burgan Formation in the west and south of the Persian Gulf (Kuwait), producing hydrocarbons from the second-largest hydrocarbon oilfield in the world, and the Nahr-Umr Formation in Qatar and Iraq. The Burgan and Nahr-Umr formations consist of fluvial sandstone in the south and east of the Persian Gulf compared to the shale-dominated volume in the north and central part (Ghasemi-Nejad et al. 2009; Noori et al. 2016). The source rock potential of this formation is largely unknown and there is a lack of published reports in the central part of the Persian Gulf. the aim of this study is to use log and seismic data for indirect estimation of thermal maturity in this area. Materials & Methods In this study, neutron, density and sonic as well as gamma-ray logs are used to predict thermal maturity. The maturity index (MI) by Zhao et al. (2007) was used to describe the level of thermal maturity based on well logs. The response of the neutron and density logs is affected by the fraction of water in a formation. The hydrocarbon density also decreases due to thermal maturation. The maturity index is calculated based on the equation from Zhao et al. (2007) (Eq. 1) N: the number of log readings or the number of samples. Øn9i: neutron porosity of rock samples with 9% density porosity or higher. The 9% is a cut-off for porosity in the calculations. Values lower than 9% are indicative of very dense formations such as anhydrite or dense dolomite known as non-source shales. The cumulative value of the calculated MI is presented as a maturity index in the Kazhdumi Formation in each well. In the second step, seismic data analysis, inversion of seismic data and log prediction are carried out. Inversion analysis starts with seismic data processing using well logs and 2D post-stack seismic data. An acoustic impedance log is created by the combination of density and sonic logs. Check-shot data are used for depth-to-time conversion resulting to the correlation of well and seismic data. Following this, an initial strata or impedance model is constructed. This model is produced using the seismic volume, available well data and defined horizons. Seismic data, the initial strata model as well as available wells are applied as the input data for inversion analysis, and then the inversion method is selected. Discussion of Results & Conclusions Generally, thermal maturity status in the Kazhdumi Formation is between immature to mature in the study area in the central part of the Persian Gulf. According to the presented data in this study, this formation is mainly immature in 2, 3, 5 and 7 fields. Field number 4 is early mature and number 1 is mature. Field 6 shows an immature to mature level indicating that the maturity varies in this field. In the next step, seismic attributes are selected by regression analysis for MI prediction. Attributes selection is a process for the extraction of seismic attributes from the raw seismic data for modeling the target log. Multi-attribute analysis is an automatic procedure for the selection of the most relevant seismic attributes to the target log. Finally, extracted attributes are used for the log prediction by Probabilistic Neural Network analysis (PNN). It is trained based on the selected attributes in the previous step and then predicts the target parameters. Acoustic impedance is recognized as the most important seismic attribute reflecting the geological properties (Chopra and Marfurt, 2005). The optimum number of attributes for MI prediction is four which shows the lowest prediction error, although the training error continuously decreases by adding more attributes. The validation plot of the target log is estimated by excluding the data step by step from the calculation. The reliability of the regression model is tested by comparing the prediction with the actual log values. Thereafter, the 2D seismic section is converted to an MI volume. The variation in MI is continuous laterally and vertically, therefore, can be tracked throughout the basin. A comparison of the computed MI with geochemical data indicated that this method is applicable for thermal maturity prediction in the study area. The increase in MI corresponds to an increase in the Tmax values, thus providing a good indicator of thermal maturity variation. The last point to consider is the significance of the selected seismic attributes and their relationship with the target parameter. Results indicated that acoustic impedance is the most important seismic attribute in MI prediction. Acoustic impedance contains information about the velocity and formation density which are both affected by the formation fluids (Broadhead et al. 2016; Atarita et al. 2017). It is inversely related to the organic matter content (Harris et al. 2019). The computed thermal maturity is inversely related to the neutron porosity and water saturation which are both controlling parameters of the acoustic impedance in a formation. Other attributes that have been used in the log prediction are time and amplitude-weighted frequency. These attributes are related to different geological properties of source/reservoir formations (Taner et al. 1994; Chen and Sidney 1997; Chopra and Marfurt 2005). Amplitude envelope (reflection strength) mainly represents acoustic impedance and is useful for identifying porosity, hydrocarbon and gas accumulation, sequence boundaries, and lithological/depositional environment variations (Chen and Sidney 1997; Hart 2002). Average frequency is defined as a signature of events and is useful for correlation, often reflecting oil and gas reservoirs by seismic attenuation (Taner et al. 1994). Amplitude-weighted frequency is a product of the amplitude envelope and the instantaneous frequency, providing a smooth estimation of instantaneous frequency by removing spikes and noises (Chen and Sidney 1997). To sum up, well logs and seismic attributes are successfully applied to predict thermal maturity in the Kazhdumi Formation in the central part of the Persian Gulf. The analysis shows that the Kazhdumi Formation is mainly immature to early mature in the central part of the Persian Gulf. Seismic data are spatially continuous which is an advantage in source rock evaluation, resulting in continuous predictions of thermal maturity in hydrocarbon fields. Using seismic data is also cost-effective and less time-consuming than geochemical testing. Abstract In this paper, a method is proposed for the prediction of thermal maturity in the source rock using indirect methods. The applied data are well logs (neutron, density, resistance, and acoustic) in 13 wells and seismic data in six oil and gas fields in the central part of the Persian Gulf. Well-logs and seismic data are much more abundant than geochemical data and cover an extensive area in the oil and gas fields. These properties compensate for the lack of geochemical data that are scattered and limited to a few wells. This study is carried out in two steps. First, the amount of thermal maturity in the Kazhdumi Formation is calculated from well logs and is presented as an index in each well. Data obtained from organic thermal evaluation analyses are used to validate the results of thermal maturity prediction. These data include Rock-Eval pyrolysis in two wells. Then, seismic data are processed and studied in two-dimensional sections at the location of the target fields. In this step, seismic attributes are extracted from the seismic data using the multi-attribute regression analysis method, and thermal maturity is calculated using these attributes. Prediction is performed by probabilistic neural network analysis, and a seismic section is extracted indicating variations in thermal maturity in the Kazhdumi Formation. Keywords: Source rock, Thermal maturity, Well log, Seismic attribute Introduction The Kazhdumi Formation is an important source rock in the Persian Gulf basin, in the south of Iran (Bordenave Burwood, 1990). Thermal maturity of the Kazhdumi Formation is low in the central Persian Gulf compared to the eastern and western sectors (Rabbani 2008; Ghasemi-Nejad et al. 2009; Rezaie