Possibility of Gas Hydrate Occurring in the Western Lower Slope of Mid Okinawa Trough
Possibility of Gas Hydrate Occurring in the Western Lower Slope of Mid Okinawa Trough
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发表时间：
2000
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通讯作者：
Meng Xian
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作者：
Meng Xian
尽管从温度和压力条件来看，深度为300米至3 000米的海洋90％的区域适合天然气水合物的存在，但海洋中并非到处都有天然气水合物。天然气水合物的赋存与地质背景密切相关。从构造角度看，海洋中的天然气水合物沿大陆边缘赋存：一类是在主动被动边缘棱柱处，另一类是在大陆坡或陆坡脚处。天然气水合物的形成也与沉积物类型、沉积速率、有机源以及热流密切相关，即快速埋藏在沉积物中的有机质含量较多，是天然气水合物形成的物质条件；天然气水合物的形成也与沉积物类型、沉积速率、有机源以及热流密切相关。沉积物孔隙度较大，多孔含水层含量较多，为天然气水合物的胶结提供了充足的空间。水合物中的气体受到有机质的生物成因和热降解作用，生物质含量较多，热流较高，有利于天然气水合物的形成。区分天然气水合物常用两种标记。海底沉积物中,一是地球物理标志,即空白反射和模拟海底反射体(BSR),这是由于水合物带与底下沉积物声波速度不同而引起的平行于海底、倾斜于层理的反射体。与海底反射相反,模拟海底反射体具有反极性;二是地球化学标志,即正CH 4 、H 2S 和CO 2 气体地球化学标志。由于天然气水合物的分解，测井剖面中应存在异常以及负盐度或氯度异常。冲绳中部海槽西下斜坡具有良好的地质环境和天然气水合物形成的条件。首先，水深超过1 000米，从温度压力来看适合天然气水合物的形成。其次，沉积物几乎为粉质泥和泥质粉砂，可为天然气水合物提供充足的胶结空间。再次，沉积物中有机质的含量远高于天然气水合物的形成。邻近地区沉积速率也较高(10～40 cm·ka),导致丰富的有机质快速埋藏,为气氢化物的形成储存了气源。最后是热流值由70~437 mw/m 2 变化,远高于其他地区,为有机质的热降解提供了充足的热量。坡缘下缘的河道地震反射剖面(A—A′),深度为1 500~1 800米,显示模拟海底反射体出现在海底2 250米深处,向上逐渐消失,与海底平行、与层理倾斜,且与海底反射相反具有负极性(反极性)。由此初步推断天然气水合物可能赋存于中冲绳海槽西下斜坡。
Although 90 percent area of the ocean with depth from 300 to 3 000 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 CH 4,H 2S and CO 2 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/m 2,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.