Search for fluid seepage structures at sea floor offshore off NE Sakhalin Island, the Sea of Okhotsk

Abstract

The International Research Project CHAOS (Russia-Korea-Japan) aimed at the study of gas hydrate formation processes associated with the fluid venting in the Sea of Okhotsk. During the cruises, large hydrate-bearing seepage structures were newly found on the continental slope of the Sakhalin Island at water depths of 400-1,000 m by multi-frequency and high-resolution geophysical surveys including Side-scan Sonar (SSS), 3.5 kHz sub-bottom profiling, sparker seismic profiling, and echo-sounding. In an area of 200 km2, SSS survey identified more than 40 seepage structures. Using short gravity core (<5 m), gas hydrate samples including massive pure gas hydrate layers (up to 35 cm thick) were successfully recovered within these seepage structures at very shallow subbottom depths. Gas hydrate sample taken at the water depth less then 400 m during the cruise is believed as the shallowest submarine gas hydrate discovered ever in the world. The total gas accumulations within all seepage structures (4.36 km2) in the area may be as much as 3.5×109 km3.