Depositional and diagenetic impact on reservoir quality of the Asmari carbonate reservoir, Naft-Sefid Oilfield, SW Iran

Abstract

The Oligo-Miocene Asmari Formation with main carbonate and evaporate lithology is the most prolific reservoir in the Dezful Embayment, south-west of Iran. Depositional and diagenetic effects on reservoir quality in the sequence stratigraphic framework were carried out on the basis of petrographic investigation, petrophysical logs, and core measurement porosity and permeability data. Petrographic analysis resulted in the identification of 12 microfacies classified in 5 sub-environments including tidal flat, lagoon, barrier, shallow open marine and basin, which indicate deposition in a homoclinal ramp setting. Tidal flat, lagoonal and barrier microfacies are mostly present in the upper and middle parts of the Asmari Formation, while outer ramp microfacies largely developed in the middle part. Anhydrite/calcite cementation, compaction, dolomitization, dissolution, and fracturing are the main diagenetic controlling factors on petrophysical properties. Fracturing, dolomitization, and dissolution are contributed to reservoir quality enhancement, while compaction and cementation had negative effects on reservoir quality. The main controlling factor on their reservoir quality was depositional characteristics and/or a facies-dependent diagenesis. The studied successions are represented by 3 third-order sequences of early Aquitanian, late Aquitanian and early Burdigalian age that largely deposited in the highstand systems tract (HST) stage. The Burdigalian sequence represent best reservoir quality due to further dolomitization and dissolution especially close to the sequence boundary.