Controlling factors on depositional facies of mid-Cretaceous carbonates (Sarvak Formation), southeastern Persian Gulf, Iran

Middle Cretaceous carbonates of the Sarvak Formation, as its stratigraphic equivalents across the Persian Gulf, are of primary importance in the late Mesozoic petroleum systems, representing one of the most significant hydrocarbon reservoirs in the Middle East. Based on petrographic examination of the samples collected from this rock unit, 10 microfacies are recognized, indicating deposition in lagoon, shoal/rudist build-up, slope, and basin settings. The vertical distribution of microfacies belts suggests that the Sarvak Formation in southeastern Persian Gulf was deposited on a carbonate ramp and/or a low-gradient rimmed shelf. Stratigraphic distribution pattern of micro- and macrofauna in the sequences examined allows for the recognition of three biozones and two biofacies, collectively corroborating a late Albian-late Cenomanian age. Additionally, recognition of sequence boundaries and maximum flooding surfaces based on combined petrographic and petrophysical data obtained from 11 subsurface sections points to the presence of three third-order sequences within the Sarvak Formation. Frequency analysis and thickness variations in the identified facies and sedimentary sequences reveal significant changes in the thickness of the Sarvak Formation. These changes are attributed to combined effects of regional tectonic activity and eustatic sea-level fluctuations. The reactivation of basement faults alongwith halokineticmovementswere significant tectonic factors influencing depositional thickness during the middle Cretaceous in the study area, as clearly evident in the 2D seismic profiles. Tectonic factors during Cenomanian, along with eustatic sea-level fluctuations, resulted in widespread erosional disconformities that are traceable in nearly all subsurface stratigraphic sections studied. The Cenomanian-Turonian boundary is identified as a significant paleoexposure surface, exhibiting evidence of karstification, leaching, and vuggy porosity in uppermost part of the Sarvak Formation.