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Inference on the Potential of Hydrocarbon Resources in the Gyeongsang Basin, South Korea, Based on Petroleum System Modeling

Abstract

Gyeongsang Basin is the largest Mesozoic terrestrial basin covering about 20,000 km2 with sediment thickness of up to 8,000 m in South Korea. Many geological and geochemical studies for hydrocarbon exploration have been carried out in this basin since the 1970s, but any conventional oil or gas reserves have not been discovered yet. Some formations such as Nakdong and Jinju Formation in this basin, however, show relatively high TOC (> 1 wt%) with maturation of late catagenesis, and thus they are appeared to be a potential source of shale resources as well as conventional hydrocarbon. We performed 1-D petroleum system modeling to constrain the timing of hydrocarbon generation and the amount of generated and expelled hydrocarbon with an integration of geological, geochemical and petrophysical results. The maturity of Nakdong Formation, the lowest formation of the basin with an average thickness of 1,100 m mainly composed of sandstones with shales, shows more than 3.0%Ro in the model, which is good agreement with the results of measured virtinite reflectance (3∼4%Ro) and Tmax (> 590 °C) of outcrop or core samples. Hydrocarbon generation of Nakdong Formation commenced during Aptian at ∼115 Ma and reached maximum oil generation window at ∼100 Ma. Total amount of generated hydrocarbon in this formation is ∼160 mg/gTOC. Most generated hydrocarbon converted to gas since Cenomanian (∼95 Ma), as a result, about 50% of it was expelled from the formation and about 60 mg/gTOC is charged to the formation. The rest (∼20 mg/gTOC) is residue of organic matters which no longer produce hydrocarbons. Jinju Formation also represents high thermal maturation (2∼3%Ro), indicated that it has been over-matured. Hydrocarbon generation of Jinju Formation began at ∼102 Ma (Albian) and reached maximum oil generation window at ∼94 Ma. After Turonian, most generated hydrocarbons converted to gas, thus about its 65% (∼105 mg/gTOC) was expelled from the Jinju Formation and ∼35 mg/gTOC of gas with some residue preserved in the formation. Since most sandstones in Gyeongsang Basin have low porosities (1∼4%) and permeabilities (< 1 md), there is a low probability of discovering conventional hydrocarbon reserves. The petroleum system modeling, however, shows some shale gas potential in this basin.