鄂霍次克海天然气水合物成藏条件分析

GEOLOGICAL FACTORS FOR THE DEVELOPMENT OF GAS HYDRATES IN OKHOTSK SEA

2006年5月由俄、韩、日、中四国共同组织的“海底冷泉与生命过程”联合调查航次，在鄂霍次克海域成功采获天然气水合物样品。从水合物发育的气源条件、温度压力条件、构造控制条件等方面，分析了谊地区天然气水合物发育所具备的基本成藏条件。指出鄂霍次克海周边的高大山系为其提供了丰富的沉积物来源，并在鄂霍次克海中形成了宽广而深厚的陆架体系。陆架区沉积地层厚度一般超过10km，且以新生代沉积为主。根据对重力柱状样品的观察和分析，并参照沉积物捕获器样品的测量结果，认为本区域沉积物总有机碳含量普遍较高。根据地震剖面解释和重力柱状样品的^14C测年结果得出，本区沉积速率较高，并与目前已知水合物区的沉积速率相当。部霍次克海地处高纬度地区，冬季海面大部分被海冰覆盖。海面以下50～120m之间常年存在一个低温益层。这个低温盖层使得海底温度一直保持在2℃左右。在这样的温度条件下，鄂霍次克海350m以深的区域都满足水合物赋存的压力每件。海底以下满足水合物温度、压力条件的沉积地层厚度为450-800m。鄂霍次克板块位于四大板块之间，并受到四大板块的挤压。由于挤压作用，在萨哈林岛东侧陆坡地区形成一系列的海底泥火山构造，从而使该区域成为天然气水合物调查研究的主要目标区。鄂霍次克海域的沉积物源、沉积厚度、沉积速率、有机碳含量等构成该区域水合物发育良好的气源条件，而温度、压力和构造控制条件等也都非常有利于天然气水合物在谊地区的发育。

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 colT umn, 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.