The role of heterogeneity controls in shaping the Asmari Formation reservoirs in the Zagros region, Iran: insights for regional characterization

Various depositional and post-depositional factors interact to shape a reservoir, typically causing heterogenies in reservoir properties on different scales. This study investigates depositional and diagenetic factors controlling reservoir properties of the Asmari Formation as the most prolific reservoir rock in Iran. Data from six surface and 12 subsurface sections show that both primary and secondary factors governed reservoir quality. Lithology is the principal primary control, with major differences between siliciclastics and carbonates. Other primary controls include rock texture, facies, and depositional setting. Dolomitization, dissolution, and cementation are the main diagenetic factors that modify the original rock fabric. Dolomitization improved reservoir quality. Early diagenetic dissolution also enhanced porosity. However, the created spaces are separated vuggy and moldic pores with no effect on permeability. Cementation and compaction are major porosity-occluding processes. Anhydrite cementation degraded reservoir quality of oolites, one of the best productive facies of Asmari. Results show that tectonics, paleo-climate, and sea-level changes were allogenic controls on reservoir configuration. Lithology variations, as a principal control on reservoir quality, resulted from tectonic movements alongside sea-level oscillations. The diagenetic path—specifically dolomitization, dissolution, and cementation—was controlled by paleo-climate and sea-level changes. Hypersaline conditions prevailed during and after deposition, with major impacts on the reservoir quality (pervasive dolomitization and dissolution), were the consequence of combined paleo-climate and sea-level effects. Regional syn- and post-depositional tectonics created widespread fractured reservoirs with higher production rates than unfractured counterparts. The results of this study can assist in regional characterization of the Asmari reservoir throughout the Zagros area.