南海东北部陆坡天然气水合物藏特征

2013年下半年,国土资源部中国地质调查局、广州海洋地质调查局在南海东北部陆坡水深664~1 420m范围内钻探13个站位,取心获取了大量、多种类型的天然气水合物(以下简称水合物)实物样品,其中甲烷气体含量超过99%。对南海东北部陆坡天然气水合物成藏特征的研究结果表明:1地震反射剖面上具有多道似海底反射层(BSR)、振幅空白带(BZ)、BSR与地层斜交、速度反转等地震异常;2测井曲线揭示含水合物沉积层具有高电阻率(200Ω·m)、高纵波速度(2 700m/s)的特征;3地震和测井异常特征暗示该区具有单、双层水合物藏组合特征;4该区域水合物藏位于海底之下埋深220m以内的粉砂质黏土及生物碎屑灰岩沉积物中,水合物饱和度为45%~100%;5自然状态下,水合物呈块状、层状、瘤状、脉状、分散状等5种赋存形式;6研究区天然气水合物赋存类型多、饱和度高、甲烷纯度高,初步估算该区天然气水合物地质储量约为1 250×108 m3(50%概率条件下),是有利的勘探目标区。

Characteristics of natural gas hydrate reservoirs on the northeastern slope of the South China Sea

From sample cores obtained in exploration drilling at 13 sites (664 – 1 420 m deep) on the northeastern slope of the South China Sea, we found a large amount of gas hydrates of multiple types, the methane gas content of which is more than 99%. After an integrated analysis of their characteristics, the following findings were obtained. (1) From the multichannel seismic profile, we detected many significant seismic anomalies, including Bottom Simulating Reflection (BSR) and Blanking Zone (BZ), BSR oblique crossing the formation, velocity inversion.（2） The borehole logging data also revealed the characteristics of high electrical resistivity (up to 200 m·Ω) and high acoustic velocities (up to 2 700 m/s) of the hydrate-bearing sediments. (3) Seismic and logging anomalies implied that both single-layer and double-layer gas hydrates occurred in the northeastern slope of the South China Sea. (4) The gas hydrate resources of this area are preserved in the sediments like silty clays or limestone grains within the depth of 220 meters below the seafloor (mbsf), where the saturations of gas hydrates range from 45% to 100%. (5) In a natural state, gas hydrates were deposited primarily in the form of massive blocks, thin beds, veins, nodular or disperse particles, among which the first four types were generally formed in the silty clay with coarser grain sizes or in the weak belts near the seafloor with many tracks of the neotectonics. In conclusion, this drilling area was considered to be a promising exploration target of natural gas hydrate resources with about 125 billion m³ because of its favorable features such as a variety of occurrence types, high porous saturation, and high methane concentration.