زیست چینه نگاری، محیط رسوبی و چینه‌ نگاری سکانسی بخش عرب در قسمت میانی حوضه‌ خلیج فارس

بخش عرب (بخش بالایی سازند سورمه) در میدان نفتی بلال واقع در بخش میانی حوضه خلیج فارس بر روی سازند دارب به صورت پیوسته و در زیر سازند هیث با ناپیوستگی فرسایشی قرار گرفته است. این بخش در میدان نفتی مورد مطالعه 130 متر ضخامت دارد و از سنگ آهک ریز تا متوسط دانه، دولومیت ریز تا متوسط دانه و انیدریت تشکیل شده است. در این مطالعه تعداد 200 نمونه به دست آمده از مغزه‌های حفاری از دیدگاه دیرینه شناختی مورد مطالعه قرار گرفته و بر اساس آن دو زیست‌زون kurnubia jurassica interval zone و clypeina jurassica range zone که گویای سن کیمریجین تیتونین هستند، شناسایی شده است. در بخش عرب به طور کلی تعداد 12 رخساره به ترتیب از دریا به خشکی شامل دو رخساره متعلق به کمربند پشته‌ سدی، سه رخساره متعلق به کمربند لاگون، دو رخساره متعلق به پهنه میان کشندی و پنج رخساره متعلق به پهنه فراکشندی شناسایی شده است. با توجه به قانون والتر و شواهد رخساره‌ای موجود، بخش عرب در پلتفرم (رمپ) کربناته هموکلینال با عمق کم نهشته شده است. بر اساس داده‌های حاصل از پرتو گاما، تغییرات رخساره‌ای و روزن‌داران کف‌زی موجود، بخش عرب طی یک سکانس رسوبی رده سوم نهشته شده است. مرز پایین این سکانس رسوبی با توجه به پیوستگی و نبود شواهد خروج از آب از نوع sb2 و مرز سکانسی بالایی با توجه به ناپیوستگی فرسایشی موجود و شواهد خروج از آب از نوع sb1 است. تفسیر تغییرات سطح آب دریای بخش مورد مطالعه در زمان رسوب‌گذاری با منحنی تغییرات سطح آب دریای جهانی تا حدود زیادی مطابقت می‌کند که ‌این امر نشان از یکسان بودن شرایط رسوب‌گذاری آنها است.

Biostratigraphy, depositional environment and sequence stratigraphy of Arab Member in the middle part of Persian Gulf

