Reservoir heterogeneity of Upper Cretaceous Sarvak Formation in the Dezful Embayment, SW Iran: Implications of flow unit distribution, electrofacies analysis and geological-based reservoir zonation

This study focuses on the origins, scale of occurrence, and predictability of reservoir heterogeneities of the carbonates of the Upper Cretaceous Sarvak Formation in five oilfields in the Dezful Embayment. Facies analysis has resulted in the recognition of fourteen microfacies types indicating deposition on a ramp-like carbonate platform. Marine, meteoric, and burial diagenetic processes (e.g.., dissolution/karstification, cementation, dolomitisation, and compaction) had major controls on reservoir properties. Two transgressive–regressive (T–R) sequences are differentiated (Cenomanian and Turonian in age) and well-correlated across the study area. Hydraulic flow units, electrofacies, and reservoir zones are defined using the core porosity–permeability data and electrical well logs. Large-to small-scale heterogeneities are differentiated and correlated in a chronostratigraphic context. The large-scale heterogeneities are mainly controlled by (a) the stratigraphic occurrences of rudist-dominated facies and (b) the meteoric diagenetic processes below the paleo-exposure surfaces. Changes in textural properties, sedimentary structures, and variations in the type and intensity of diagenetic alterations were responsible for small-scale heterogeneities. Reservoir zones of the Sarvak Formation are formed within the dissolved rudist-dominated facies in the RSTs (regressive systems tracts) of third-order sequences. Non-reservoir units are formed within the compacted and cemented mud-dominated facies of transgressive systems tracts (TSTs). Consequently, reservoir heterogeneities of the Sarvak Formation are depositional–diagenetic in origin and are predictable in a sequence stratigraphic framework. This geological-based heterogeneity model can be incorporated into the reservoir models of the Sarvak Formation in the Dezful Embayment and, in a larger scale, throughout the Zagros area.