Reservoir zonation in the framework of sequence stratigraphy: A case study from Sarvak Formation, Abadan Plain, SW Iran

Located in the northeast of the Arabian Plate, Zagros Basin hosts some of the most significant hydrocarbon reservoirs in Iran, including the Sarvak Formation. It has been dated back to the Middle Cretaceous and exhibits a dominantly carbonate lithology. In this study, petrographic and petrophysical data of the Sarvak Formation were analyzed. The results of the petrographic study showed that the formation was deposited in a homoclinal carbonate-ramp depositional environment and exposed to diagenetic processes in three depositional environments: marine, meteoric, and deeply buried environments. The stratigraphic modified Lorenz plot (SMLP) method was used to distinguish between the reservoir and non-reservoir zones. By integrating the petrographic and petrophysical findings, stratigraphic relationships among the identified zones were assessed. Accordingly, three third-order sedimentary sequences were identified on the Middle Cenomanian-to-Middle Turonian section within the studied formation. Two disconformities were detected in the Upper Sarvak, namely Cenomanian-Turonian disconformity (CTD) and Middle Turonian disconformity (MTD). The highest-quality reservoir zones were related to the CTD. The porosity resulted from the karstification, meteoric diagenesis, and significant dissolution beneath the CTD was found to have greatly increased the reservoir quality. Later on, however, the sediments were overmatured under the impact of the MTD, which is known to take a long time to occur, with their porosity largely decreased, ending up with a much lower reservoir quality. In general, the sedimentary facies were found to be the most important factors controlling the distribution of the reservoir zones across the Sarvak Formation, followed by the diagenetic processes. The findings provided an insight into the factors controlling the reservoir quality in the Sarvak Formation within the study area and the Arabian Plate.