Facies Characterization, Diagenetic Processes, and Reservoir Quality of the Asmari Formation in the Lorestan Region, Zagros Basin (Iran): A Case Study from a Subsurface Well

The Asmari Formation, with Oligo-Miocene age, one of the most important hydrocarbon reservoirs for production and extraction in Iran, in the Zagros Basin. In the Lorestan region, this formation displays a mixed carbonate-evaporite nature, within early Miocene deposits. In this study, a subsurface well from the Lorestan sector was analyzed in detail to reconstruct the sedimentary environment, identify microfacies, and assess the reservoir quality based on sedimentary and diagenetic controls. A total of ۱۷ sedimentary microfacies were identified, deposited in a homoclinal ramp-type carbonate platform. The facies distribution and depositional conditions experienced substantial changes across the formation's vertical profile, with a distinct transition from common coral red algae in the lower sequence to rare aragonitic-walled red algae (Peyssonellia-type) in the upper sequence. Porosity assessment revealed that facies such as packstone to grainstone dominated by aragonitic red algae exhibit the highest average porosity (۹.۲%), attributed to extensive moldic porosity resulting from algae dissolution. In contrast, low-energy lagoonal and deeper open marine facies typically showed limited reservoir potential due to micritization and cementation. Diagenetic processes, including marine cementation, mechanical compaction, dolomitization, neomorphism, and selective dissolution, significantly controlled the evolution and spatial distribution of porosity. A sequence stratigraphic framework highlights the major influence of sea-level fluctuations on facies transitions and diagenetic overprints, with evaporite intervals acting as potential sealing units. Overall, this study introduces a unique facies model for the Asmari Formation in Lorestan, documenting the replacement of coral red algae with aragonitic red algae, and emphasizes the critical role of diagenesis in enhancing or diminishing reservoir properties in carbonate-evaporite systems.

کلیدواژه ها:

Asmari Formation ، Zagros Basin ، Microfacies ، Diagenesis ، Porosity Distribution ، Sequence Stratigraphy ، Homoclinal Ramp ، Aragonitic Red Algae ، Reservoir Quality