Kavanrudi et al. 2015; Rabbani et al. 2014; Baniasad et al. 2019). This formation is over-mature in proximity to the Hormoz Strait. The TOC content of this formation depends on the variety of depositional environments across the basin, increasing to the northwest with a maximum of 6 wt% in the proximity of the Hormoz strait (Rezaie Kavanrudi et al. 2015; Noori et al. 2016). The kerogen type is mainly ⅡS and type Ⅲ in different areas (Ghasemi-Nejad et al. 2009). The stratigraphic equivalents of the Kazhdumi Formation are the Burgan Formation in the west and south of the Persian Gulf (Kuwait), producing hydrocarbons from the second-largest hydrocarbon oilfield in the world, and the Nahr-Umr Formation in Qatar and Iraq. The Burgan and Nahr-Umr formations consist of fluvial sandstone in the south and east of the Persian Gulf compared to the shale-dominated volume in the north and central part (Ghasemi-Nejad et al. 2009; Noori et al. 2016). The source rock potential of this formation is largely unknown and there is a lack of published reports in the central part of the Persian Gulf. the aim of this study is to use log and seismic data for indirect estimation of thermal maturity in this area. Materials & Methods In this study, neutron, density and sonic as well as gamma-ray logs are used to predict thermal maturity. The maturity index (MI) by Zhao et al. (2007) was used to describe the level of thermal maturity based on well logs. The response of the neutron and density logs is affected by the fraction of water in a formation. The hydrocarbon density also decreases due to thermal maturation. The maturity index is calculated based on the equation from Zhao et al. (2007) (Eq. 1) N: the number of log readings or the number of samples. Øn9i: neutron porosity of rock samples with 9% density porosity or higher. The 9% is a cut-off for porosity in the calculations. Values lower than 9% are indicative of very dense formations such as anhydrite or dense dolomite known as non-source shales. The cumulative value of the calculated MI is presented as a maturity index in the Kazhdumi Formation in each well. In the second step, seismic data analysis, inversion of seismic data and log prediction are carried out. Inversion analysis starts with seismic data processing using well logs and 2D post-stack seismic data. An acoustic impedance log is created by the combination of density and sonic logs. Check-shot data are used for depth-to-time conversion resulting to the correlation of well and seismic data. Following this, an initial strata or impedance model is constructed. This model is produced using the seismic volume, available well data and defined horizons. Seismic data, the initial strata model as well as available wells are applied as the input data for inversion analysis, and then the inversion method is selected. Discussion of Results & Conclusions Generally, thermal maturity status in the Kazhdumi Formation is between immature to mature in the study area in the central part of the Persian Gulf. According to the presented data in this study, this formation is mainly immature in 2, 3, 5 and 7 fields. Field number 4 is early mature and number 1 is mature. Field 6 shows an immature to mature level indicating that the maturity varies in this field. In the next step, seismic attributes are selected by regression analysis for MI prediction. Attributes selection is a process for the extraction of seismic attributes from the raw seismic data for modeling the target log. Multi-attribute analysis is an automatic procedure for the selection of the most relevant seismic attributes to the target log. Finally, extracted attributes are used for the log prediction by Probabilistic Neural Network analysis (PNN). It is trained based on the selected attributes in the previous step and then predicts the target parameters. Acoustic impedance is recognized as the most important seismic attribute reflecting the geological properties (Chopra and Marfurt, 2005). The optimum number of attributes for MI prediction is four which shows the lowest prediction error, although the training error continuously decreases by adding more attributes. The validation plot of the target log is estimated by excluding the data step by step from the calculation. The reliability of the regression model is tested by comparing the prediction with the actual log values. Thereafter, the 2D seismic section is converted to an MI volume. The variation in MI is continuous laterally and vertically, therefore, can be tracked throughout the basin. A comparison of the computed MI with geochemical data indicated that this method is applicable for thermal maturity prediction in the study area. The increase in MI corresponds to an increase in the Tmax values, thus providing a good indicator of thermal maturity variation. The last point to consider is the significance of the selected seismic attributes and their relationship with the target parameter. Results indicated that acoustic impedance is the most important seismic attribute in MI prediction. Acoustic impedance contains information about the velocity and formation density which are both affected by the formation fluids (Broadhead et al. 2016; Atarita et al. 2017). It is inversely related to the organic matter content (Harris et al. 2019). The computed thermal maturity is inversely related to the neutron porosity and water saturation which are both controlling parameters of the acoustic impedance in a formation. Other attributes that have been used in the log prediction are time and amplitude-weighted frequency. These attributes are related to different geological properties of source/reservoir formations (Taner et al. 1994; Chen and Sidney 1997; Chopra and Marfurt 2005). Amplitude envelope (reflection strength) mainly represents acoustic impedance and is useful for identifying porosity, hydrocarbon and gas accumulation, sequence boundaries, and lithological/depositional environment variations (Chen and Sidney 1997; Hart 2002). Average frequency is defined as a signature of events and is useful for correlation, often reflecting oil and gas reservoirs by seismic attenuation (Taner et al. 1994). Amplitude-weighted frequency is a product of the amplitude envelope and the instantaneous frequency, providing a smooth estimation of instantaneous frequency by removing spikes and noises (Chen and Sidney 1997). To sum up, well logs and seismic attributes are successfully applied to predict thermal maturity in the Kazhdumi Formation in the central part of the Persian Gulf. The analysis shows that the Kazhdumi Formation is mainly immature to early mature in the central part of the Persian Gulf. Seismic data are spatially continuous which is an advantage in source rock evaluation, resulting in continuous predictions of thermal maturity in hydrocarbon fields. Using seismic data is also cost-effective and less time-consuming than geochemical testing.

https://jssr.ui.ac.ir/article_27899_cba939e3f766c52a74565e745e6b7865.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 Sequence stratigraphy and microfacies of the Sarvak Formation, west of the Hendijan–Bahregansar–Nowrooz Palaeohigh Sequence stratigraphy and microfacies of the Sarvak Formation, west of the Hendijan–Bahregansar–Nowrooz Palaeohigh 23 44 27991 10.22108/jssr.2023.138811.1266 FA Solmaz Sadeghi PhD student of Stratigraphy and Palaeontology, 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 Bijan Beiranvand Senior Researcher, Research Institute of Petroleum Industry (RIPI), Tehran, Iran Journal Article 2023 08 29 Abstract The 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. Introduction The 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 & Methods A 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 & Conclusions Seven 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. Abstract The 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. Introduction The 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 & Methods A 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 & Conclusions Seven 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.

