Stratigraphy and depositional environment of the Asmari Formation in selected locations in the Persian Gulf

The Oligo-Miocene Asmari Formation is the most important reservoir unit in onshore SW Iran. It is more than 500 m thick in the central Zagros fold-and-thrust belt, decreasing in thickness southwards towards the offshore to less than 200 m in oil fields to the SE and NW of the Persian Gulf, and it does not extend over the Qatar Arch in the central area. Despite its reduced thickness offshore, the formation serves as the primary reservoir unit in a number of important structurally trapped fields, producing natural gas to the SE and oil in the NW. The formation also has stratigraphic potential because of its lithological heterogeneity. The Asmari succession in the Gavarzin gas field to the SE and the Abouzar oil field in the NW part of the Persian Gulf was cored, and the sedimentology and palaeontology of each section was recorded in detail. The Gavarzin core section consisted of 160 m of limestones comprising 10 sedimentary facies, with Ruplian–Chattian index foraminifera. The lower half of the formation is dominated by coralgal limestones of Rupelian age, whereas the upper half comprises Chattian-aged foraminiferal limestones with interbedded shales and marls. Deposition is interpreted to have occurred on a carbonate ramp with coral and red algal patch reefs representing a proximal mid-ramp area. Two third-order sedimentary sequences were identified: the RuS (Rupelian) and the ChS-1 (early Chattian) sequences. The Abouzar core section to the NW is 135 m thick and comprises three members: lower Asmari carbonates, Ghar Member sandstones and upper Asmari carbonates. The 90 m-thick Ghar Member is the main oil reservoir and is roughly twice the thickness of the two carbonate sections combined. The lower Asmari carbonates contain Chattian index foraminifera and, in the absence of Burdigalian microfossils, the Ghar Member and upper Asmari carbonates were assigned to the Aquitanian. The succession comprises eight microfacies and petrofacies, interpreted to have been deposited on a shallow-water carbonate ramp with a significant influx of clastics. Three third-order sequences have been defined of late Chattian (Ch-S sequence) and Aquitanian (AqS-1 and AqS-2 sequences) age. The regional stratigraphy and depositional history of the Asmari was assessed by correlating the Gavarzin and Abouzar sequences with equivalent sequences in 10 additional fields, along two offshore transects to the SE (transect A) and NW (transect B). Seismic reflector profiles highlight a clinoforming sequence on the SE transect, prograding towards the Lengeh Trough during the Rupelian and early Chattian. This is onlapped by Fars salt. The salt unit is barren of microfossils but is probably Chattian and Aquitanian in age. The NW transect suggests that the Asmari Formation and Ghar Member sandstones were largely confined to the Binak Trough.