Biostratigraphy, microfacies, depositional environment and sequence stratigraphy of the Sarvak Formation in one of the oilfields, southwest Iran

Sarvak Formation is one of the units in the Bangestan lithostratigraphic group with Albian-Turonian age. The shallow sea at the end of the Albian to Cenomanian time has deposited large amounts of the shallow marine carbonates (Sarvak Formation) over a large area of the Zagros basin (Motiei, 1995). The Sarvak Formation has two different facies. At the type section and coastal Fars, shallow marine facies of this formation mostly formed while in the Lorestan area, the facies are belonging to deeper water (Motiei, 1995). The Sarvak Formation is a reservoir of hydrocarbon in the Zagros sedimentary basin. Therefore, a detailed study of this formation, based on microfacies and sequence stratigraphy, will provide useful and valuable information for further exploration in this basin. The purpose of this study was to evaluate the biostratigraphy, microfacies, depositional environment and sequence stratigraphy of the Sarvak Formation.

To approach the objectives of this research, two stages have been taken: Preparation of 195 thin sections from drill cores and cuttings (National South Oil Company), then Laboratory studies of thin sections prepared by binocular microscopy and identification of allochems and orthochems and the preparation of photomicrographs. The identified biozones correspond to biozones named after Wynd (1965). In this study, Flugel (2010) method used to describe the microfacies, and also Reading (1996) and Dunham (1962) methods used to determine rock texture and nomenclature Identification, photography and the creation of the Atlas of microfossils, their distribution in stratigraphic column, representation of identified biozones, and sedimentary sequence lithology in the studied area has been made possible through the study of graphs and logs, thin sections, and the use of various research satellites such as Bolli (1945; 1959; 1966), Postuma (1971) and Caron (1983). Sequence stratigraphy of the Sarvak Formation has been studied based on sequence stratigraphic methods and principles (Haq et al., 1987; Van Wagoner et al., 1988; Emery and Myers, 1996; Simmons et al., 2007).

Based on this study, 5 genus and 2 species of pelagic microfossils and 25 genus and 28 species of benthic microfossils were identified. Therefore, three biozones including Nezzazata - alveolinids assemblage zone # 25, Rudist debris zone # 24, and Oligosteginid flood zone # 26 were introduced that are corresponding to the biozones introduced by Wynd (1965). Based on the studied faunal assemblage, the age of the Sarvak Formation is from Cenomanian to Turonian in the studied wells. The upper boundary of the Sarvak Formation with the Gurpi Formation is discontinuous due to the presence of glauconitic nodules at the base of the Gurpi. According to the regional geology of this area and information from the studied wells, the Ilam Formation and the 30th Wynd Biological Zone (Wynd, 1965), as well as the biodegradation of Wynd (1965) at the top of the Sarvak Formation, are absent. The drilling has not reached the base of the Sarvak Formation, therefore the lower boundary is not clear. 7 microfacies identified in the Sarvak Formation in the studied wells. These microfacies are formed in 3 main facies belts including 1- open sea, 2- shoal, and 3- lagoon. Investigation of carbonate rocks of the Sarvak Formation in the study area shows that the sedimentary environment of the Sarvak Formation in the studied field was a shallow carbonate platform of the carbonate ram type. The Sarvak Formation in the study area consists of 1 complete sedimentary sequence and 2 incomplete sequences, which are generally HST and TST in the studied sections. The HST facies in this sequence originate from a variety of marine microfacies and finished with the lagoon. The TST facies is mainly related to the open marine. Finally, the HST facies is related to the inner and middle parts of the platform; while the TST facies is related to the outer and middle parts of the platform.