https://jssr.ui.ac.ir/article_27991_4c7ee52312f04f0adb57a4777b5afc5a.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 Comparison of the function of ELM and RBF models for estimating the porosity of the Asmari Formation, in one of the offshore fields of the northwest Persian Gulf Comparison of the function of ELM and RBF models for estimating the porosity of the Asmari Formation, in one of the offshore fields of the northwest Persian Gulf 45 58 27911 10.22108/jssr.2023.137083.1256 FA Farshad Tofighi MSc student, Department of Mining, Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran 0009-0007-1391-0379 Parviz Armani Associate professor, Department of Geology, Faculty of Sciences, Imam Khomeini International University, Qazvin, Iran Ali Chehrazi PhD of geology, Iranian Offshore Oil Company, Tehran, Iran Andisheh Alimoradi Assistant professor, Department of Mining, Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran Journal Article 2023 03 16 Abstract Nowadays, the use of artificial intelligence is common to increase the accuracy of the study and, close to reality, is used in the oil industry to increase the accuracy of studying and understanding the relationship between various parameters. The main purpose of this study is to compare the performance of the two methods of Extreme Learning Machine (ELM) and Radial Basis Function (RBF) in porosity estimation, which is static oil modeling. The data from seven wells in the offshore field (Hendijan Oilfield) of the northwestern Persian Gulf were examined. In this regard, post-stack seismic attributes which have a significant relationship with porosity and porosity log for each well were used to compare the performance of the ELM and RBF networks under the same conditions. Eventually, it reveals that ELM is quite sensitive to the data set and needs more data points to prepare a map (quantitatively), but is better than RBF in terms of classification (qualitative). On the other hand, RBF is one of the most powerful algorithms in mapping, especially in low numbers of data points, which can be challenging for others. Abstract Nowadays, the use of artificial intelligence is common to increase the accuracy of the study and, close to reality, is used in the oil industry to increase the accuracy of studying and understanding the relationship between various parameters. The main purpose of this study is to compare the performance of the two methods of Extreme Learning Machine (ELM) and Radial Basis Function (RBF) in porosity estimation, which is static oil modeling. The data from seven wells in the offshore field (Hendijan Oilfield) of the northwestern Persian Gulf were examined. In this regard, post-stack seismic attributes which have a significant relationship with porosity and porosity log for each well were used to compare the performance of the ELM and RBF networks under the same conditions. Eventually, it reveals that ELM is quite sensitive to the data set and needs more data points to prepare a map (quantitatively), but is better than RBF in terms of classification (qualitative). On the other hand, RBF is one of the most powerful algorithms in mapping, especially in low numbers of data points, which can be challenging for others.

https://jssr.ui.ac.ir/article_27911_816ad68166ecc51b8add09ec44141e03.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 The role of relative sea-level fluctuations on dolomitization of carbonate reservoirs Case study: the Oligocene–Miocene Asmari Formation The role of relative sea-level fluctuations on dolomitization of carbonate reservoirs Case study: the Oligocene–Miocene Asmari Formation 59 80 27923 10.22108/jssr.2023.138904.1268 FA Armin Omidpour National Iranian South Oil Company, Ahvaz, Iran Asadollah Mahboubi Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran Roghayeh Fallah-Bagtash Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran Journal Article 2023 08 26 Abstract The Oligocene–Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The Asmari Formation has been deposited along a homoclinal ramp-type platform with a gentle slope divisible into an inner ramp, mid ramp, outer ramp and basinal settings. The distribution of dolomite through the Asmari carbonate platform in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized while, the mid- and outer ramp facies associations are moderate to least dolomitized intervals. Six third-order depositional sequences were recognized within the Oligo–Miocene succession. They are bounded by sequence boundaries (SB) with significant evidence of subaerial exposure, diagenetic alteration, oxidizing conditions, and an abrupt change in facies or facies bathymetry. The stratigraphic distribution of facies associations proves more dolomite percent is formed near the sequence boundary as well as in the high stand system tract (HST) deposits. The main dolomitization model of the Asmari carbonate platform is seepage-reflux. Thus, the highest dolomite percentages occur near the sequence boundary when the sea level was low in the shoreface facies (inner ramp), the lowest percentage and dominant fabric destructive dolomite in the offshore facies, and fabric selective dolomite near the maximum flooding surface (MFS) due to the slow rate of dolomitization and low volume of dolomitizing fluids. Keywords: Dolomitization models, Seepage-reflux, Sequence stratigraphy, Asmari Formation, Oligocene–Miocene. Introduction Dolomitization can affect the reservoir quality of carbonate platforms in both constructive and destructive ways, and according to the model and time of dolomitization, it can lead to an increase or decrease in the porosity and permeability of the reservoir (Rahimpour-Bonab et al. 2010). Dolomitization affected more than 90% of the studied Asmari reservoir sequence. As such, it has played the principal role in shaping the spatial pore space architecture, flow capacity and heterogeneity of the final Asmari reservoir quality (Aqrawi et al. 2006; Fallah-Bagtash et al. 2022; Omidpour et al. 2022). The Oligo–Miocene carbonates of the Asmari Formation form the giant reservoirs in the southwestern fields of Iran, including the Shadegan Oil Field in the Dezful Embayment (Aqrawi et al. 2006). In this formation, the best reservoir units occur within the dolomitized intervals. Therefore, in the Asmari reservoir with poor primary reservoir properties, dolomitization enhanced reservoir quality (Aqrawi et al. 2006; Noorian et al. 2020; 2021; Khazaie et al. 2022; Fallah-Bagtsh et al. 2022). The Asmari Formation has been the subject of many studies, including facies changes and sedimentary environment, paleoenvironmental and sequence stratigraphic reconstruction, investigation of diagenetic processes and reservoir quality in the Zagros Basin (Ehrenberg et al. 2007; Van Buchem et al. 2010; Khodaveisi et al. 2014; Adabi et al. 2016; Noorian et al. 2022; Omidpour and Fallah-Bagtash 2022; Fallah-Bagtash et al. 2021; 2022; Omidpour et al. 2021; 2022; 2023; Ahmadi et al. 2023). These carbonates were deposited in shallow facies belts along a carbonate ramp platform (Fallah-Bagtash et al. 2021; 2022; Omidpour et al. 2021; 2022). Based on previous studies, the age of this formation is Oligocene (Rupelian) to Early Miocene (Burdigalian) in different parts of the Zagros Basin (Ehrenberg et al. 2007; Laursen et al. 2009). According to biostratigraphic studies, the Asmari Formation in the Shadegan Oil Field has been deposited from Oligocene (Chattian) to Lower Miocene (Aquitanian–Burdigalian) (Omidpour et al. 2021). Due to the importance of the Asmari Formation as a most known reservoir in the Zagros region, especially in the Dezful Embayment, and considering the role of dolomitization in the reservoir quality of carbonate reservoirs, a well in the Shadegan Oil Field has been subjected to detailed sedimentology and sequence