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

The 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 (Al-Husseini, 2008).

A total of 200 rock samples were selected for analysis. Thin-sections 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), Boudagher-Fadel (2008), Chiocchini (2008), and Hughes (2018), and they were stained with Alizarin Red-S 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 non-skeletal grain identification follow Flugel (2010). Parasequences of the Arab Member were recognized according to the sedimentary cycles and their relative sea-level equivalents (Haq et al., 1988; Van Wagoner et al., 1988-1990).

This 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 Kimmeridgian-Tithonian age for the Arab Member in this part of the Persian Gulf Basin. Petrographic investigations is led to the identification of 12 microfacies (Mf1-Mf12). 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 third-order 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 gamma-ray logs. By sequence, stratigraphic analysis one third-order sequence was recognized in the studied member that consists of transgressive systems tract (TST) and highstand systems tract (HST).

Based on the determined biozones (Kurnubia jurassica Interval Zone and Clypeina jurassica Range Zone), the age of the studied succession is Kimmeridgian-Tithonian. 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 third-order sequence consists of transgressive systems tract (TST) and highstand systems tract (HST).