【摘要】 通过对南海973航次在该区域的多道地震剖面的分析,结合该区域的地质背景,认为台湾增生楔具有天然气水合物存在和分布的地球物理特征,在地震剖面上出观海底反射层(BSR)、振幅空白(BZ)、极性反转等地震识别标志。BSR所在区域位于南海向菲律宾海板块俯冲的增生楔上,南海新生代沉积不仅提供了富含有机质的丰富物源,而且类似于活动大陆边缘的构造体系又为天然气水合物的形成提供了良好的条件。更多还原

【Abstract】 Gas hydrate is a crystalline solid, composed of water and gas. Natural gas hydrate is usually methane hydrate. It is stable within a certain high-pressure low-temperature range and can exist in permafrost regions and continental margins where sea depth exceeds 300m. For its high reservoir amount and burning efficiency the gas hydrate may become a major energy resource. It also can trigger terrain collapse and landslide because its instability, any small change in temperature or pressure would cause breakdown. The release and absorption of large volumes of methane from sea-floor hydrates may have had considerable impacts in modifying the Earth’s cilmate. The South China Sea (SCS) is one of the biggest marginal seas in western Pacific. Rich conventional hydrocarbon resources are located in it especially in the north and south margins. From June to September 2001, an oceanographic Survey # 973 by TANBAO marine vehicle was made in the area from northeast part of SCS to the Philippine Sea Basin, crossing Taiwan Accretionary Prism tectonically, undertaken by the Marine Geology Survey of Guangzhou, China. The multi-channel seismic profile along the line was studied in scope of the geological background and geophysical features of gas hydrates. Taiwan Accretionary Prism lies from northeast of SCS to southwest of Taiwan. It is on the conjunction area between the Eurasian and Philippine Sea Plate, resulted from the SCS’s subduction beneath the Philippine Sea Plate. The prism is composed of the Gaoping Slope and Hengchuan Sea Ridge. It extends to north and connects with Taiwan in Hengchuan Peninsula. To the east is the Taiwan Orogenic Belt which experienced active thrusting. To the west are series of faulted basins that developed in the objective continental margin. The structure of the Taiwan Accretionary Prism offers good conditions for the formation of gas hydrate. The SCS’s subduction to the Philippine Sea Plate formed a small trench-arc system similar to the positive continental margin in the northeast of the basin. The material sources are mainly from the sediments of the north margin of SCS. After Oligocene the SCS’ s north margin was in thermal subsidence and the sediments were mainly of terrestrial and marine facies. The abundant terrestrial materials, high subsidence rate, and rich organic matters favored the hydrocarbon generation. With the continuous of the subduction, some Cenozoic sediments subducted to the deep while some accumulated beyond the Manila Trench that formed the Taiwan Accretionary Prism. On one hand, when the subducted materials reached the appropriate depth, they would experience decomposition and generated gases. On the other hand, with the growth of the accretionary prism, increasing pressure and temperature favored the organic matters decomposition and gas generation. From the above all analysis we believe that there were abundant gas sources mainly from the deep thermal decomposition and some from shallow biodegradation. The plentiful thrust faults in Taiwan Accretionary Prism offer good channels through which the gases can migrate to the appropriate place where the gas hydrate can form and reserve. In appropriate temperature and pressure, the gases would fill up the interspace of the sediments and form the gas hydrate stability zone (GHSZ). For its low osmosis, the GHSZ can be a cap below which plenty of free state gases accumulated. The base limit of the GHSZ often marked by a very strong reflector called a bottom-simulating reflector (BSR) on seismic profiles resulting from the negative acoustic impedance contrast between hydrate-bearing strata and free gas-bearing strata or water saturated strata. The sediments above the BSR experienced cementation and formed blanking zone (BZ). With the uprising of the Taiwan Accretionary Prism, the pressure would reduce and the GHSZ be destroyed. So in Hengchuan sea ridge, the gas hydrate decreases for the pressure reduction and bacterial oxidation. Accretionary Prism is a main structure unit of an active continental margin. Many abnormal reflectors (BSR) are proven on seismic profiles of the aceretionary prisms all over the world. BSRs are usually oblique to the bedding planes of sediments, parallel to the seafloor and the presence of high amplitude strong reflectors, reverse polarity, and sometimes amplitude blanking zones above them. The accretionary prism favors the gas hydrate’s generation.更多还原