Investigating the reliability of Gamma Deviation log in sequence stratigraphic studies: Dariyan Formation, the central Persian Gulf

In this study, 110 m of carbonate rocks from one well of the Dariyan Formation in the central part of the Persian Gulf were investigated by using 390 thin-sections and gamma ray data. In order to determine the depositional sequences of this formation, facies analysis led to the identification of nine microfacies in five sub-environments deposited in a carbonate ramp platform. The sea-level changes at the time of deposition of the Dariyan Formation have resulted in formation of three third-order complete depositional sequences. To determine the boundaries by gamma deviation method, each gamma ray data was subtracted from the average data in the whole well and then the obtained values were added together to calculate cumulative deviation from the mean value. Three units and the associated main boundaries were identified by the first level analysis. The second level analysis, which was done separately on all three units, other sequence boundaries (except the boundary at a depth of 1127.2 m) were identified. Comparing the determined boundaries resulting from this method with sequence stratigraphic studies based on the core analysis results indicates that this tool is valuable in determining sequence stratigraphic boundaries in carbonate formations.