Shallow Gas Hydrate Accumulation in the Eastern Margin of Japan Sea: A Potential Natural Gas Resource

Ryo Matsumoto², Antonio F. Freire¹, Mikio Satoh³, Akhiro Hiruta², and Osamu Ishizaki^2
1Natural Environmental Studies, University of Tokyo, Kashiwa, Japan
²Earth and Planetary Sciences, University of Tokyo, Bunkyo-ku, Japan
³Fuel Resource Geology Research Group, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan

Integrated geological and geophysical explorations for the last five years have identified massive accumulation of gas hydrate in methane plume field offshore Joestu, eastern margin of Japan Sea. The field is characterized by active venting of methane: a number of 500-700 m high methane plumes have been identified in mounds and pockmark zones on the crest of the Umitaka spur and Joestu knoll, which were formed by recent contraction tectonics of Japan Sea. ROV surveys found massive and fracture-filling gas hydrates exposed in depression structures formed by collapse and slides. Methane is largely thermogenic of about -40‰ of d13C. Seismic surveys recognized chimney structures, approximately 500-700 m in diameter, under the mounds and pockmark zones on the spur and knoll, suggesting a sufficient methane is being supplied from deep-seated gas pools. BSRs widely develop at around 140 m below sea floor, in particular, within the gas chimneys, in which the high amplitude BSRs exhibit apparent pull-up structures probably due to massive and dense accumulation of gas hydrate above. These lines of evidence strongly suggest that thermogenic methane forms disc-shaped, massive gas hydrate buildup within the shallow part of gas chimney structures, perhaps a potential natural gas resources near future.

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