Gas hydrate in the Qilian Mountain permafrost and its distribution characteristics

The Scientific Drilling Project of Gas Hydrate in Qilian Mountain permafrost，which covered the Juhugeng ore district of the Muli coalfield within Tianjun County, Qinghai Province, was implemented during 2008-2009. Four scientific drilling holes, i.e., DK-1, DK-2, DK-3 and DK-4 had been completed. The gas hydrate samples recovered from drillings have confirmed the existence of gas hydrate in Qilian Mountain permafrost. The drilling results show that "fracture-filling" and "pore-filling" are the main gas hydrate types existent in the permafrost. Based on a study of ten aspects of gas hydrate characteristics, the authors consider that gas hydrates mainly occur in Middle Jurassic Jiangcang Formation and hosted under the permafrost zone in the 133.0~396.0m interval. The reservoir rocks of gas hydrate are composed of siltstone, oil shale, mudstone, fine sandstone and a small amount of medium-grained sandstone. Vertical distribution of gas hydrate does not show continuity in individual holes, and lateral distribution is not apparent. Statistics of Rock Quality Designation Index（RQD）for cores show that the RQD lower value zone together with the gas hydrate reservoir segment has fairly good consistency. It is thus concluded that the fracture system plays an important role in the control of the distribution of gas hydrate in this area.