Palaeoecology and microfacies correlation analysis of the Oligocene–Miocene Asmari formation, in the Gachsaran oil field, Dezful Embayment, Zagros Basin, Southwest Iran

Abstract

The Asmari formation is a thick carbonate succession of Oligocene–Miocene age in the Zagros Basin, southwest Iran. This research is focused on the microfacies analysis and paleoecology interpretation (based on the distribution of benthic foraminifera) of carbonate sediments of the Asmari formation. Three sections of this formation studied in the Gachsaran oil field (wells 25, 31 and 37), Dezful Embayment, Zagros Basin (southwest Iran). These sections are composed of limestone, dolomite, and limy dolomite. The Asmari formation has a conformable contact with the overlying (Gachsaran formation) and the underlying (Pabdeh formation) units. Based on petrologic studies, identification of larger foraminifera and facies analysis of the Asmari formation allowed the recognition of 14 facies types. There are grouped into four depositional environments which correspond to the inner, middle, outer ramp, and intrashelf basin. According to this study, deposition of the Asmari formation with association of hyaline, lamellar, perforate large, and flat foraminifera first started in the intrashelf basin environment during the Early Oligocene. In the latest Oligocene–earliest Miocene time, the Asmari platform margin prograded basinward, sediments of shallow water were deposited.