Formation Condition and Distribution Prediction of Gas Hydrate in Qinghai-Tibet Plateau Permafrost

Permafrost is one of the two major geological environments for gas hydrate occurrences. The Qinghai-Tibet Plateau has a mean altitude over 4000m and the permafrost area is about 1.4×106km2. Based on the thickness of permafrost and the geothermal gradient in the Qinghai-Tibet Plateau permafrost, the occurrence and distribution of gas hydrate in permafrost were predicted by using the thermodynamic method based on the temperature and pressure of natural gas hydrate formation. The thermodynamic phase equilibrium for both thermogenic and biogenic gas hydrates in Qinghai-Tibet Plateau permafrost implies that the gas hydrate is accumulated within the stable zone whose top boundary is buried at ca. 27~560m and bottom boundary is buried at ca. 27~2070m. The potential of natural gas resources as caged in hydrates in the Qinghai-Tibet Plateau permafrost is estimated at about 1.2×1011 ~ 2.4 × 1014m3. Gas hydrate is propitious to occur where permafrost is thicker and geothermal gradient is lower in Tibet Plateau permafrost. The seasonal change of air temperature in Qinghai-Tibet Plateau affects only sediments within a depth of ca. 10m and would not affect gas hydrates that are buried below 30m. On the global warming, the gas hydrates in Qinghai-Tibet Plateau would be unstable. Similar to the degradation of permafrost, the distribution area of gas hydrates will be gradually reduced with the lowering of the top boundary and the rising of the bottom boundary, and the gas hydrates will finally vanish in the Qinghai-Tibet Plateau permafrost.