Depositional setting and diagenetic processes and their impact on the reservoir quality in the late Visean-Bashkirian Kashagan carbonate platform (Pre-Caspian Basin, Kazakhstan)

The Kashagan isolated carbonate platform developed from the Visean to the Bashkirian in the Pre-Caspian Basin. Sedimentation is organized in shallowing-upward cycles marked at the top by subaerial exposure surfaces. This study is based on analyses of 12 cored wells, integrated sedimentology, petrography, fluid inclusion, stable isotopes, and trace element data. We reconstructed the pore-system evolution in the platform interior and margin. Near-surface diagenesis includes marine cement fringes followed by meteoric dissolution and cementation. Meteoric diagenesis was stronger in the Bashkirian sequence and progressively decreased down section in the Visean. The main karst surfaces are evidenced by root traces, sharp delta C-13 depletion, and meteoric water-fluid inclusions in calcites. Porosity in the platform interior follows a cyclic pattern due to the interaction of the freshwater dissolution and cementation and compaction that destroyed the porosity at the tops and bases of the cycles. Burial diagenesis, represented by some calcite cementation, did not significantly alter the cyclic porosity distribution in the platform interior. In the platform margin, porosity distribution is more heterogeneous due to more poorly defined sedimentary cyclicity and stronger burial diagenetic overprint linked to exotic fluid circulation. Basin-sourced fluids caused dissolution, cementation, and local dolomitization. These cements show bright luminescence, a high manganese content, depleted PO, a high homogenization temperature, and low-salinity fluid inclusions. The complex diagenetic evolution diagenesis produced better matrix porosity in the platform interior and heterogeneous porosity with larger pores and fractures but with poor matrix porosity in the platform margin.