stratigraphy studies. The present study aims at investigatin the depositional history, the effect of dolomitization on the facies and depositional sequences of the Asmari Formation, and finally the relationship between relative sea level changes and dolomitization in the Asmari carbonate platform. Material & Methods The present study is based on a petrographic analysis of 524 thin sections from core samples of SG-11 well drilled in the Asmari Formation at Shadegan Oil Field. All thin sections were stained with potassium ferricyanide and Alizarin Red-S to distinguish calcite and dolomite minerals (Dickson 1965). Carbonates were classified based on the schemes of Dunham (1962) and Embry and Klovan (1971). Facies analysis and interpretation of the depositional environment were performed using the standard microfacies classification by Wilson (1975), Burchette and Wright (1992), and Flügel (2010). Sequence stratigraphic interpretations of the Asmari Formation were based on Hunt and Tucker (1993) method. Stratal surfaces have been identified according to the changes in lithofacies, fossils and their position relative to each other during the interval, and finally petrophysical logs such as gamma log (SGR and CGR) along these surfaces. Discussion of Results & Conclusion The Asmari reservoir with Oligocene (Chattian) –Miocene (Aquitanian–Burdigalian) age in Shadegan Oil Field with a thickness of 363 meters, includes limestones with interlayers of dolostones, sandstones, shales and evaporites. The detailed thin-section analysis of the carbonate samples resulted in the distinction of 26 carbonate-evaporate microfacies types and 12 facies associations in the studied successions. The Oligocene–Miocene succession was deposited along a homoclinal carbonate ramp setting within inner, mid, and outer ramp, and basin sub-environments. The distribution of dolomite through the facies and depositional sequences of the Oligocene–Miocene succession and the lateral and vertical heterogeneity in the percentage of dolomite indicate that the carbonate platform of the Asmari reservoir has undergone multiple dolomitization, which can be arranged into five dolomitization models from near surface to deep burial settings. The distribution of dolomite through the Oligocene–Miocene succession in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized. Mid- and outer ramp facies associations are moderate to least dolomitized intervals. In turn, the stratigraphic distribution of these facies associations proves more dolomite percent was formed near the sequence boundary as well as in HST deposits. Thin-layered sabkha dolomites are formed at or just below the sediment-water interface in mud-supported facies soon after deposition or during shallow burial. The matrix dolomites (medium to coarse crystalline dolomites) are the most abundant type of dolomites which were formed during the intermediate burial stages of the Asmari succession. These dolomites formed from warmer and more saline basinal fluids and/or from the dissolution of high-magnesium calcite or earlier dolomites, or recrystallization of fine crystalline dolomites. The very coarse crystalline dolomites and other dolomites associated with the shaley facies, formed in a deeper burial setting by hydrothermal processes, utilizing hot and slightly-saline fluids that were affected by brine enrichment. Investigation of the relationship between relative sea level changes and dolomitization in carbonate platform shows that towards the sequence boundaries (near shoreface facies), due to the large volume of dolomitizing fluids and high nucleation rate, the dolomitization rate is high, leading to a relatively small dolomite crystals size. On the other hand, toward the MFS (the offshore facies), the dolomitization rate is slowed due to the low volume of dolomitizing fluids and slow nucleation rate, resulting in relatively coarser dolomite crystal sizes. Abstract The Oligocene–Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The Asmari Formation has been deposited along a homoclinal ramp-type platform with a gentle slope divisible into an inner ramp, mid ramp, outer ramp and basinal settings. The distribution of dolomite through the Asmari carbonate platform in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized while, the mid- and outer ramp facies associations are moderate to least dolomitized intervals. Six third-order depositional sequences were recognized within the Oligo–Miocene succession. They are bounded by sequence boundaries (SB) with significant evidence of subaerial exposure, diagenetic alteration, oxidizing conditions, and an abrupt change in facies or facies bathymetry. The stratigraphic distribution of facies associations proves more dolomite percent is formed near the sequence boundary as well as in the high stand system tract (HST) deposits. The main dolomitization model of the Asmari carbonate platform is seepage-reflux. Thus, the highest dolomite percentages occur near the sequence boundary when the sea level was low in the shoreface facies (inner ramp), the lowest percentage and dominant fabric destructive dolomite in the offshore facies, and fabric selective dolomite near the maximum flooding surface (MFS) due to the slow rate of dolomitization and low volume of dolomitizing fluids. Keywords: Dolomitization models, Seepage-reflux, Sequence stratigraphy, Asmari Formation, Oligocene–Miocene. Introduction Dolomitization can affect the reservoir quality of carbonate platforms in both constructive and destructive ways, and according to the model and time of dolomitization, it can lead to an increase or decrease in the porosity and permeability of the reservoir (Rahimpour-Bonab et al. 2010). Dolomitization affected more than 90% of the studied Asmari reservoir sequence. As such, it has played the principal role in shaping the spatial pore space architecture, flow capacity and heterogeneity of the final Asmari reservoir quality (Aqrawi et al. 2006; Fallah-Bagtash et al. 2022; Omidpour et al. 2022). The Oligo–Miocene carbonates of the Asmari Formation form the giant reservoirs in the southwestern fields of Iran, including the Shadegan Oil Field in the Dezful Embayment (Aqrawi et al. 2006). In this formation, the best reservoir units occur within the dolomitized intervals. Therefore, in the Asmari reservoir with poor primary reservoir properties, dolomitization enhanced reservoir quality (Aqrawi et al. 2006; Noorian et al. 2020; 2021; Khazaie et al. 2022; Fallah-Bagtsh et al. 2022). The Asmari Formation has been the subject of many studies, including facies changes and sedimentary environment, paleoenvironmental and sequence stratigraphic reconstruction, investigation of diagenetic processes and reservoir quality in the Zagros Basin (Ehrenberg et al. 2007; Van Buchem et al. 2010; Khodaveisi et al. 2014; Adabi et al. 2016; Noorian et al. 2022; Omidpour and Fallah-Bagtash 2022; Fallah-Bagtash et al. 2021; 2022; Omidpour et al. 2021; 2022; 2023; Ahmadi et al. 2023). These carbonates were deposited in shallow facies belts along a carbonate ramp platform (Fallah-Bagtash et al. 2021; 2022; Omidpour et al. 2021; 2022). Based on previous studies, the age of this formation is Oligocene (Rupelian) to Early Miocene (Burdigalian) in different parts of the Zagros Basin (Ehrenberg et al. 2007; Laursen et al. 2009). According to biostratigraphic studies, the Asmari Formation in the Shadegan Oil Field has been deposited from Oligocene (Chattian) to Lower Miocene (Aquitanian–Burdigalian) (Omidpour et al. 2021). Due to the importance of the Asmari Formation as a most known reservoir in the Zagros region, especially in the Dezful Embayment, and considering the role of dolomitization in the reservoir quality of carbonate reservoirs, a well in the Shadegan Oil Field has been subjected to detailed sedimentology and sequence