GEOLOGICAL FACTORS FOR THE DEVELOPMENT OF GAS HYDRATES IN OKHOTSK SEA

An international joint cruise under the frame of CHAOS among Russia, Korea, Japan and China was carried out during May of 2006. The main purpose of this cruise was the study of gas hydrates in Okhotsk Sea. Gas hydrate sample was successfully recovered by this cruise. For understanding the gas hydrate prospect in the Okhotsk Sea, sedimentary sources, thickness of sedimentary column, concentration of total organic carbon of sediment, rate of sedimentation, temperature condition of gas hydrates development, tectonic control of gas hydrates development were examined. This paper shows that, except Kuril Islands, the other three sides of Okhotsk Sea were surrounded by elevated mountain ranges which provided large numbers of particles both organic and inorganic to the Okhotsk Sea. Large shelves wider than 300 km in the north and 100 km in the west and east sides were formed by quick sediments accumulation. Thickness of sediment column mainly from Cenozoic can be as thick as 10 km in shelf area. Sedimentary rate of Okhotsk Sea from both ¹⁴C data and seismic profile interpretation is as high as the rates of other gas hydrate areas. The concentrations of total organic carbon from bottom samples are all over 0.5% which is believed as an ability threshold for the sediment to form gas hydrates. The lowest value 0.65% appeared at the side of Kuril basin, the others outside of Kuril basin are all over 1.0% which is two times the threshold value. Ice covers most of the parts of Okhotsk Sea in winter, and a cold cap between 50~120 m thick covers the water column all over the year, so a cold bottom temperature around 2℃ remains all the year,comparing 5~10℃ in winter time in the shelves of East China Sea and South China Sea. Mud volcanoes which are the most suitable structure for the development of cold seepage and gas hydrates are widely developed on the Sahalin slope area due to pressure from the surrounding plates.
Based on the abundant sedimentary sources from the adjacent mountain ranges, thick sedimentary column, high concentration of total organic carbon of sediment from Cenozoic, high sedimentary rate, low bottom temperature condition, and suitable tectonic control, we believe that Okhotsk Sea will be the most perspective area for the gas hydrates development.