University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 3 2023 09 23 Petrographic and geochemical evidence of diagenetic alterations in the Sarvak Formation in an oilfield from the Abadan Plain, west of Iran Petrographic and geochemical evidence of diagenetic alterations in the Sarvak Formation in an oilfield from the Abadan Plain, west of Iran 79 102 28166 10.22108/jssr.2024.139935.1274 FA Ramin Abbasi M.Sc. Student, Soft-Rock Department, School of Geology, College of Science, University of Tehran, Tehran, Iran Hamzeh Mehrabi Assistant Professor, Soft-Rock Department, School of Geology, College of Science, University of Tehran, Tehran, Iran 0000-0002-2211-4899 Emad Yahyaei M.Sc. Student, Soft-Rock Department, School of Geology, College of Science, University of Tehran, Tehran, Iran Hossein Rahimpor Professor, Soft-Rock Department, School of Geology, College of Science, University of Tehran, Tehran, Iran Journal Article 2023 11 29 Abstract The Sarvak Formation, the second important oil reservoir in the Zagros Basin, has a complicated diagenetic history. This study focuses on the diagenetic processes of this formation in the Abadan Plain. To achieve this goal, petrographic investigations of core samples, thin sections, X-ray diffraction, and scanning electron microscopy are integrated with the results of elemental geochemical data. Intensive meteoric dissolution (karstification), paleosol formation, dissolution-collapsed brecciation, meteoric cementation, and silicification are major meteoric diagenetic processes. Such intensive meteoric diagenesis along with the dominance of kaolinite and montmorillonite, as predominant clay types within the paleosol, all indicate a warm and humid paleoclimatic condition at the time of exposure. These diagenetic alterations provided special trends of variations in the elemental contents of altered carbonates. They include a clear increase in Na, Mn, Fe, and Rb along with the decrease in Sr contents recorded below the Cenomanian–Turonian and mid-Turonian disconformities. The variations in Mg contents depend on the original mineralogy of carbonates that can result in variable trends in response to the meteoric diagenesis. Below the mid-Turonian disconformity, the development of mud-dominated facies hampered the free fluid circulation and, consequently, diagenetic alterations and their related geochemical trends are limited within the Turonian sequence. Keywords: Sarvak Formation, Trace elements, Palaeoclimate, Meteoric diagenesis, Abadan Plain Introduction The Sarvak Formation, as a member of the Bangestan Group, is an important oil reservoir in Iran, especially in the Abadan Plain (Motiei 1993). It is mainly composed of carbonate rocks deposited on the northeast margin of the Arabian Plate (Fard et al. 2006). In carbonate successions, complex diagenesis history is strongly influenced by the combined effects of tectonics, eustacy, and palaeoclimate (Ahr 2008). The Zagros Basin has experienced an active tectonic setting and a warm and humid palaeoclimatic condition, during the Late Cretaceous (Mehrabi 2023). At this time, subaerial exposure of carbonate successions has resulted in the development of disconformities at the Cenomanian–Turonian boundary (CT-ES) and middle Turonian (mT-ES) (Rahimpour-Bonab et al. 2013). This study focuses on the petrographic and geochemical investigations of diagenetic features in the Sarvak Formation in an oilfield in the Abadan Plain. Material & Methods In this study, elemental geochemical analyses of 27 samples taken from the Sarvak Formation are integrated with a petrographic study of 831 thin sections in a well, located in the Abadan Plain. Bulk rock and micrite samples were taken for geochemical analyses. After macro- and microscopic studies, 1.5 to 2 mg of powder was prepared using a dental bit (tungsten carbide). This powder was cleaned to remove the organic matter and oil staining and, then, reacted with pure phosphoric acid to produce the carbon dioxide gas. A Kiel III Thermo Finnigan 252 mass spectrometer is used at the Arak University for elemental analysis. Discussion of Results & Conclusions The main results of this study are as follows: Dissolution and Karstification: Intensive meteoric dissolution in the forms of the vuggy, channel, and moldic pores are frequently recorded in the upper parts of the Sarvak Formation. These pores are subsequently filled by meteoric or burial cements or remain unfilled to provide high reservoir potential beneath the palaeoexposure surfaces. Silicification and Brecciation: Replacement of silica within the skeletal fragments (mostly rudists) or in the form of cement filling the burrows and intra-skeletal pores are recorded at the topmost parts of the Sarvak Formation. It shows close association with other meteoric diagenetic features including dissolution, meteoric cementation, and paleosols. Brecciation is also recorded in microscopic studies. A clay-rich matrix filled between these breccias. Development of paleosols: Bauxite and laterite are common paleosols recorded at the topmost parts of the Sarvak Formation. High Fe and Al contents