stratigraphy studies. The present study aims at investigatin the depositional history, the effect of dolomitization on the facies and depositional sequences of the Asmari Formation, and finally the relationship between relative sea level changes and dolomitization in the Asmari carbonate platform. Material & Methods The present study is based on a petrographic analysis of 524 thin sections from core samples of SG-11 well drilled in the Asmari Formation at Shadegan Oil Field. All thin sections were stained with potassium ferricyanide and Alizarin Red-S to distinguish calcite and dolomite minerals (Dickson 1965). Carbonates were classified based on the schemes of Dunham (1962) and Embry and Klovan (1971). Facies analysis and interpretation of the depositional environment were performed using the standard microfacies classification by Wilson (1975), Burchette and Wright (1992), and Flügel (2010). Sequence stratigraphic interpretations of the Asmari Formation were based on Hunt and Tucker (1993) method. Stratal surfaces have been identified according to the changes in lithofacies, fossils and their position relative to each other during the interval, and finally petrophysical logs such as gamma log (SGR and CGR) along these surfaces. Discussion of Results & Conclusion The Asmari reservoir with Oligocene (Chattian) –Miocene (Aquitanian–Burdigalian) age in Shadegan Oil Field with a thickness of 363 meters, includes limestones with interlayers of dolostones, sandstones, shales and evaporites. The detailed thin-section analysis of the carbonate samples resulted in the distinction of 26 carbonate-evaporate microfacies types and 12 facies associations in the studied successions. The Oligocene–Miocene succession was deposited along a homoclinal carbonate ramp setting within inner, mid, and outer ramp, and basin sub-environments. The distribution of dolomite through the facies and depositional sequences of the Oligocene–Miocene succession and the lateral and vertical heterogeneity in the percentage of dolomite indicate that the carbonate platform of the Asmari reservoir has undergone multiple dolomitization, which can be arranged into five dolomitization models from near surface to deep burial settings. The distribution of dolomite through the Oligocene–Miocene succession in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized. Mid- and outer ramp facies associations are moderate to least dolomitized intervals. In turn, the stratigraphic distribution of these facies associations proves more dolomite percent was formed near the sequence boundary as well as in HST deposits. Thin-layered sabkha dolomites are formed at or just below the sediment-water interface in mud-supported facies soon after deposition or during shallow burial. The matrix dolomites (medium to coarse crystalline dolomites) are the most abundant type of dolomites which were formed during the intermediate burial stages of the Asmari succession. These dolomites formed from warmer and more saline basinal fluids and/or from the dissolution of high-magnesium calcite or earlier dolomites, or recrystallization of fine crystalline dolomites. The very coarse crystalline dolomites and other dolomites associated with the shaley facies, formed in a deeper burial setting by hydrothermal processes, utilizing hot and slightly-saline fluids that were affected by brine enrichment. Investigation of the relationship between relative sea level changes and dolomitization in carbonate platform shows that towards the sequence boundaries (near shoreface facies), due to the large volume of dolomitizing fluids and high nucleation rate, the dolomitization rate is high, leading to a relatively small dolomite crystals size. On the other hand, toward the MFS (the offshore facies), the dolomitization rate is slowed due to the low volume of dolomitizing fluids and slow nucleation rate, resulting in relatively coarser dolomite crystal sizes.

https://jssr.ui.ac.ir/article_27923_7a0ef69b7aa6e153a25497f27c0e98d3.pdf

University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 Palaeoesedimentary environment, sequence stratigraphy and geochemistry of the Taleh Zang Formation in Ritt Anticline, southeastern Lorestan Basin Palaeoesedimentary environment, sequence stratigraphy and geochemistry of the Taleh Zang Formation in Ritt Anticline, southeastern Lorestan Basin 81 98 28004 10.22108/jssr.2023.138826.1267 FA Iraj Maghfouri Moghaddam Associate Professor, Department of Geology, Faculty of Sciences, Lorestan University, Lorestan, Iran 0000-0002-0116-1768 Iraj Sedaghatnia PhD student, Bu Ali Sina University, Hamedan, Iran; Central Laboratory ,Lorestan University, Khorram Abad, Iran Journal Article 2023 08 29 The carbonate deposits of the Taleh Zang Formation are exposed extensively in the southeastern to northern Lorestan zone, SW Iran. The thickness of the shallow water carbonate Taleh Zang Formation in Ritt Anticline is 84.5 meters. It overlies on top of the turbidity Amiran Formation and it is overlain by the clastic-dominated Kashkan Formation. Petrographic examinations revealed seven facies deposited in tidal flat, lagoon, shoal and open marine. Detailed analysis of sedimentary facies indicates that in the Late Paleocene interval, the depositional system of the Taleh Zangi Formation was a ramp carbonate platform. Changes in the depositional facies and cycle stacking patterns indicate one transgressive-regressive sea-level cycle from the bottom to the top of the section, which is equivalent to the last eustatic sea level rise in the Late Paleocene (Thanetian). This depositional sequence is separated by type 2 and 1 sequence boundaries at its lower and upper boundaries, respectively. Elemental geochemical evidence indicates a closed to weakly open digenetic system, with low water-rock interaction for carbonates of the Taleh Zang Formation. The relatively open digenesis system in this formation could be due to the influence of meteoric fluids in the erosional discontinuity at the border between the two formations, the Taleh Zang and Kashkan formations. Keywords: Taleh Zang Formation, Geochemistry, Sedimentary environment, Sedimentary sequence, Lorestan zone Introduction The use of Paleogene biozones (Shallow benthic zone = SBZ) of the Mediterranean basin (introduced by Serra Kilel et al. 1998) in the biostratigraphic studies of the Taleh Zang Formation started several years ago (Bagherpour and Vaziri 2012). Using these biozones, the deposits of Taleh Zang Formation were separated into stages and even substages, which made it possible to compare their palaeoecological conditions and palaeoenvironment with global events (e.g. in South Lorestan by Zohdi and Adabi, 2008; in the south of Kermanshah by Shalavand et al. 2020). Recent studies show that the facies changes of the Taleh Zang Formation are affected by two important factors, one is global changes especially the evolution of the Tethys carbonate platforms (Scheibner and Speizer 2008) and the other is the location of the section of the Tele Zang Formation in the Lorestan Zone. In a way, the age of this formation becomes younger from the north-east to the south-west of the Lorestan region (Jafarizadeh et al. 2023), which is the result of drastic changes in the biological contents and palaeoecological conditions of this formation in different regions. Therefore, it seems that the study of the Taleh Zang Formation in different anticlines of the Lorestan zone plays a significant role in clarifying the evolution of this zone. In the current research, the sedimentary environment and sequence stratigraphy of this formation have been investigated from the perspective of sedimentology and elemental geochemistry of the Taleh Zang formation in Ritt Anticline. The study of the biostratigraphy of the Tele Zang Formation in this anticline has been done by (Zakerzadeh