International study efforts on gas hydrates near cold seeps NE off the Sakhalin in the Sea of Okhotsk

Hitoshi Shoji1), Young Keun Jin2), Anatoly Obzhirov3) and Boris Baranov4)
1)Kitami Institute of Technology, Kitami, Japan 2)Korea Polar Research Institute, Incheon, Korea
3)V. I. Il’ichev Pacific Oceanological Institute FEB RAS, Vladivostok 4)Oceanological Institute RAS, Moscow


Gas hydrates in marine sediments are potential energy resources and important reservoirs of greenhouse effect gas. Field observations conducted in 1991 revealed the existence of near-sea-bottom hydrates close to cold seeps NE off the Sakhalin in the Sea of Okhotsk.
The New Energy Resources Research Center, Kitami Institute of Technology has been collaborating with scientists from Russia (V. I. Il’ichev Pacific Oceanological Institute FEB RAS and Oceanological Institute RAS), Korea (Korea Polar Research Institute, KORDI) and other countries, to investigate the distribution, amount and characteristics of hydrate accumulations and the role of the fluid discharge for the hydrate formation. Multidisciplinary field operations include hydro-acoustic survey, side-scan-sonar survey with sub-bottom profiling, sparker seismic survey, and sediment coring and water column sampling. Core and water analyses include measurements of gas composition, stable isotopes, carbonates, ionic concentrations in pore water, and SEM, Raman and calorimetric observations on gas hydrates.

Geophysical survey results reveal that gas chimney images in seismic reflection profiles were traced to connect BSR line and seepage structures. Both pull-up and disturbed structures of BSR around the gas chimney images were interpreted as to be indications of significant heat flows caused by ascending fluid. On the other hand, almost no pull-up/disturbance of BSR was observed at one seepage structure, suggesting little water seepage. Core analyses results reveal that major composition of guest gas in hydrates is methane of microbial origin. Water enriched in deuterium is seeping from a depth below the sea bottom with gas at seepage structures, although little water seepage is suggested at one structure site.
The seep activity may vary with time. The structure with little water seepage, locating at the edge of a dense area of the seepage structures, might serve as an indicator for the long-term activity of the fluid seepage system off the Sakhalin.