A Study of microfacies and diagenetic processes of the Asmari Formation in Khesht Oil Field with emphasis on reservoir characteristic: a case study from Zagros basin, Fars, SW Iran

The Oligo-Miocene Asmari Formation is the most known oil reservoir in the southwest of Iran and the Middle East. In the present study, in order to understand the reservoir quality controlling factors of the Asmari Formation, detailed petrographic studies were carried out in the Khesht Oil Field to identify microfacies, diagenetic processes affecting these microfacies and their paragenetic sequence. Six carbonate microfacies are indicated based on about two hundred microscopic thin section analyses from core samples in this field deposited in tidal flat and lagoon sub-environments on a ramp type platform. The results  show that important diagenetic processes that have affected the Asmari Formation carbonates  includ micritization, cementation (marine, meteoric, and burial cements), dissolution (meteoric and burial dissolution), neomorphism, mechanical and chemical compactions, fracturing, dolomitization (dolomicrite, dolomicrosparite, dolosparite and dolomite cement) and anhydritization (in different types). Dissolution and fracturing have enhanced porosity and permeability values, while cementation and compaction have decreased the reservoir potential of the Asmari Formation by occluding the pore spaces. Dolomitization has had a dual impact on the reservoir quality of the Asmari Formation.