are measured from these horizons. Fe-oxide staining is distinguished within the weathered and karstified units of this formation, below the disconformable surfaces. Moreover, kaolinite and montmorillonite are common clay minerals within these paleosols that indicate a warm and humid climatic condition at the time of exposure. Trace Elements: Elemental contents of Sr, Rb, Fe, Na, Ca, Mn, and Mg are measured from the Sarvak Formation. Manganese (Mn): Mn values of analyzed samples range from 5 to 809 ppm, with an average of 407 ppm. It shows a uniform trend across the Cenomanian sequence, with a sharp increase at the beginning of Turonian as 809 ppm. Sodium (Na): This element varies from 791 to 15439 ppm in the analyzed samples. An increase in Na is recorded at the base of the Cenomanian that changes to lower values in the upper Cenomanian. Two peaks of Na are recorded around the Cenomanian–Turonian boundary. Lower values of Na are measured from the Turonian sequence. Strontium (Sr): The Sr content of the studied well changes from 28 to 604 ppm, with a mean of 367 ppm. Decreased Sr contents are recorded in the lower part of the Cenomanian sequence that changes to higher concentrations in the upper part of this sequence. Generally, the Sr content of the Cenomanian sequence is low. Around the C-T boundary, perturbations in Sr concentrations are recorded with sharp decreases below the disconformities. Paragenetic Sequence: The paragenetic sequence of the Sarvak Formation includes its deposition in the marine realm, experiencing two stages of meteoric diagenesis, and passing through shallow to deep burial realms. Elemental Evidence: The effects of meteoric waters on marine carbonates commonly result in an increase in Mn concentrations in the altered carbonates (Brand and Veizer, 1980). Such an increase in Mn content is recorded at the C–T and mT paleoexposures. The variations in Na concentrations are strongly facies dependent. Sharp increasing peaks of Na are recorded at disconformable surfaces indicating a meteoric diagenetic effect (Brand and Veizer 1980). Decreased values of Sr are expected within the meteorically-altered carbonates, because the Sr concentration in meteoric waters (0.1–0.01 ppm) is much lower than the marine carbonates (1000–9400 ppm) (Brand and Veizer 1980). In the Sarvak Formation, sharp decreases in Sr are measured from the karstified intervals below the paleoexposure surfaces. Geochemical Correlation: The geochemical profile of the Sarvak Formation in the studied well is correlated with a previous study in the Dezful Embayment (Mehrabi et al. 2022). As shown, there is a close correlation between these sections, especially regarding the trace elemental concentrations. Both C–T and mT disconformities are distinguished across the Zagros Basin, including the Abadan Plain, Dezful Embayment, Izeh, and Fars zones. Consequently, similar geochemical trends are formed as a result of intensive meteoric diagenetic alterations below these disconformable surfaces. Abstract The Sarvak Formation, the second important oil reservoir in the Zagros Basin, has a complicated diagenetic history. This study focuses on the diagenetic processes of this formation in the Abadan Plain. To achieve this goal, petrographic investigations of core samples, thin sections, X-ray diffraction, and scanning electron microscopy are integrated with the results of elemental geochemical data. Intensive meteoric dissolution (karstification), paleosol formation, dissolution-collapsed brecciation, meteoric cementation, and silicification are major meteoric diagenetic processes. Such intensive meteoric diagenesis along with the dominance of kaolinite and montmorillonite, as predominant clay types within the paleosol, all indicate a warm and humid paleoclimatic condition at the time of exposure. These diagenetic alterations provided special trends of variations in the elemental contents of altered carbonates. They include a clear increase in Na, Mn, Fe, and Rb along with the decrease in Sr contents recorded below the Cenomanian–Turonian and mid-Turonian disconformities. The variations in Mg contents depend on the original mineralogy of carbonates that can result in variable trends in response to the meteoric diagenesis. Below the mid-Turonian disconformity, the development of mud-dominated facies hampered the free fluid circulation and, consequently, diagenetic alterations and their related geochemical trends are limited within the Turonian sequence. Keywords: Sarvak Formation, Trace elements, Palaeoclimate, Meteoric diagenesis, Abadan Plain Introduction The Sarvak Formation, as a member of the Bangestan Group, is an important oil reservoir in Iran, especially in the Abadan Plain (Motiei 1993). It is mainly composed of carbonate rocks deposited on the northeast margin of the Arabian Plate (Fard et al. 2006). In carbonate successions, complex diagenesis history is strongly influenced by the combined effects of tectonics, eustacy, and palaeoclimate (Ahr 2008). The Zagros Basin has experienced an active tectonic setting and a warm and humid palaeoclimatic condition, during the Late