Deposition, Diagenesis, and Geochemistry of Upper Cretaceous Carbonates (Sarvak Formation) in the Zagros Basin and the Persian Gulf, Iran

Abstract

Upper Cretaceous carbonate rocks of the Sarvak Formation are among the most important hydrocarbon reservoirs of the Middle East. This paper summarizes the depositional history, diagenetic alteration, and geochemistry of this formation in south and southwest Iran. The combined effects of tectonic activities and the paleoclimate had remarkable control over the depositional–diagenetic evolution of this formation. Large changes in facies types, their frequency, and thickness were recorded across the study area and attributed to the tectonic activities. The complex diagenetic history of this formation includes minor marine, intensive meteoric, and burial diagenesis. Two pronounced paleoexposure events were recorded at the Cenomanian–Turonian boundary (CT-ES) and the middle Turonian (mT-ES), which are associated with remarkable changes in facies and diagenetic characteristics and special geochemical trends. Elemental (Mg, Fe, Rb, Mn, Sr), isotopic (carbon, oxygen), and 87Sr/86Sr ratios provide useful tools for delineating the paleoexposure surfaces, their associated diagenetic alterations, and chemo-sequence stratigraphic interpretation. Variable exposure durations resulted in immature to over-matured karstified profiles below the CT-ES and mT-ES, respectively. Meteorically dissolved rudist-dominated facies provided the best reservoir zone of the Sarvak Fm. in the regressive systems tract of the Cenomanian sequence, just below the CT-ES. Microporous and dolomitized mud-dominated facies formed the second important reservoir intervals, especially within the transgressive systems tracts. The results of this study revealed that the reservoir characteristics of the Sarvak Fm. were considerably controlled by paleoexposure events and, therefore, it can be considered as a typical paleokarst hydrocarbon reservoir in Iran.