南海北部陆坡流体运移差异性的原因分析——以神狐天然气水合物钻探区和LW3-1井区为例

神狐天然气水合物(以下简称水合物)钻探区和LW3-1井区均位于南海北部陆坡珠江口盆地,平面相距约25km,但其天然气(水合物)的成因却不相同。为此,首先对神狐海域高精度2D/3D地震资料进行了精细解释,共识别出底辟构造(气烟囱)、大尺度断层、上新世中尺度正断层和更新世滑脱断层等4种含气流体的运移通道。在此基础上,对比分析了神狐水合物钻探区和LW3-1井区运移通道的平面分布特征。结果表明:气烟囱虽在两个区域内都有发育,但在形态、规模、内部结构、与围岩接触关系等方面却存在着明显的不同;晚中新世以来,神狐水合物钻探区以上新世正断层和更新世滑脱断层为主,前者被认为是流体侧向运移的通道,后者则沟通了气烟囱和海底,可被视为流体的逃逸通道,而LW3-1井区则主要发育沟通深部的大尺度断层,且剖面上呈现为地堑/地垒的特征。结论认为:LW3-1井区中沟通深部烃源层的气烟囱和大尺度断层,其垂向的流体输送能力要强于神狐水合物钻探区,这便是造成这两个相邻区域流体运移条件不同的主要原因。

A comparative study on the differences of fluid migration between the Shenhu and LW3-1 drilling areas for natural gas hydrate,northern South China Sea

The Shenhu area and LW3-1 area are located in the Pearl River Mouth Basin, Northern South China Sea, with only 25 km apart in plane, but the origins of natural gas hydrate forming in the two areas are different. To explain this phenomenon, the high-resolution seismic data (2D/3D) in the Northern South China Sea were interpreted integrally. Four types of gas fluid migration pathways were first identified: diapirs (gas chimneys), large-scale faults, the Pliocene normal scale faults, and the Pleistocene detachment faults. On this basis, the horizontal distribution characteristics of these pathways in the Shenhu and LW3-1 areas were analyzed and compared in detail. The following findings were obtained. (1) Although gas chimneys are developed in both two areas, there are obvious differences in shapes, scales, inner structures, and contact relations with surrounding rocks. (2) The Pliocene normal faults and the Pleistocene detachment faults were well developed in the Shenhu area since the Late Miocene; the former ones are considered to be the lateral pathways for fluid migration, while the latter ones be the "escaping pathways" to seafloor because of their good connection between gas chimneys and seafloor. (3) Large scale faults are well developed deep in the LW3-1 area, and are shown as grabens/horsts in profile. It is concluded that gas chimneys and large-scale faults connecting source rocks in the LW3-1 area have a higher vertical transport capacity compared to those in the Shenhu area, which may be the main reason why the fluid migration conditions in the two areas with only 25 km apart are totally different.