et al. 2023), whose results are the basis for determining the age of the Taleh Zang Formation in the present study. Material & Methods A total of 60 samples were collected. Thin sections were prepared from the collected samples and subsequently analyzed using an optical microscope. Facies description was based on field observation and microfacies characteristics including skeletal and non-skeletal components, depositional texture, and grain size. The microfacies classification follows (Flügel 1010) and (Wilson 1975). The study area is located in Kerki Village about 63 southeast of Khorram Abad city. The section was measured in detail at 33°04′1″ N and 48°14′23″E. Twenty samples of 50 grams were selected and the powdered. The samples were subjected to elemental analysis in the Central Laboratory of Lorestan University. In the present study, elemental geochemical methods (use of major and minor elements present in carbonate samples) and petrographic studies were used to determine the sedimentary environment and the depositional sequence of the Taleh Zang Formation. Discussion of Results & Conclusions Based on biogenic composition, textural and lithological characteristics, seven facies were identified in the carbonate succession of the Taleh Zang Formation. These facies are related to four sub-environments including tidal flat, lagoon, shoal and open marine, of the carbonate ramp. The facies of the tidal zone (MF1) include dolomicrites (microcrystalline dolomites). The absence of skeletal particles and a small amount of detrital particles are the characteristics of this sub-environment. The microfacies of the lagoon environment (MF2-5) include sandy wackestone to packstone; bioclastic benthic foraminifera pelloid wackestone; bioclastic benthic foraminifera–green algae wackestone to packstone; bioclastic pelloid packstone. The facies of the shoal consists of Coral boundstone (MF6). The open marine facies represented by bioclastic coral-corallinacean floatstone (MF7). According to the field and laboratory evidence, the Taleh Zang Formation in this section consists of a third-order depositional sequence with a thickness of 84.5 meters. This depositional sequence is mostly carbonate and consists of limestones and to a lesser content dolomitic limestone. The lower boundary of this depositional sequence with the Amiran Formation is sequence boundary (SB) type 2. The upper boundary of this sequence was defined by the clastic of the Kashkan Formation that showed disconformity (SB1). Considering the discontinuity between the Taleh Zang and the Keshkan formations and the closed to slightly open digenesis system of the studied deposits, the strata of the high strand system tracts (HST) facies are slightly affected by meteoric digenesis fluids, and their Sr content is slightly higher than that of the transgressive system tracts (TST) facies. It may be due to the partial dissolution of aragonite shells in the environment, while the Mn values of these sediments are higher than those of the TST facies. The change process of Sr element to Na and Mn versus Sr indicates their primary aragonite mineralogy. Comparison of Sr/Ca values versus Mn and Mg elements indicates a closed to slightly open digenesis system with low water-rock interaction for the carbonates of Taleh Zang Formation. The relatively open digenesis system in this formation can be due to the influence of meteoric fluids, which is affected by the erosional discontinuity at the boundary of the Taleh Zang and Kashkan formations The carbonate deposits of the Taleh Zang Formation are exposed extensively in the southeastern to northern Lorestan zone, SW Iran. The thickness of the shallow water carbonate Taleh Zang Formation in Ritt Anticline is 84.5 meters. It overlies on top of the turbidity Amiran Formation and it is overlain by the clastic-dominated Kashkan Formation. Petrographic examinations revealed seven facies deposited in tidal flat, lagoon, shoal and open marine. Detailed analysis of sedimentary facies indicates that in the Late Paleocene interval, the depositional system of the Taleh Zangi Formation was a ramp carbonate platform. Changes in the depositional facies and cycle stacking patterns indicate one transgressive-regressive sea-level cycle from the bottom to the top of the section, which is equivalent to the last eustatic sea level rise in the Late Paleocene (Thanetian). This depositional sequence is separated by type 2 and 1 sequence boundaries at its lower and upper boundaries, respectively. Elemental geochemical evidence indicates a closed to weakly open digenetic system, with low water-rock interaction for carbonates of the Taleh Zang Formation. The relatively open digenesis system in this formation could be due to the influence of meteoric fluids in the erosional discontinuity at the border between the two formations, the Taleh Zang and Kashkan formations. Keywords: Taleh Zang Formation, Geochemistry, Sedimentary environment, Sedimentary sequence, Lorestan zone Introduction The use of Paleogene biozones (Shallow benthic zone = SBZ) of the Mediterranean basin (introduced by Serra Kilel et al. 1998) in the biostratigraphic studies of the Taleh Zang Formation started several years ago (Bagherpour and Vaziri 2012). Using these biozones, the deposits of Taleh Zang Formation were separated into stages and even substages, which made it possible to compare their palaeoecological conditions and palaeoenvironment with global events (e.g. in South Lorestan by Zohdi and Adabi, 2008; in the south of Kermanshah by Shalavand et al. 2020). Recent studies show that the facies changes of the Taleh Zang Formation are affected by two important factors, one is global changes especially the evolution of the Tethys carbonate platforms (Scheibner and Speizer 2008) and the other is the location of the section of the Tele Zang Formation in the Lorestan Zone. In a way, the age of this formation becomes younger from the north-east to the south-west of the Lorestan region (Jafarizadeh et al. 2023), which is the result of drastic changes in the biological contents and palaeoecological conditions of this formation in different regions. Therefore, it seems that the study of the Taleh Zang Formation in different anticlines of the Lorestan zone plays a significant role in clarifying the evolution of this zone. In the current research, the sedimentary environment and sequence stratigraphy of this formation have been investigated from the perspective of sedimentology and elemental geochemistry of the Taleh Zang formation in Ritt Anticline. The study of the biostratigraphy of the Tele Zang Formation in this anticline has been done by (Zakerzadeh et al. 2023), whose results are the basis for determining the age of the Taleh Zang Formation in the present study. Material & Methods A total of 60 samples were collected. Thin sections were prepared from the collected samples and subsequently analyzed using an optical microscope. Facies description was based on field observation and microfacies characteristics including skeletal and non-skeletal components, depositional texture, and grain size. The microfacies classification follows (Flügel 1010) and (Wilson 1975). The study area is located in Kerki Village about 63 southeast of Khorram Abad city. The section was measured in detail at 33°04′1″ N and 48°14′23″E. Twenty samples of 50 grams were selected and the powdered. The samples were subjected to elemental analysis in the Central Laboratory of Lorestan University. In the present study, elemental geochemical methods (use of major and minor elements present in carbonate samples) and petrographic studies were used to determine the sedimentary environment and the depositional sequence of the Taleh Zang Formation. Discussion of Results & Conclusions Based on biogenic composition, textural and lithological characteristics, seven