Relationship Between Depositional Facies and Reservoir Characteristics of the Oligo-Miocene Asmari Formation, Aghajari Oilfield, SW Iran

The present study focuses on facies analysis and the reservoir characteristics of the Asmari Formation in the Aghajari oilfield (located in the Zagros fold-thrust belt, southwest Iran), and how they are linked to depositional environments and reservoir characteristics. This formation consists of about 380 m carbonates with some siliciclastic intercalations. Based on the integration of 1200 m of core description and petrographic analyses of 2500 thin sections, 9 carbonate and 1 evaporite microfacies along with 2 siliciclastic petrofacies were identified. Facies analysis shows that the Asmari Formation sediments were deposited in a homoclinal carbonate ramp within open marine (facies association A), shoal (facies association B), restricted lagoon (facies association C), tidal flat (facies association D), and beach (petrofacies E) environments. Macroscopic and microscopic studies reveal that the main porosity types include interparticle, growth framework, vuggy, intercrystaline, moldic and fracture. Based on Routine core analysis (RCA), porosity of the studied reservoir varies from 0.001 to 25.8%, with an average of 12.5%. The permeability ranges from zero to 1514 mD with an average of 94.27 mD. There is a good relationship between trend of mercury injection capillary pressures (MICP) and depositional environment. Therefore, facies deposited in the same depositional environment show relatively similar trends of Pc. Although, there is a possibility that some post-depositional processes have altered these normal trends. Facies association A exhibits the lowest reservoir quality due to nature of fine grain matrix, and cementation, whereas beach loosely cemented-sandstones (PF E) have the best reservoir qualities. The relation between MICP data and reservoir quality index (RQI) confirms that carbonate facies associations B and A have the best and worse reservoir qualities, respectively. Therefore, more focus on petrofacies E and facies association B is cost effective for the future drilling wells in this oilfield and similar field in the Zagros basin and Middle-East.