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, e.g., 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 discussed. The active continental margin provided 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. Coarse-grained turbidity sediments, i.e., silt and sandy silt have good porosity and permeability. Furthermore, the sediment thickness on the accretionary wedge is large, which provides a good storage space for hydrate accumulation. Numerical simulations of the Blake Ridge, and Niger Delta hydrate accumulation showed 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. Organic matter content, gas production rate, geothermal gradient and sedimentation rate at the passive continental margin had 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.