facies were identified in the carbonate succession of the Taleh Zang Formation. These facies are related to four sub-environments including tidal flat, lagoon, shoal and open marine, of the carbonate ramp. The facies of the tidal zone (MF1) include dolomicrites (microcrystalline dolomites). The absence of skeletal particles and a small amount of detrital particles are the characteristics of this sub-environment. The microfacies of the lagoon environment (MF2-5) include sandy wackestone to packstone; bioclastic benthic foraminifera pelloid wackestone; bioclastic benthic foraminifera–green algae wackestone to packstone; bioclastic pelloid packstone. The facies of the shoal consists of Coral boundstone (MF6). The open marine facies represented by bioclastic coral-corallinacean floatstone (MF7). According to the field and laboratory evidence, the Taleh Zang Formation in this section consists of a third-order depositional sequence with a thickness of 84.5 meters. This depositional sequence is mostly carbonate and consists of limestones and to a lesser content dolomitic limestone. The lower boundary of this depositional sequence with the Amiran Formation is sequence boundary (SB) type 2. The upper boundary of this sequence was defined by the clastic of the Kashkan Formation that showed disconformity (SB1). Considering the discontinuity between the Taleh Zang and the Keshkan formations and the closed to slightly open digenesis system of the studied deposits, the strata of the high strand system tracts (HST) facies are slightly affected by meteoric digenesis fluids, and their Sr content is slightly higher than that of the transgressive system tracts (TST) facies. It may be due to the partial dissolution of aragonite shells in the environment, while the Mn values of these sediments are higher than those of the TST facies. The change process of Sr element to Na and Mn versus Sr indicates their primary aragonite mineralogy. Comparison of Sr/Ca values versus Mn and Mg elements indicates a closed to slightly open digenesis system with low water-rock interaction for the carbonates of Taleh Zang Formation. The relatively open digenesis system in this formation can be due to the influence of meteoric fluids, which is affected by the erosional discontinuity at the boundary of the Taleh Zang and Kashkan formations

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University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 Identifying organic-rich layers and reconstructing trophic levels in the Dalichai Formation in north Damghan Identifying organic-rich layers and reconstructing trophic levels in the Dalichai Formation in north Damghan 99 113 28066 10.22108/jssr.2023.139476.1272 FA Elahe Zarei Assistant Professor, School of Earth Sciences, Damghan University, Damghan, Iran Journal Article 2023 11 01 Abstract The Dalichai Formation in the Ahvanu section, north of Damghan city, with a thickness of 78 meters, comprises alternations of bluish-gray marls and limestones. Microfacies and palynofacies studies, along with fossil evidence, including foraminifera diversity and abundance, the ratio of epifaunal to infaunal benthic foraminifera, and the ratio of surface to deep infaunal, were utilized to identify organic-rich layers and reconstruct trophic levels. These factors confirm a mesotrophic environment with average oxygen and nutrient levels for the Dalichai Formation (Bajocian to Lower Callovian) in the studied section. The primary reasons for low organic matter in the Dalichai Formation are attributed to low production and poor preservation of organic matter. However, dark-colored deposits with high organic matter at the Bajocian–Bathonian boundary and the lower and upper Callovian boundaries suggest increased production rates in anaerobic and eutrophic conditions. Macrofossil fragments (ammonites and echinoids) within limestone deposits at the uppermost part of the Dalichai Formation, exhibiting low organic matter, and the presence of Zeophycus trace fossils, suggest an oligotrophic environment with dysoxic conditions in the Upper Callovian. Keywords: Trophic level, Organic facies, Dalichai Formation, Foraminifera, Palynomorphs Introduction The stratigraphic section of Ahvano, studied near the village in the north of Damghan, has coordinates E 54° 10΄ 42˝ longitude and N 36° 12΄44˝ latitude. The Dalichai Formation, 78 meters thick, exhibits shale-marl lithology with limestone interlayers, characteristic of the structural range of eastern Alborz. The lower boundary adjoins the Shemshak Formation with a parallel discontinuity, while the upper boundary is overlaid by the limestones of the Lar Formation. The presence of facies rich in organic matter depends not only on suitable conditions for their production but also on two other fundamental factors: the rate of sedimentation and the presence of oxygen-poor waters to protect them (Bombardiere and Gorin 2000). To investigate and determine the trophic level and its role in the formation of the organic facies of the Delichai Formation in the studied section, various tools, including palynomorphs, foraminifers, and microfacies studies, were employed. Material & Methods For palynological studies, 18 samples were taken at intervals of one to five meters. The samples were prepared using the standard Travers method (Travers 2007). Each sample (50 grams) underwent treatment with hydrochloric acid and 30% hydrofluoric acid, followed by neutralization with a heavy solution of hydrogen chloride (ZnCl2). The treatment has been done at the biostratigraphy laboratory of the Damghan University. Thin sections were prepared and employed for the foraminifera study. Statistical analyses involved selecting 20 fields for each sample, and graphs were created using Excel. Discussion of Results & Conclusions Microfacies studies, palynological analyses, and fossil evidence, encompassing foraminifera diversity and abundance, the ratio of epifaunal to infaunal forms, and the ratio of surface infaunal to deep infaunal forms, were employed to reconstruct trophic levels during the Dalichai Formation deposition in the Ahvanu section in eastern Alborz. The presence of Nannoceratopsis spp. from dinoflagellate proximate forms, low percentages of amorphous organic materials (AOM), and benthic foraminifer presence, especially epifaunal species like Ophthalmidium sp., Triloculina sp., Quinquluculina sp., and Glomospira sp., with terrestrial debris influx, indicates a gradual rise in sea level in the late Bajocian deposits, creating a mesotrophic environment. However, at the Bajocian–Bathonian boundary, a significant decrease in the diversity and abundance of epifaunal benthic foraminifers is observed, with an increase in forms such as Nodosaria sp., Textularia sp., and Lenticulina sp. Palynological studies support these conditions, revealing the appearance and abundance of planktonic foraminifers Globigerina spp. and radiolarians, indicating a eutrophic environment (Baumgartner 2013). From sample number 10 to sample number 13 in microfacies A2 (peloid foraminifera wackestone) and microfacies A3 (bioclast Wackestone), there is an increase in the ratio of epifaunal benthic forms to infaunal, with the presence of radiolarian and sponge spicules, indicative of a mesotrophic environment. However, from sample number 13 to 15, a gradual decrease in the ratio of epifaunal benthic forms to infaunal is observed, with an increase in the abundance of deep infaunal forms such as Nodosaria sp. and Dentalina sp., leading to a decline in benthic foraminiferal diversity. In palynological slides, better preservation of marine palynomorphs is noted. These conditions indicate the persistence of a mesotrophic environment until the lower Callovian. Gradually, in the limestone deposits at the uppermost part of the Dalichai Formation in the section