Cretaceous (Mehrabi 2023). At this time, subaerial exposure of carbonate successions has resulted in the development of disconformities at the Cenomanian–Turonian boundary (CT-ES) and middle Turonian (mT-ES) (Rahimpour-Bonab et al. 2013). This study focuses on the petrographic and geochemical investigations of diagenetic features in the Sarvak Formation in an oilfield in the Abadan Plain. Material & Methods In this study, elemental geochemical analyses of 27 samples taken from the Sarvak Formation are integrated with a petrographic study of 831 thin sections in a well, located in the Abadan Plain. Bulk rock and micrite samples were taken for geochemical analyses. After macro- and microscopic studies, 1.5 to 2 mg of powder was prepared using a dental bit (tungsten carbide). This powder was cleaned to remove the organic matter and oil staining and, then, reacted with pure phosphoric acid to produce the carbon dioxide gas. A Kiel III Thermo Finnigan 252 mass spectrometer is used at the Arak University for elemental analysis. Discussion of Results & Conclusions The main results of this study are as follows: Dissolution and Karstification: Intensive meteoric dissolution in the forms of the vuggy, channel, and moldic pores are frequently recorded in the upper parts of the Sarvak Formation. These pores are subsequently filled by meteoric or burial cements or remain unfilled to provide high reservoir potential beneath the palaeoexposure surfaces. Silicification and Brecciation: Replacement of silica within the skeletal fragments (mostly rudists) or in the form of cement filling the burrows and intra-skeletal pores are recorded at the topmost parts of the Sarvak Formation. It shows close association with other meteoric diagenetic features including dissolution, meteoric cementation, and paleosols. Brecciation is also recorded in microscopic studies. A clay-rich matrix filled between these breccias. Development of paleosols: Bauxite and laterite are common paleosols recorded at the topmost parts of the Sarvak Formation. High Fe and Al contents are measured from these horizons. Fe-oxide staining is distinguished within the weathered and karstified units of this formation, below the disconformable surfaces. Moreover, kaolinite and montmorillonite are common clay minerals within these paleosols that indicate a warm and humid climatic condition at the time of exposure. Trace Elements: Elemental contents of Sr, Rb, Fe, Na, Ca, Mn, and Mg are measured from the Sarvak Formation. Manganese (Mn): Mn values of analyzed samples range from 5 to 809 ppm, with an average of 407 ppm. It shows a uniform trend across the Cenomanian sequence, with a sharp increase at the beginning of Turonian as 809 ppm. Sodium (Na): This element varies from 791 to 15439 ppm in the analyzed samples. An increase in Na is recorded at the base of the Cenomanian that changes to lower values in the upper Cenomanian. Two peaks of Na are recorded around the Cenomanian–Turonian boundary. Lower values of Na are measured from the Turonian sequence. Strontium (Sr): The Sr content of the studied well changes from 28 to 604 ppm, with a mean of 367 ppm. Decreased Sr contents are recorded in the lower part of the Cenomanian sequence that changes to higher concentrations in the upper part of this sequence. Generally, the Sr content of the Cenomanian sequence is low. Around the C-T boundary, perturbations in Sr concentrations are recorded with sharp decreases below the disconformities. Paragenetic Sequence: The paragenetic sequence of the Sarvak Formation includes its deposition in the marine realm, experiencing two stages of meteoric diagenesis, and passing through shallow to deep burial realms. Elemental Evidence: The effects of meteoric waters on marine carbonates commonly result in an increase in Mn concentrations in the altered carbonates (Brand and Veizer, 1980). Such an increase in Mn content is recorded at the C–T and mT paleoexposures. The variations in Na concentrations are strongly facies dependent. Sharp increasing peaks of Na are recorded at disconformable surfaces indicating a meteoric diagenetic effect (Brand and Veizer 1980). Decreased values of Sr are expected within the meteorically-altered carbonates, because the Sr concentration in meteoric waters (0.1–0.01 ppm) is much lower than the marine carbonates (1000–9400 ppm) (Brand and Veizer 1980). In the Sarvak Formation, sharp decreases in Sr are measured from the karstified intervals below the paleoexposure surfaces. Geochemical Correlation: The geochemical profile of the Sarvak Formation in the studied well is correlated with a previous study in the Dezful Embayment (Mehrabi et al. 2022). As shown, there is a close correlation between these sections, especially regarding the trace elemental concentrations. Both C–T and mT disconformities are distinguished across the Zagros Basin, including the Abadan Plain, Dezful Embayment, Izeh, and Fars zones. Consequently, similar geochemical trends are formed as a result of intensive meteoric diagenetic alterations below these disconformable surfaces.

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