Source Rocks for Gases from Gas Hydrate and Their Burial Depth in the Qilian Mountain Permafrost, Qinghai: Results from Thermal Stimulation

The composition and isotope ratio of gases from gas hydrate and organic geochemistry of mudstone, oil shale, and coal within a gas hydrate-bearing interval in the Qilian Mountain permafrost suggest that gases from gas hydrate are derived from the deep, but the lithology and burial depth of source rocks for gases from gas hydrate are not yet known. In this study, samples of mudstone, oil shale, and coal were analyzed with thermal stimulation within the gas hydrate-bearing interval in the Qilian Mountain permafrost. Results showed that gas composition and carbon isotopes of hydrocarbon gases thermally stimulated from mudstone at temperatures of 350–400 °C and from oil shale at temperatures of 380–400 °C are similar to those of gases from gas hydrate, indicating that the deep mudstone and oil shale are potential source rocks for gases from gas hydrate. The gas composition of hydrocarbon gases thermally stimulated from coal was similar to that of gas hydrate, but their carbon isotope values were very different, suggesting that coal was not necessarily related to a gas source for gas hydrate. The burial depths of source rocks for gases from gas hydrate were further estimated to be 1500–2000 m on the basis of the relationship among thermally stimulated temperature, vitrinite reflectance, and burial depth in the study area.