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Characterization of Organic Matter in Shale from the Gyeongsang Basin, Korea

Abstract

Because shale gas is one of the important natural energy resources in the future, many studies related to it have been carried out in Korea since 2010. To date, however, there are no results for the shale gas resource in Korean non-marine basins. The purpose of this study is to characterize organic matter in Gyeongsang Basin, one of the Korean non-marine basins, and to document the potential of shale gas resource in this basin. Gyeongsang Basin is the largest Cretaceous non-marine sedimentary basin (∼20,000 km2) in Korea, which is divided into the Sindong, Hayang, and Yucheon groups with decreasing age. During the deposition of the Sindong Group, alluvial fan (Nakdong Formation), fluvial (Hasandong Formation) and lake (Jinju Formation) environments developed from the basin margin to the center. Black shale is observed at the Nakdong and Jinju Formation. We collected shale samples from outcrops and core (YB-1) drilled to ∼1.2 km from the surface and analyzed several geochemical analyses to characterize the organic matter in both formations. The results of Rock-Eval analysis show that most samples in these formations have low S2 (< 1 mgHC/gRock) and Tmax (< 400 °C), and their TOC content is less than 2.5 wt%. It is difficult to discriminate organic matter type in the HI-OI diagram due to low S2. These results indicated that organic matter has been over-matured to generate gas, which might have stored at shale in both formations or migrated into other formations, and residual organic matter cannot produce hydrocarbons any more. The low concentration of residual methane in shale (<100 ppm) is evident that the generated gas migrates into the other formations, which is consistence with the result of 1-D petroleum system modelling; more than 65% of produced gas in the Jinju Formation is discharged. It is appeared that various lines of above results point to low potential of shale gas resource in the Gyeongsang Basin. The relationship between TOC/TN and δ13Corg of organic matters from the Nakdong and Jinju Formation is mostly located in the region of the terrestrial land plant, illustrating that the organic matters in these formations are predominantly originated from a terrestrial source (Type III).