دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 39 2 2023 06 22 Sequence stratigraphy and microfacies of the Sarvak Formation, west of the Hendijan–Bahregansar–Nowrooz Palaeohigh چینه نگاری سکانسی و ریزرخساره‌های سازند سروک در غرب بلندای قدیم هندیجان- بهرگانسر- نوروز 23 44 27991 10.22108/jssr.2023.138811.1266 FA سولماز صادقی دانشجوی دکتری گروه زمین شناسی، دانشکده علوم زمین، دانشگاه خوارزمی، تهران، ایران حسین هاشمی دانشیار گروه زمین شناسی، دانشکده علوم‏ زمین، دانشگاه خوارزمی، تهران، ایران بیژن بیرانوند استادیار پژوهشگاه صنعت نفت، تهران، ایران 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. سازند سروک (کرتاسه میانی) یکی از مهم‌ترین مخازن نفتی در جنوب غرب ایران به شمار می‌رود. گسل هندیجان- بهرگانسر- نوروز به طول بیش از 700 کیلومتر یک گسل تقریباً عمودی-راست‌گرد لغزشی در روند عربی (NNE-SSW) است که ضمن عبور از شمال غربی خلیج‌فارس و تاثیرگذاری بر تکامل زمین شناسی منطقه نقش مهمی در تحولات چینه شناسی و رخساه های رسوبی سازند سروک داشته است. از اینرو در مطالعه اخیر به بررسی ریز رخساره‌ها، محیط رسوبی و چینـه نگـاری سکانـسی سـازند سـروک در غرب بلندای قدیم هندیجان- بهرگانسر- نوروز در میادین هندیجان، بهرگانسر و ماهشهر پرداخته شد. برای این منظور تعداد 186مقطع نازک مورد مطالعات پتروگرافی، رسوب شناسی و چینه شناسی قرار گرفت. این مطالعه مجموعاً منجر به شناسایی ۷ ریزرخساره در ۴ کمربند رخساره ای پهنه جزر و مدی، لاگون، سد، و دریای باز گردید. وجود ریف های پشته ای و نبود رسوبات توربیدایتی یک پلاتفرم کربناته کم عمق از نوع رمپ را برای تشکیل رسوبات سازند سروک در این ناحیه پیشنهاد می کند. همچنین چینه نگاری سکانسی رسوبات مورد مطالعه در شمال غرب گسل هندیجان- بهرگانسر- نوروز منجر به شناسائی پنج سکانس رسوبی رده سوم شد. بر اساس نتایج به دست آمده، اگرچه منطقه مورد مطالعه در اوایل سنومانین از نظر ساختاری پایدار بود اما از اواخر سنومانین تحت تاثیر فازهای تکتونیکی مهم در امتداد یک برجستگی قدیمی شروع به بالاآمدن کرد. همچنین بالاآمدگی صفحه عربی در اوایل تورونین، موجب عقب نشینی آب دریا و خروج رسوبات سنومانین از آب و در نتیجه وقوع یک فاز فرسایشی مهم در مرز سنومانین و تورنین در بیشتر مناطق (میادین بهرگانسر و هندیجان) شد. در میدان ماهشهر به دلیل نبود سنگواره های شاخص ، اطلاعات جامع و با استناد به اطلاعات مقاطع لرزه نگاری، لاگ های پتروفیزیک و مطابقت ریز رخساره‌ها با چاههای مشابه احتمالارسوبات کربناته بخش فوقانی سازند سروک در روند شمال شمال غرب – جنوب جنوب شرق، به صورت آنلپ در کناره‌های این بلندای قدیمه ته‌نشین شدند.

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