Depositional environment, geochemistry and diagenetic control of the reservoir quality of the Oligo-Miocene Asmari Formation, a carbonate platform in SW Iran

Abstract

The Oligo-Miocene Asmari Formation in SW Iran represents sedimentation on a carbonate platform. Thin-section analysis allowed distinguishing 26 microfacies, which can be grouped into twelve microfacies associations that represent four main depositional environments: open-marine, outer-ramp, middle-ramp and inner-ramp settings. The carbonates have undergone a complex diagenetic history, from penecontemporaneous shallow-marine consolidation to deep-burial diagenesis. The most important processes that affected the carbonates are dolomitization (in different stages), cementation (by anhydrite and calcite), dissolution (fabric-selective and fabric-destructive), fracturing, stylolitization and neomorphism. Minor diagenetic processes that modified the sediments are pyritization, silicification, glauconitization, micritization and bioturbation. Diagenetic processes such as dolomitization, dissolution and fracturing improved the reservoir quality, whereas cementation and compaction have reduced the reservoir quality of the Asmari Formation. Whole-rock oxygen and carbon isotope analyses of limestone samples show that the isotopic composition of the carbonates was hardly affected by diagenesis and that the carbonates remained roughly in isotopic equilibrium with the Paleogene seawater. Some samples have, however, been affected significantly by diagenesis during deep burial in a closed to semi-closed diagenetic system.