Formation Conditions and Characteristics of Natural Gas Hydrate in Muli Permafrost

Natural gas hydrate has been found in Qilian Mountain Muli permafrost.It is the first discovery of gas hydrate in mid-low latitude permafrost zone.Compared with foreign permafrost gas hydrate,Muli permafrost gas hydrate has some characteristic,including shallow buried depth,thin permafrost thickness,and high heavy hydrocarbon gas components.As the isotope analysis of gas hydrocarbon,about the source of gas,the oil-type gas predominates in the region,associated with coal-type gas local layers.The main factors influencing gas hydrate formation of the Muli permafrost regions have been analyzed,and the permafrost characteristics and gas components have great influence on the permafrost gas hydrate formation.Heavy hydrocarbon-rich gas components are more conducive to hydrate accumulation in Muli region,but pure methane gas is hard to form gas hydrate in the region.The formation lithology of gas hydrate reservoir intervals are mainly siltstone,oil shale,mudstone and fine sandstone,furthermore,mudstone and oil shale account for more than 90%.Rock fractures develop relatively,and fractured gas hydrates are the main reservoir type.Authors preliminary think that Muli permafrost natural gas hydrate reservoir belongs to heavy hydrocarbon thermal gas-low temperature freezing-strata dominant dynamic accumulation model,which thermal gases of the lower strata,under the action of the fault tectonic channel,migrate into the gas hydrate stability zone and form layered gas hydrate.