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The Geochemical Context of Gas Hydrate in the Eastern Nankai Trough
Author(s) -
Waseda Amane,
Uchida Takashi
Publication year - 2004
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/j.1751-3928.2004.tb00188.x
Subject(s) - clathrate hydrate , geology , methane , geochemistry , total organic carbon , organic matter , methanogenesis , hydrocarbon , terrigenous sediment , hydrate , isotopes of carbon , kerogen , environmental chemistry , source rock , sedimentary rock , geomorphology , chemistry , organic chemistry , structural basin
. Geochemical studies for gas hydrate, gas and organic matter collected from gas hydrate research wells drilled at the landward side of the eastern Nankai Trough, offshore Tokai, Japan, are reported. Organic matter in the 2355 m marine sediments drilled to Eocene is mainly composed of Type III kerogen with both marine and terrigenous organic input. The gas hydrate‐bearing shallow sediments are immature for hydrocarbon generation, whereas the sediments below 2100 mbsf are thermally mature. The origins of gases change from microbial to thermogenic at around 1500 mbsf. Carbon isotope compositions of CH 4 and CO 2 , and hydrocarbon compositions consistently suggest that the CH 4 in the gas hydrate‐bearing sediments is generated by microbial reduction of CO 2 . The δ 13 C depth‐profiles of CH 4 and CO 2 suggest that the microbial methanogenesis is less active in the Nankai Trough sediments compared with other gas hydrate‐bearing sediments where solid gas hydrate samples of microbial origin were recovered. Since in situ generative‐potential of microbial methane in the Nankai Trough sediments is interpreted to be low due to the low total organic carbon content (0.5 % on the average) in the gas hydrate‐bearing shallow sediments, upward migration of microbial methane and selective accumulation into permeable sands should be necessary for the high concentration of gas hydrate in discrete sand layers.