Formation of permafrost and its influence on natural gas hydrate accumulation in Muli Region, Qinghai Province

The characteristics of the existing permafrost and natural gas hydrates were summarized and analyzed by studying the natural gas hydrate formation in Muli Region, Qinghai Province. On the basis of the evidence of the Quaternary climate survey and the available geological exploration data on the Qinghai-Tibet Plateau, FLAC^3D was used to simulate and calculate the formation of permafrost. The calculation results show that the existing permafrost probably formed during the late Holocene Neoglacial(4 000-3 000 to 1 000 a BP), and a stable boundary at bottom of the permafrost layer formed after about 170 a of cooling at a depth of 122 m. Furthermore, the temperature gradient in the permafrost layer is 1.64℃/hm. The calculated results are quite consistent with the results obtained in the field exploration. In addition, on the basis of the formation of the permafrost layer, temperature and pressure curves of the formation of A/B class natural gas hydrates and the current status of natural gas hydrate accumulation, the reasons for the relatively scattered distribution of natural gas hydrates in Muli Region were analyzed. First, due to the geological structure, the hydrocarbon gas only migrated to the depth of around 140 m, and the natural hydrate underwent a phase change in situ. Second, it is possible that the hydrocarbon gas migrated to the shallower strata. However, as the repeated evolution of permafrost over many years, the gas hydrate entered the atmosphere in gaseous form after being decomposed dynamically. The related results can provide some ideas for the exploration and exploitation of the natural gas hydrate in Muli Region.