冲绳海槽西侧陆坡及其邻区天然气水合物成藏构造特征

STRUCTURAL CHARACTERISTICS OF GAS HYDRATE DEPOSITION IN THE WEST SLOPE OF THE OKINAWA TROUGH AND ITS ADJACENT AREA

为了解特殊地质体与天然气水合物成藏构造特征之间的关系,作者以冲绳海槽西侧陆坡和槽底为例,利用地震资料和地球化学方法对该地区上新世和第四纪沉积物厚度、有机碳含量和断裂系统进行了分析。研究表明,在冲绳海槽西侧及槽底都发育了较厚的上新统和第四系沉积层,上新统厚度约为1000—1500m,第四系厚约为1000—3500m,其中海槽南段存在巨厚的第四系,而且冲绳海槽西侧陆坡沉积物中有机质含量高达0.75%—1.25%,并且沉积速率高达10—40cm/ka,有利于有机质的保存和转化。陆坡发育有与海槽走向一致的NE-SW向断裂系统,以及横切海槽的一系列NW-SE向水平错动扭性断裂系统,其中NE-SW向断层在海槽南段方向变为NEE-SWW,这些断裂系统为烃类气(流)体的运移创造了有利条件。因此,在冲绳海槽西侧陆坡发育的海底峡谷、滑塌体、断块隆脊、泥底辟等特殊地质体与天然气水合物的形成密切相关。

As a special case, the relationship between special geologic structures and hydrate formation the west slope of Okinawa Trough and the trough floor were studied. The thickness of sedimentary deposit of the Pliocene and Quaternary, the organic carbon content and the fault system were studied based on seismic and geo- chemical data. The results show that the sedimentation rate of the study area was high between 10-40 crn/ka, as the Pliocene was 1000-1500 m thick; and the Quaternary was 1000-3500 m thick in the southern Okinawa Trough. The west slope of the trough and the adjacent area were abundant in organic matter of which organic carbon content occupied 0.75%-1.25%. The high sedimentation rate and great thickness should be favorable for organic matter burial and transformation. On the other hand, faults in NE-SW parallel to the trough strike crosscut frequently those in NW-SE direction, of which those in NE-SW changed into NEE-SWW in the south part of the trough. These faults provided advantageous condition for gas hydrate accumulation. Therefore, there are several kinds of structures related to the gas hydrate presence in the study area, such as submarine canyons, slides and slumps, mud diapirs and faulted block ridges.