东沙海区天然气水合物形成及分布的地质因素

分析了东沙海区似海底反射波(BSR)分布区的地质构造特征,以了解水合物的地质构造控制因素.似海底反射波通常被认为与水合物密切相关,主要出现在下陆坡沉积物碎屑流和浊流发育区,邻近在海陆过渡带,也是活动断裂构造密集发育和快速沉积作用(重力流沉积作用)地区.该区位于被动大陆边缘,发育巨厚的沉积物.这些沉积物能够供应大量的流体(天然气和水),在渗透性相对高的碎屑流和浊流沉积物中形成水合物.此外,沿洋陆壳过渡带,岩石圈热结构表现较高的热流值,容易形成似海底反射波.相反,在缺乏似海底反射波分布的广大上陆坡区,较薄沉积作用和构造剥蚀等因素不利于似海底反射波发育和天然气水合物的形成.

Geological constraint on the distribution of gas hydrate in the Dongsha continental slope of South China Sea

The geological and tectonic characteristics for generating gas hydrate in the Dongsha continental slope of the South China Sea were analyzed.It is suggested that tectonics and sedimentation play the important roles in the generation of methane hydrate.The range of occurrence of bottom simulating reflector(BSR) in the Dongsha continental slope of the South China Sea was mapped by means of some high-resolution seismic data.The BSR clearly appeared on the seismic profiles of the lower slope,whereas it was few discovered in the upper slope.The BSR was mainly distributed in the transition zone between the continental crust and oceanic crust southwestward the Dongsha Islands.The background of the region was characterized by the passive continental margin.The BSR were distributed in the range of debris flow and turbidity sedimentation.This means that debris and turbidity are sufficiently high to allow the vertical transport of large volumes of methane and to form the gas hydrate.The occurrence of BSR was resulted from anomaly overpressure fluid including gas and water,because of the rapid sedimentary rate revealed by ODP drilling and piston cores during the Late Cenozoic.The high heat flow values indicate that the active tectonics and deep crust fluid in the region benefit to the generation of gas hydrate.Otherwise,few BSR and gas hydrate occurred in the upper slope,because of the thin sedimentary layers and serious tectonic erosion.