珠江口盆地东部海域含天然气水合物沉积层的地球物理特征

Geophysical characteristics of gas hydrate bearing sediments in the eastern sea area of the Pearl River Mouth Basin

为了弄清珠江口盆地东部海域为何存在渗漏型和扩散型两种天然气水合物(以下简称水合物)类型以及其间的地球物理特征有何区别,根据钻井资料和区域地质条件,对该区地震和测井资料进行了精细分析,总结了不同水合物沉积地层的地球物理特征和构造特征。结果表明:(1)含水合物层的测井曲线具有高电阻率、高声波速度、高中子孔隙度、低补偿密度和低自然伽马值的"三高两低"的异常特征;(2)含水合物层在地震反射剖面上具有似海底反射(BSR)、振幅空白带、BSR与地层斜交及眼球状结构等特征;(3)当地震剖面上存在模糊、杂乱反射且BSR之下气烟囱发育、BSR之上有断裂通道时,通道内容易形成渗漏型水合物矿体,而BSR上面连续性较好的振幅空白带所反映的沉积地层则是扩散型水合物矿藏的主要储集层;(4)渗漏型水合物矿藏上、下两层水合物的物性特征存在着显著区别,下层水合物碳氢气体浓度较高,在断层通道内高度聚集形成块状水合物,上层水合物碳氢气体浓度下降,分布范围较广;(5)扩散型水合物由于气体容积扩散导致气体浓度下降,分布范围较大。

This paper aims to figure out why there are two different types of hydrate deposits： leakage and diffusion discovered in the eastern sea area of the Pearl River Mouth Basin and what distinct geophysical characteristics could help identify them from each other. Based on the seismic and logging data analysis and the drilling data and regional geological conditions, both geophysical and tec- tonic characteristics were analyzed and summarized of the two different hydrate-bearing sediments. The following findings are achieved. （1） On the logging curves, the hydrate-bearing sediments display the anomalous characters of high resistivity, high acoustic veloci- ty, high neutron porosity, low compensated density and low natural gamma. （2） On the seismic profile, the gas hydrate reservoirs are as- sociated with multiple bottom-simulating reflectors （BSRs）, amplitude blank zone, BSR oblique-crossing the strata and ＂augen＂ struc- tures. （3） On the chaotic or clutter reflection, there are well-developed gas chimneys under the BSR and fault passages up above the BSR, where leakage-type hydrate deposits are easily formed, while the diffusion-type hydrate deposits exist in those amplitude blanking zones with good continuity up above the BSR. （4） Obvious difference could be seen in physical properties between the overlying and under- lying strata of leakage-type hydrate deposits： the hydrocarbon gas with a high concentration migrates and accumulates within the fault passage to form massive hydrate in the underlying strata, while in the overlying strata, the hydrocarbon content becomes lower and lower but in a wide distribution. （5） The diffusion-type hydrate is in more widespread distribution due to its enlarging gas volume but with low- er gas concentration.