流体迁移和海底地形与天然气水合物的形成

SEA FLOOR TOPOGRAPHY OF SHALLOW GAS HYDRATE AREA——DATA FROM OKHOTSK SEA

使用重力取样器、渔网、深潜器等手段,已经在海底及以下浅表层的区域采获天然气水合物样品,但关于浅表层水合物的发育机制、分布规律、与海底地形的关系等问题还缺乏基本认识。根据2006年鄂霍次克海天然气水合物调查航次的调查数据,发现萨哈林东北陆坡区,特别是中、下陆坡区发育大量海底凸起。这些凸起一般呈不对称的丘形,宽几百米,高几十米。与海底沙波、沙脊不同,海底凸起为孤立海底地形,在南北方向上并不连续。海底剖面仪结果清楚地显示古陆坡凸起的发育。现今海底陆坡凸起的幅度普遍地要小于古陆坡凸起的幅度,个别地方古今陆坡凸起的形态有所变化,但大部分古、今陆坡凸起是一一对应的,基本形态没有根本变化。在萨哈林陆坡地区存在两个方向的挤压应力场,分别是由德鲁根盆地向萨哈林陆坡方向的挤压应力场和萨哈林陆坡沿萨哈林走滑断裂向南的挤压应力场,海底陆坡凸起是这两大应力场复合作用的结果。浊反射区中的游离气是底辟构造中的超高压多相物质向上迁移形成的,浊反射区上方对应的海底凸起应该是宏观构造挤压和局部底辟发育叠合的结果,浊反射区上方的海底凸起,在形态等方面应该和其他仅由挤压构造原因形成的凸起有所区别,比如顶部发育裂口等。在底辟构造中,由于游离气体的向上迁移,在整个水合物稳定域中从下到上,直至海底都可能形成水合物。

Shallow gas hydrates have already been sampled by gravity corer,fishing net,and submersible machines on the sea floor and in the sediment near the sea floor.However the formation mechanism of shallow gas hydrate,its distribution and relationship with sea floor topography remain unclear.Based on the side scan sonar and sub-bottom profile data from the 2006 gas hydrate cruise in the Okhotsk Sea,we found that the occurrence of shallow gas hydrates are closely related to some domelike structures.Domelike structures are common on the slope,especially along the Middle and Lower Sakhalin Slope.They are about several hundred meters wide and several ten meters high.Different from sea floor sand waves and sand ridges,the domelike structures are isolated structures with a slightly longer lower wing and a shorter upper wing.Sub-bottom profile recorder showed that buried dome structures are also well developed on the slope beneath a 30 cm modern sediment layer.A joint compress stress field from the Deryugin basin to the Sakhalin Slope and from the north Sakhalin Slope to the South Sakhalin Slope along the strike slip fault on the Sakhalin Slope was the main cause of the formation of the dome structures,and the diapir structures among the dome structures.The acoustic turbidity beneath the diapir structures clearly show the migration of free gases from deep to the gas hydrate stability zone and to the sea water body from the mini crater on the top of the dome structures.Gas hydrates formed due to the availability of free gas within the diapir structures from the bottom of gas hydrate stability zone up to the sea floor.