Seismic and sequence stratigraphy of the Oligocene-Miocene Asmari reservoir in the Marun oilfield, SW Iran

Abstract

The Oligocene-Lower Miocene Asmari Formation shows considerable reservoir heterogeneity because of variations in the lithology, depositional facies and diagenesis. This paper aim to investigate reservoir heterogeneities using seismic, core and well logs data. Twelve carbonate microfacies and three siliciclastic petrofacies are identified in the Asmari Formation based on well log and core data from 7 wells in the Marun field, which generally indicate a shallowing-up profile. Microfacies changes suggest that the Asmari Formation was deposited in a restricted lagoonal and carbonate ramp setting with periodic clastic sediment supply due to relative sea level fall. Six third-order sequences are recognized in the Asmari Formation from studies of core and well log data, while interpretation of 3D seismic data shows that the formation consists of two second-order seismic sequences. The lowest sequence boundary between the Pabdeh and Asmari Formations (SBI) is a type-2 boundary, and the six others are type-1 sequence boundaries. Six distinct packages of reflectors are interpreted on seismic data through the Cenozoic. The Pabdeh and Asmari Formations can be divided into three packages. Package 1 corresponds to transgressive and highstand systems tract deposits, which mainly consist of shales, marls, and carbonates with interbedded sandstones. This package is overlain by mounded and lenticular seismic facies (package 2) and high-amplitude and continuous seismic reflectors (package 3). Package 2 is predominantly sandstone with interbedded shale, and package3 mainly consists of carbonates. Inversion of seismic data shows that high-porosity zones are present in the western and southern Marun field. A lower high-porosity zone corresponds to lowstand fluvial-deltaic sediments and the upper zone to the beach and shallow marine sandstones.