--> Abstract: Sedimentology and Geochemistry of the Taishu Group in Tsushima Islands and Its Source Rock Potential, by Myong H. Park, Ji H. Kim, Il M. Kang, Yungoo Song, and Tae J. Cheong; #90082 (2008)

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Sedimentology and Geochemistry of the Taishu Group in Tsushima Islands and Its Source Rock Potential

Myong H. Park1, Ji H. Kim2, Il M. Kang3, Yungoo Song4, and Tae J. Cheong2
1Petroleum Technology Institute, Korea National Oil Corporation, Anyang, South Korea
2Petroleum and Marine Resources Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea
3New Ventures Department, Korea National Oil Corporation, Anyang, South Korea
4Department of Earth System Sciences, Yonsei University, Seoul, South Korea

Sedimentological and geochemical studies on the sedimentary rocks taken from Tsushima Islands (located between Eastern and Western channels of Korea Strait) were carried out to evaluate possible depositional environments and potential of the shales from Tertiary Taishu Group as source rocks of hydrocarbon. The Taishu Group consists largely of the Lower, Middle and Upper formations, and its gross thickness is over 5,000 meters. Each formation alternates primarily sandstone and shale. The Lower Formation consists mostly of bedded sandstone and shale alternations, while the Upper Formation is dominated by more thick-bedded sandstone and homogeneous mudstone than the lower formations. The depositional environment for the Taishu Group was mainly a tide-dominated coast. In detail, it seems to have changed from prograding bay-head deltas to protected bays or lagoons.

X-ray diffraction (XRD) for bulk powder samples demonstrates that the shales of the Taishu Group are mainly composed of quartz (41.2~60.2%), mica (13.6~21.5%), chlorite (9.1~21.0%) and plagioclase (7.1~11.3%) with a small quantity of ankerite. In the fine clay fraction of <0.2µm, progressive changes of illite/smectite are identified from short range ordering (R≥1) for the Upper Formation to long range order (R≥3) for the Middle and Lower formations. These indicate that the diagenetic temperature (or maximum burial temperature) of the Upper Formation was approximately 120~160C, which corresponds to the thermal condition of oil generation. On the other hand, diagenetic temperature of the Middle and Lower formations reached to the thermal condition for wet gas and condensate (>160C). However, the pyrolysis analysis did not detect S2 peak and Tmax in the shales (Cheong et al., in prep.), indicating that organic matter seems to be overmatured. Thus, further study is clearly needed to solve this discrepant problem.

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