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

Abstract 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. Keywords: Dariyan Formation, Gamma ray, Standard deviation, Sequence stratigraphy. Introduction Given the importance of oil and gas reservoirs in the world, careful study of facies, sedimentary environments and detection of depositional sequences are very important for identifying oil and gas reservoirs. Accordingly, interpretation of facies associations from thin-sections and reconstructing the sedimentary model are recommended (Reading 1986). Sequences are determined by examining the relationship between facies and depositional environments with the unconformities in sedimentary succession (Catuneanu et al. 2009). Various types of data, such as the outcrops, cores, well logs, cuttings and seismic data are used. However, due to limitations in the production of core data, it is not possible to use them in all cases. Therefore, well log data that are available in most reservoirs can be used as the best tool for sequence stratigraphic studies. Despite the great importance of gamma ray log in determination of sequence stratigraphic framework, few studies have been performed on this subject due to the low amplitude changes of this log in carbonate formations (Aigner et al. 1995; Heimhofer et al. 2010; Peyravi et al. 2010; Ghasemi-Nejad et al. 2015; Mansouri-Daneshvar et al. 2015; Gholami Zadeh et al. 2019). Some previous studies (Tavakoli 2014, 2017; Ghiasvand 2016) determined the sequence boundaries suggested by the gamma deviation log. The present study was performed on the Dariyan Formation (Late Barremian to Early Aptian) from the Khami Group. The purpose of this study was to investigate the reliability of gamma deviation log in sequence stratigraphy analysis of carbonate successions (third-order depositional sequences). Materials and Methods For the present 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 understand the geological properties and determine the sequence stratigraphy framework of the Dariyan Formation, petrographic studies were carried out and samples were classified according to Dunham (1962) facies classification. Then, using the transgressive-regressive sequence method, depositional sequences were determined. To determine the boundaries by gamma deviation method, each gamma ray data (CGR-SGR) 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) Tavakoli 2014). Finally, the defined sequence boundaries were compared with the results of the gamma deviation log and the method was used in well B with no core data. Discussion of Results and Conclusions Facies analysis led to the identification of nine microfacies (T1, L1, L2, L3, S1, S2, R1, R2, O1) in five sub-environments (tidal zone, lagoon, shoal, proximal open marine and open marine) deposited in a carbonate ramp platform (inner- to mid-ramp). Sea-level changes at the time of deposition of the Dariyan Formation have resulted in formation of three third-order complete depositional sequences with thicknesses of 25, 30.2 and 54.8 m from bottom to the top. Three lithostratigraphic units (Lower Dariyan, Hawar, Upper Dariyan) and the main boundaries were identified by the first level analysis. The second level analysis, which was performed separately on each of the first three units, revealed other sequence boundaries (except the boundary at a depth of 1127.2 m). The reason for the uncertainty of this boundary was very limited change of facies which could not change the gamma ray values. Comparing the determined boundaries with sequence stratigraphic studies based on the core analysis indicates that this tool is valuable in determining sequence stratigraphic boundaries in carbonate succession. The method can also be used in wells with no core data (such as well B).