东海陆坡及邻近槽底天然气水合物成藏条件分析及前景

Geological Factors for the Development and Newly Advances in Exploration of Gas Hydrate in East China Sea Slope and Okinawa Trough

在西太平洋边缘海中,东海是唯一没有获得天然气水合物样品的边缘海。利用已有的地震资料、海底温度资料等,从沉积物来源、沉积地层厚度、烃源岩条件、沉积速率、海底温度—压力条件等方面对东海水合物成藏条件进行了分析。认为冲绳海槽沉积物源丰富,沉积厚度大,且发育烃源岩地层。冲绳海槽较高的沉积速率主要分布于冲绳海槽槽底沉积中心,以及西部陆坡连接海底峡谷底部的三角洲区域。根据冲绳海槽实测的海底温度数据,整个冲绳海槽地区600m以深的范围都能够满足水合物发育的温度、压力条件。以温度梯度为30℃/km计算,冲绳海槽中水合物稳定域的最大厚度为650m。冲绳海槽盆地中普遍发育的底辟构造、背斜构造等局部构造,以及网格状断裂系统,为烃类气流体的向上及侧向运移创造了有利条件,成为天然气水合物发育的有利区带。根据已经发现的BSR特征来看,东海地区天然气水合物前景广阔。

Up to now, East China Sea is the only marginal sea in which gas hydrate samples haven＇ t been revealed in the West Pacific. Based on the geological and geophysical data obtained both from our past several cruises and from other publications, geological factors including sedimentary sources, sedimentary thickness, source rock, sedimentation rate, temperature and pressure distribution that control gas hydrate development have been analysed. The paper points out that East China Sea has abundant sedimentary sources supplied mainly by Changjiang River and partly by Huanghe River which have the capability of transporting 1,498 × 10^6 t of sediment each year from China's Mainland to East China Sea, Bohai Sea and Huanghai Sea. Several sedimentary centers have been clearly revealed along East China Sea Slope and west part of Okinawa Trough by our seismic data. Sediment thicknesses of those centers are all over 5 km with the earliest sediment layer since around Eocene or even Oligocene time. We believe that source rock is existing among the deeper part of the sediment center either Eocene layer or Oligocene layer which have already been confirmed as good quality source rock in East China Shelf Basin by drilling works, and hydrocarbons might be generated earlier from the source rock in those centers than in shelf basin due to higher temperature gradient in slope and trough area as comparing with that of the shelf area. Sedimentation rate along the slope and trough varies from several cm/ka to 400 cm/ka, with higher values mainly constraining in the sediment centers and delta area which is connecting to the end of slope canyons and distributing at the most west part of trough basin. Based on our measured sea floor temperature data, gas hydrates can only developed in the slope and trough area with water depth over than 600 m. Gas hydrates stability zone can be as thick as 650 m when take temperature gradient as 30 ℃/km, but thinning to northeast along the axis of Okinawa Trough and to northwest up to the slope as the water depth changing shallower and sea floor temperature changing warmer. Local geological structures like mud diapir, anticline, fracture system are quite common in the study area, and they mainly control the fluid migration forming a favorable environment for the gas hydrate development and preservation in the study area. Based on our analysis, and BSRs found on some seismic profiles, we believe that the reason for gas hydrates unrevealling in East China Sea as comparing with other marginal seas is mainly due to the relatively lower exploration level.