Permafrost structures and their effects on the accumulation of the natural gas hydrates in the Qiangtang Basin, northern Xizang

The permafrost regions constitute much of the Qiangtang Basin on the Qinghai-Xizang Plateau, and are considered to be favourable areas for the accumulation of the natural gas hydrates with a great potential. Based on the data from the test drillings, three structural types are recognized for the permafrost regions, of which the most common one is the structural type composed of the active layers, ice-bearing sediments permafrost layer, icebearing basement permafrost layer, and ice-free basement permafrost layer. While in the case of the ice-free basement permafrost layer, the pore fluid pressures in the underlying ice-free permafrost layers tend to be closely related to the structures of the micropores or microfractures in the overlying the ice-bearing permafrost layers, which may facilitate the sealing of the near-surface hydrocarbon gas and accumulation of gas hydrates. The geochemical signatures of ice in the ice-bearing permafrost layers have reflected that the water from the melted atmospheric snowfall directly percolates into the soil and rock layers, and results in the gradually freezing of the strata from shallower to deeper depths due to the decrease of temperatures. Meanwhile, the mineralization degrees and antion and cation concentrations in water may give a reference for the assessment of physical and chemical properties of the sediments. As indicated by the experimental simulation, the methane permeability tends to decrease with the increase of ice saturation in the ice-bearing permafrost layers, and may go into ice-free permafrost layers as the ice saturation exceeds 80%. Influenced by the climatic changes, the permafrost layers may exercise a major control not only on the accommodation spaces of the gas hydrates but also on the migration of the near-surface hydrocarbon gases. It follows that within the permafrost regions in the Qiangtang Basin on the Qinghai-Xizang Plateau, there may be the petroleum systems composed of the fault-controlled deep-seated hydrocarbon reservoirs, medium-deep gas hydrate reservoirs and shallow-seated gas reservoirs.