东海天然气水合物的地震特征

SEISMIC CHARACTERISTICS OF GAS HYDRATES IN THE EAST CHINA SEA

使用中国科学院海洋研究所“科学一号”调查船于2001年以及20世纪80年代在东海地区采集的多道地震资料,以海域天然气水合物研究为目的,对这些资料进行了数据处理并获得了偏移地震剖面.通过对地震剖面的解释,在6条剖面上确定了6段异常反射为BSR,均有振幅强、与海底相位相反的特点.6段BSR基本上都没有出现和沉积地层相交的现象.分析认为,这与东海地区第四纪以来的沉积特征有关,并不能由此否认这些异常反射是BSR.6段BSR出现的水深为750～2 000 m,埋深在0.1～0.5 s（双程时间）之间.随着海底深度的增大,BSR埋深有增大的趋势.计算结果显示,6段BSR所处的温度和压力条件都满足水合物稳定赋存所需要的温度和压力条件.本文的BSR主要与北卡斯凯迪亚盆地以及智利海域水合物的温度、压力条件相似,而与日本南海海槽、美国布莱克海台等海域水合物的温度、压力条件相差比较大.在地震剖面上,6段BSR所处的局部构造位置都和挤压、断层有关,有利于水合物的发育;在空间上,它们主要分布在东海陆坡近槽底的位置以及与陆坡相近的槽底.在南北方向上,除分布在吐噶喇断裂和宫古断裂附近外,还与南奄西、伊平屋和八重山热液活动区相邻.热液活动和水合物虽然没有直接的成因关系,但岩浆活动为水合物气源的形成提供了热源条件,为流体和气体的运移、聚集提供了通道条件,从而有利于水合物的发育与赋存.根据地震剖面反射特征推断,剖面A1A2和A14A23发育BSR的位置应该有气体或者流体从海底流出,可能是海底冷泉发育的位置.剖面A14A23上BSR发育处,振幅比的异常增大和BSR埋深的降低是相关联的.这种关联支持该处发育海底冷泉的推测.

For the purpose of gas hydrate study, multi-channel seismic data were collected by Institute of Oceanology, Chinese Academy of Sciences （IOCAS） using “R/V Science 1” in the East China Sea in 2001. Seismic data from 2001 together with those from 1980s were processed to get the migration profiles. 6 sections of abnormal reflections were determined as BSR based on the migration profiles interpretation. All the BSRs given have strong amplitude and reverse phase as comparing to the sea floor reflector and with buried depth within 0.1 to 0.5 seconds （two way travel time） ,which agrees with the data from published references. The buried depth of our BSRs increases with the increase of the sea floor depth from 750 m to around 2 000 m. There is no BSR that cuts the normal sediment layer as the case in other places,and this is because the upper Quaternary sediment layer in the East China Sea is parallel to the sea floor,leading the BSR to being parallel to the sea floor too. Thus it does not mean that our BSR is a faint one. The initial temperature and pressure calculation of our BSRs based on a very simple model for sediment velocity and temperature gradient shows that all our BSRs are within the stability domain in the gas hydrate phase diagram. Assuming that BSR is the base of gas hydrate stability zone in the gas hydrate phase diagram, our BSRs show a similar temperature and pressure condition to that of North Cascadia and Chilian Margin, but different from that of Nankai Trough and Blake Ridge. Among the whole Okinawa Trough, our BSR is either near main faults（Tokara fault and Miyako fault）or near hydrothermal activity fields distributed along the central graben of Okinawa Trough. On the seismic profile, our BSRs are locally related with tectonic compression and faulting, which can lead to the formation of gas hydrates. Although there is no direct relationship between hydrothermal activity and gas hydrate occurrence, shallow magma in hydrothermal field supplies heat for methane production from organic material in sediment and fault system in the hydrothermal area serves as conduits for the migration of methane from deeper part of the sediment to the gas hydrate reservoir. We believe that hydrothermal envi- ronment will help with the formation of gas hydrates. We can see from the seismic characteristics of profiles A1A2 and A14A23 that cold seepage occurs on the seafloor above the BSR of these two profiles. Coherence of the abnormal increase of amplitude ratio and the abnormal decrease of is considered as an evidence of cold seepage on profile A14A23 where BSR occurs.