冲绳海槽中段西陆坡下缘天然气水合物存在的可能性分析

海洋中的天然气水合物主要发育在有机质供应充分、沉积速率快、热流值较高、水深大于300m的大陆斜坡和活动边缘的增生楔发育区;沉积物类型主要以泥质砂岩、砂质泥岩和浊积岩为主。似海底反射层(BSR)和极性反转是识别天然气水合物层的关键标志。冲绳海槽中段西陆坡下缘水深大于1000m;沉积物类型主要为粉砂质泥和泥质粉砂,在部分层位见浊积层。与东海陆架相比,西陆坡下缘的有机质含量、沉积速率的热流值都较高,其范围分别为0.75%～1.25%、10～40cm/ka和70～437mw/m2;单道地震剖面具有明显的似海底反射层(BSR)和极性反转特征,因此,推断冲绳海槽中段西陆坡下缘可能存在天然气水合物层。

Possibility of Gas Hydrate Occurring in the Western Lower Slope of Mid-Okinawa Trough

Although 90 percent area of the ocean with depth from 300 to 3000 meters is suitable to occurrence of gas hydrate in view of temperature and pressure condition, gas hydrate dose not occur everywhere in ocean. The occurrence of gas hydrate is tightly related to geological setting. From tectonic point of view, gas hydrate in the ocean occurs along continenatl margin: one kind is at active passive margin prism, the other is at the continental slope or slope foot. The fromation of gas hydrate is also tightly related to sediment type, sedimentation rate and organic source, as well as heat flow, i. e. more content of organic matter quickly buried in sediments is material condition for the formation of gas hydrate; more coarse sediments, consequently more porosity and more content of porous aquifer supply plentiful space for the cementation of gas hydrate. Gas in hydrate is subjected to biogenesis and thermal degradation of organic matter, so that more content of biomass and higher heat flow are favorable for the formation of gas hydrate. Two kinds of marks are commonly used for distingushing gas hydrate. In seabed sediments,one is geophysical mark, i. e. blank reflection and simulated seafloor reflector (BSR) which is caused by the different acoustic wave velocity between hydrate zone and underneath sediments and parallel to seafloor and oblique to bedding. In contrast to seafloor reflection, simulated seafloor reflector has reversed polarity. The other is geochemical mark, i. e. positive CH4, H2S and CO2 gas geochemical anomalous, as well as negative saltness or chlorinity anomalous should be present due to the decomposition of gas hydrate in the log profile. The western lower slope of Mid-Okinawa Trough possesses good geological setting and conditions for the formation of gas hydrate. First of all, the water depth is more than 1 000 meters, which is suitable to the formation of gas hydrate from the temperature-perssure point of view. Secondly, sedimentss almost silty mud and muddy silt which may supply plentiful cemented space for gas hydrate. Thirdly, the content of organic matter in sediment is much higher than adjacent area, the sedimentation rate is also higher( 10～40 cm· ka), causing the abundant organic matter buried quickly, so that the gas source is stored for the formation of gas hydeate. The last is that the values of heat flow change from 70 to 437 mw/m2, much higher than the other area, supplying plentiful heat for thermal degradation of organic matter. A channel seismic reflection profile(A－A＇)at the lower slope margin,with depth from 1 500 to 1 800 meters,shows that a simulated seafloor reflector occurs at depth of 2 250 meters beneath sea floor and gradually disappears up slope, which is parallel to seafloor and oblique to bedding, and also has negative polarity (reversed polarity) in contrast to seafloor reflection. Therefore, we initially deduce that the gas hydrate may occur at the western lower slope of Mid-Okinawa Trough.