Reservoir characteristics of natural gas hydrates in China

There is abundant natural gas hydrate resource in both permafrost and marine regions of China, showing similar mechanisms in basin-based occurrence, migration and accumulation, but vast differences between hydrate reservoirs in the two regions are found in reservoir endowment characteristics, enrichment patterns and resource potential. In order to provide basic data for mining gas hydrates based on the geology-engineering-environment integration, such variations in distribution patterns and occurrence characteristics between two regions were analyzed and summarized from various aspects, including tectonic structure, deposition, geothermal, heat flow, geochemistry, geophysical response, grow habit, pore structure/permeability, mechanical strength and saturation. Meanwhile, the hydrates in the permafrost are mainly buried in the Mesozoic strata, and the gas in hydrates is from thermogenic gas. The hydrate occurrence is controlled by faults and fracture structures with excellent trap conditions. Reservoir temperature, geothermal gradient, heat flow and pore pressure show lower values compared to those of marine hydrate reservoirs. Most of hydrates occur in sandstone pores and/or shale/mud fractures. The logging response of hydrate-bearing layers in the permafrost in general shows the characteristics of low density, low API value, high resistivity and high velocity. The formation frameworks are rocks with relatively high mechanical strength, low porosity, low permeability and low hydrate saturation. The hydrates in the marine areas mainly located in the Cenozoic strata, formed by mixed origin gas. Their occurrences are usually controlled by mud diapirs, gas chimneys and faults, with no obvious trap structures. All of the reservoir temperature, geothermal gradient, heat flow and pore pressure show high features. Most of marine hydrates distribute in fossil-rich clayey silt and/or silty clay, while the seismic reflection wave shows obvious BSR mark. The logging response shows the characteristics of high resistivity and high acoustic velocity. The formation frameworks in the marine regions are unconsolidated sediments with low mechanical strength, high porosity, low permeability and relatively high hydrate saturation.