2002 年 67 巻 1 号 p. 38-51
The MITI Sanriku-Oki well confirmed the presence of the gas-charged Cretaceous to Eocene formations, and proved the gas potential of those formations in the Pacific Ocean off Sanriku province of the northern Honshu Island. This study deals with the geologic structure, tectonic evolution and hydrocarbon systems of the Cretaceous to Eocene formations, which were deposited in the forearc basin setting. The area of investigation is from off Sanriku province of northern Honshu to off Hidaka province of Hokkaido Island. Structural interpretation was made utilizing 11,190 line kilometers of seismic sections. Based upon the structural interpretation, it was made clear that the severe folding, uplifting and erosion that gave rise to the trapping structures took place during Oligocene, and that fracturing of the basement volcanic and granitic rocks occurred during middle Miocene. In the Sanriku-Oki Sub-basin off Sanriku province, the A3-Coal-Marker (upper Cretaceous) and B2-Top (middle Eocene) horizons form a N-S trending synclinorium about 70km wide that gently plunges to the north. Coaly gas prone source rocks intercalated between the B2-Top and A 3-Coal-Marker are considered to have been matured since Oligocene in the axial part of the synclinorium. Several domal traps, fault dependent traps and stratigraphic traps beneath the Oligocene unconformity (sub-crop traps) are possibly gas charged. Combined gas in place in those traps can be as much as 15 TCFs. In the Yufutu-Oki Sub-basin off southern Hokkaido Island, coaly gas prone source rocks are buried deep in the Hidaka Trough and have been well maturated since late Miocene. Several basement paleo-highs on the Tomakomai Ridge are possible hydrocarbon traps if weathered or fractured. Combined gas in place in those basement traps can be as much as 6.4 TCFs.