主动、被动大陆边缘天然气水合物成藏模式对比

不同大陆边缘（主动、被动）沉积物中天然气水合物（以下简称水合物）赋存的控制因素与成藏模式有所差异，开展二者之间的对比研究对于指导水合物勘探具有重要的意义。为此，以主动大陆边缘卡斯凯迪亚（Cascadia）和日本南海（Nankai）海槽、被动大陆边缘布莱克海台（Blake Ridge）和尼日尔三角洲盆地（Niger Delta Basin）等典型水合物成藏区为研究对象，借助于综合大洋钻探（IODP）航次资料解剖和数值模拟分析等手段，从应力场的角度探讨了上述两种背景下含甲烷流体的驱动样式，进而对比分析了主动、被动大陆边缘水合物的成藏模式。研究结果表明：①主动大陆边缘以侧向挤压应力为气体垂向运移提供了驱动力和通道，引诱深部游离气和原位生物气沿断裂运移，气体运移通道主要为俯冲—增生产生的断层、断裂和滑塌体；②主动大陆边缘粉砂和砂质粉砂等粗粒浊流沉积孔隙度大、渗透性好，并且增生楔上沉积物厚度大，是水合物成藏较为有利的储集空间；③较之于主动型大陆边缘，被动大陆边缘虽然缺少俯冲带造成的侧向应力，但在其内巨厚沉积层塑性物质及高压流体、陆缘外侧火山活动等的共同作用下，产生垂向加积和拉张应力，形成的扩散型水合物聚集速率主要取决于甲烷的供给速度；④被动大陆边缘有机质含量、产气速率、地温梯度及沉积速率对水合物含量空间分布具有差异性影响，泥火山或底辟构造等为水合物的形成与赋存提供了理想的场所。

A comparative study on natural gas hydrate accumulation models at active and passive continental margins

The comparative study on natural gas hydrate accumulation models between active and passive continental margins as well as their controlling factors is of great significance to the guidance of natural gas hydrate exploration. Based on the data and research results of international typical active continental margin hydrate accumulation areas such as the Cascadia margin of the Northeast Pacific, the Nankai trough, etc. and passive continental margin areas like the Blake Ridge, the models of the gas hydrate accumulation system are summarized and numerically simulated, and a preliminary comparison of active and passive continental margin reservoir accumulation models was also carried out. The following results were obtained. (1) The active continental margin provides a driving force and channel for vertical gas migration, which induces deep free gas and in-situ biogas to migrate along the fault. The migration channels are mainly faults, fractures and slumps produced by subduction-accretion. (2) Coarse-grained turbidity sediments such as silt and sandy silt have good porosity and permeability. Moreover, the sediment thickness on the accretionary wedge is large, which provides a good storage space for hydrate accumulation. (3) Numerical simulations of the Blake Ridge, and Niger Delta hydrate accumulation show that the passive continental margin lacks the lateral stress caused by the subduction zone compared with the active continental margin. However, due to the plastic materials in the thick sedimentary layer, high-pressure fluids and volcanic activities outside the continental margin, vertical accretion and tensile stress are generated and the accumulation rate of diffusion-type hydrates mainly depends on the methane supply rate. (4) Organic matter content, gas production rate, geothermal gradient and sedimentation rate at the passive continental margin have different effects on the spatial distribution of hydrate content. Mud volcanoes or diapir structures provide an ideal place for the formation and occurrence of hydrates.