IntroductionThe Arab Member (upper part of Surmeh Formation) in the Balal oil field in the middle part of the Persian Gulf is 130 meters thickness. It consists of thin to medium grain limestone, thin to medium crystal dolomite, and anhydrite. It is equivalent to the Arab Formation of Saudi Arabia, Bahrain, Qatar, and the United Arab Emirates (AlHusseini, 2008). Materials and MethodsA total of 200 rock samples were selected for analysis. Thinsections were prepared at the Ferdowsi University of Mashhad and the Iranian Offshore Oil Company. The preservation of the microfossils is good, and identification follows Loeblish Tappan (1988), BoudagherFadel (2008), Chiocchini (2008), and Hughes (2018), and they were stained with Alizarin RedS solution using the Dickson (1965) method to distinguish calcite from dolomite. According to Dunham (1962) and Embry Klovan (1971), the carbonate microfacies were classified. Skeletal and nonskeletal grain identification follow Flugel (2010). Parasequences of the Arab Member were recognized according to the sedimentary cycles and their relative sealevel equivalents (Haq et al., 1988; Van Wagoner et al., 19881990). DiscussionThis research aims to determine the age of the studied member, microfacies, and sequence stratigraphy of the upper Surmeh Formation in Balal oil field in the Persian Gulf. The investigation of benthic foraminifera and green algae is led to the identification of 12 species belonging to 12 genera of benthic Foraminifera and 2 species 2 genera of Algae. Based on the index taxa, two biozones are distinguished within the Arab Member: Kurnubia jurassica Interval Zone and Clypeina jurassica Range Zone, which indicate the KimmeridgianTithonian age for the Arab Member in this part of the Persian Gulf Basin. Petrographic investigations is led to the identification of 12 microfacies (Mf1Mf12). These microfacies are grouped into four facies associations that are assigned to four depositional environments (supratidal, intertidal, lagoon, and shoal) sited on the inner part of a homoclinal carbonate ramp. Facies analysis was carried out mainly by using the standard facies model for ramps by Flügel (2010). One thirdorder sequence can be substantiated in this study. It should be noted that the lagoonal facies are considered to represent the maximum flooding surfaces (MFS) for depositional sequence due to the lack of other deeper facies. In addition, sequence boundaries are indicated based on a combination of methods, including core observations, microfacies analyses, and the presence of evaporite minerals along with gammaray logs. By sequence, stratigraphic analysis one thirdorder sequence was recognized in the studied member that consists of transgressive systems tract (TST) and highstand systems tract (HST). ConclusionBased on the determined biozones (Kurnubia jurassica Interval Zone and Clypeina jurassica Range Zone), the age of the studied succession is KimmeridgianTithonian. Petrographic investigations are led to the identification of 12 microfacies. According to Walter’s law and facies evidence, the Arab Member sited on the inner part of a homoclinal carbonate ramp. The Arab Member deposited in one depositional sequence based on seismic data, facies changes, and benthic foraminifera; One thirdorder sequence consists of transgressive systems tract (TST) and highstand systems tract (HST). AcknowledgmentAuthors acknowledge the Research Institute of Petroleum IndustryAhvaz Center Iranian offshore Oil for their exceptional support of this project. ReferencesAlHusseini, M.I., 2008. Launch of the Middle East geologic time scale. GeoArabia, 13 (4): 185188.BoudagherFadel, M.K., 2008. Evolution and geological significance of larger benthic foraminifera. First edition, Palaeontology and Stratigraphy, 571p.Chiocchini, M., Chiocchini, R.A., Didaskalou, P., Potetti, M., 2008. Micropaleontological and biostratigraphical researches on the Mesozoic of the LatiumAbruzzi carbonate platform (Central Italy). Memorie descrittive della, Carta Geologica D’Italia, 6: 6575.Dickson, J.A.D., 1965. A modified staining technique for carbonates in thin section. Nature, 205, 587p.Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham, W.H., (ed.), Classification of carbonate rocks. American Association of Petroleum Geologists Memoir, 1: 108121.Embry, A.F., Klovan, E.J., 1972. Absolute water depth limits of the Devonian paleoecological zones. Geololgy Rundschau, 61: 672686.Flügel, E., 2010. Microfacies of Carbonate Rocks, Analysis, Interpretation and Application. Springer, Heidelberg, Dordrecht, London, New York, 984p.Haq, B.U., Hardenbol, J., Vail, P.R., 1988. Mesozoic and Cenozoic chronostratigraphy and cycles of sealevel change. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.S.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (eds.), Sealevel Changes: an Integrated Approach. Society of Economic Paleontologists and Mineralogists, 42:71108.Hughes, G.W., 2018. A new thinsection based micropaleontological biozonation for Saudia Arabia Jurassic carbonates. Micropaleontoloy, 64 (56): 331364.Loeblish, A.R., Tappan, H; 1988b. Foraminiferal genera and their classification (Plates). Van Nostrand, New York, 212p. Van Wagoner, J.C., Mitchum, R.M.Jr., Campion, K.M., Rahmanian, V.D; 1990. Siliciclastic sequence stratigraphy in well logs, core and outcrops, concepts for highresolution correlation of time and facies. American Association of Petroleum Geologists, Methods in Exploration Series, 7: 155.VanWagoner, J.C., W., Posamentier, Mitchum, H. R.M.Jr., Vail, P.R., Sarg, J.F., Loutit, T.S., Hardenbol, J., 1988. An Overview of the Fundamentals of Sequence Stratigraphy and Key Definitions. In: Wilgus, C.K., Hastings, B.S., Posamentier, H., Van Wagoner, J., Ross, Ch.A., Kendall, Ch.G.St.C. (eds.), Sea Level Changes: An Integrated Approach. SEPM Special Publication, 42: 3945.