under study, there is an increase in the ratio of epifaunal to infaunal forms, fragments of macrofossils (ammonites, echinoids, and bryozoans) with a micrite matrix. Together with the influence of Zoophycus fossils, these observations signify a shift from a mesotrophic to an oligotrophic environment during the Callovian. Microfacies studies, palynological analysis, and fossil evidence, including foraminiferal diversity, epifaunal to infaunal ratios, and surface-to-deep infaunal ratios, were utilized to reconstruct the trophic levels during the Dalichai Formation deposition in the Ahano section. These indicators confirm a mesotrophic environment for the Dalichai Formation deposits in the studied area. Towards the end of the Dalichai Formation, there is a gradual rise in the epifaunal to infaunal ratio, and the presence of macrofossils (ammonites, echinoids, and bryozoans) in micrite background, suggesting a shift from mesotrophic to oligotrophic conditions during the Callovian. However, at the Bajocene–Batonian and early–late Callovian boundaries, there's a notable decrease in the diversity and abundance of epifaunal benthic foraminifers. Planktonic foraminifers (Globigerina sp.), along with increased bioturbation, Posidonia, and radiolarian abundance, indicate elevated production and organic matter preservation, suggesting a transition to a eutrophic environment. Abstract The Dalichai Formation in the Ahvanu section, north of Damghan city, with a thickness of 78 meters, comprises alternations of bluish-gray marls and limestones. Microfacies and palynofacies studies, along with fossil evidence, including foraminifera diversity and abundance, the ratio of epifaunal to infaunal benthic foraminifera, and the ratio of surface to deep infaunal, were utilized to identify organic-rich layers and reconstruct trophic levels. These factors confirm a mesotrophic environment with average oxygen and nutrient levels for the Dalichai Formation (Bajocian to Lower Callovian) in the studied section. The primary reasons for low organic matter in the Dalichai Formation are attributed to low production and poor preservation of organic matter. However, dark-colored deposits with high organic matter at the Bajocian–Bathonian boundary and the lower and upper Callovian boundaries suggest increased production rates in anaerobic and eutrophic conditions. Macrofossil fragments (ammonites and echinoids) within limestone deposits at the uppermost part of the Dalichai Formation, exhibiting low organic matter, and the presence of Zeophycus trace fossils, suggest an oligotrophic environment with dysoxic conditions in the Upper Callovian. Keywords: Trophic level, Organic facies, Dalichai Formation, Foraminifera, Palynomorphs Introduction The stratigraphic section of Ahvano, studied near the village in the north of Damghan, has coordinates E 54° 10΄ 42˝ longitude and N 36° 12΄44˝ latitude. The Dalichai Formation, 78 meters thick, exhibits shale-marl lithology with limestone interlayers, characteristic of the structural range of eastern Alborz. The lower boundary adjoins the Shemshak Formation with a parallel discontinuity, while the upper boundary is overlaid by the limestones of the Lar Formation. The presence of facies rich in organic matter depends not only on suitable conditions for their production but also on two other fundamental factors: the rate of sedimentation and the presence of oxygen-poor waters to protect them (Bombardiere and Gorin 2000). To investigate and determine the trophic level and its role in the formation of the organic facies of the Delichai Formation in the studied section, various tools, including palynomorphs, foraminifers, and microfacies studies, were employed. Material & Methods For palynological studies, 18 samples were taken at intervals of one to five meters. The samples were prepared using the standard Travers method (Travers 2007). Each sample (50 grams) underwent treatment with hydrochloric acid and 30% hydrofluoric acid, followed by neutralization with a heavy solution of hydrogen chloride (ZnCl2). The treatment has been done at the biostratigraphy laboratory of the Damghan University. Thin sections were prepared and employed for the foraminifera study. Statistical analyses involved selecting 20 fields for each sample, and graphs were created using Excel. Discussion of Results & Conclusions Microfacies studies, palynological analyses, and fossil evidence, encompassing foraminifera diversity and abundance, the ratio of epifaunal to infaunal forms, and the ratio of surface infaunal to deep infaunal forms, were employed to reconstruct trophic levels during the Dalichai Formation deposition in the Ahvanu section in eastern Alborz. The presence of Nannoceratopsis spp. from dinoflagellate proximate forms, low percentages of amorphous organic materials (AOM), and benthic foraminifer presence, especially epifaunal species like Ophthalmidium sp., Triloculina sp., Quinquluculina sp., and Glomospira sp., with terrestrial debris influx, indicates a gradual rise in sea level in the late Bajocian deposits, creating a mesotrophic environment. However, at the Bajocian–Bathonian boundary, a significant decrease in the diversity and abundance of epifaunal benthic foraminifers is observed, with an increase in forms such as Nodosaria sp., Textularia sp., and Lenticulina sp. Palynological studies support these conditions, revealing the appearance and abundance of planktonic foraminifers Globigerina spp. and radiolarians, indicating a eutrophic environment (Baumgartner 2013). From sample number 10 to sample number 13 in microfacies A2 (peloid foraminifera wackestone) and microfacies A3 (bioclast Wackestone), there is an increase in the ratio of epifaunal benthic forms to infaunal, with the presence of radiolarian and sponge spicules, indicative of a mesotrophic environment. However, from sample number 13 to 15, a gradual decrease in the ratio of epifaunal benthic forms to infaunal is observed, with an increase in the abundance of deep infaunal forms such as Nodosaria sp. and Dentalina sp., leading to a decline in benthic foraminiferal diversity. In palynological slides, better preservation of marine palynomorphs is noted. These conditions indicate the persistence of a mesotrophic environment until the lower Callovian. Gradually, in the limestone deposits at the uppermost part of the Dalichai Formation in the section under study, there is an increase in the ratio of epifaunal to infaunal forms, fragments of macrofossils (ammonites, echinoids, and bryozoans) with a micrite matrix. Together with the influence of Zoophycus fossils, these observations signify a shift from a mesotrophic to an oligotrophic environment during the Callovian. Microfacies studies, palynological analysis, and fossil evidence, including foraminiferal diversity, epifaunal to infaunal ratios, and surface-to-deep infaunal ratios, were utilized to reconstruct the trophic levels during the Dalichai Formation deposition in the Ahano section. These indicators confirm a mesotrophic environment for the Dalichai Formation deposits in the studied area. Towards the end of the Dalichai Formation, there is a gradual rise in the epifaunal to infaunal ratio, and the presence of macrofossils (ammonites, echinoids, and bryozoans) in micrite background, suggesting a shift from mesotrophic to oligotrophic conditions during the Callovian. However, at the Bajocene–Batonian and early–late Callovian boundaries, there's a notable decrease in the diversity and abundance of epifaunal benthic foraminifers. Planktonic foraminifers (Globigerina sp.), along with increased bioturbation, Posidonia, and radiolarian abundance, indicate elevated production and organic matter preservation, suggesting a transition to a eutrophic environment.

https://jssr.ui.ac.ir/article_28066_2d1c5748ea65b7e0b39665c281804b60.pdf