2010 年 75 巻 1 号 p. 42-53
The Japanese Government has acquired regional reconnaissance seismic data in territorial waters since 1969 to evaluate the country's hydrocarbon potential. Total length of seismic lines is about 100,000 km and many surveys cover relatively deep water areas where gas hydrate stability zone (GHSZ) is expected to exist. BSRs related to methane hydrates in the offshore areas surrounding Japan were studied jointly by the group comprised of JNOC (at present, JOGMEC) and 10 private sectors based on these archived seismic data, and the areal extent of BSRs was reported to be 44,000 km2 in 2000.
The country's research into methane hydrates has been accelerated since 2001 by establishment of the research consortium (MH21) under administrative guidance of METI. To investigate into methane hydrate, extensive 3D seismic surveys were conducted in the eastern Nankai Trough and many LWD wells were drilled there. Through the exploration campaign, certain new knowledge on seismic attributes related to the concentration of methane hydrates was obtained. In the light of this advanced knowledge about the appearance of BSRs, MH21 has decided that the archived seismic data should be investigated for the comprehensive understanding of methane hydrates as the potential future energy resources. Old seismic data processed suitably for loading in a “work station” was interpreted in the sophisticated system, though the volume of them was very limited.
A method of high density velocity analysis was applied to the seismic data which recorded clear BSR appearing at abnormally shallow depth as 200msec below the sea floor in the Sea of Japan and in the Sea of Okhotsk, comparing to 500 to 700 msec in the Pacific side. This may represent that the geotectonic setting of the Japanese Islands controls GHSZ and this understanding seems to be important for the future exploration of MH.
Present study has revealed that the areal extent of BSRs in offshore Japan is 122,000 km2.