木里冻土区天然气水合物形成条件及特征

祁连山木里冻土区是第一个发现天然气水合物的中纬度高山冻土区,与国外发现的冻土区天然气水合物相比,祁连山木里冻土区天然气水合物具有埋深浅、冻土层薄、重烃气体组分高等特征。气体碳氢同位素分析显示,区域内气体来源主要以油型气为主,并在局部伴有煤型气。分析了木里冻土区天然气水合物形成的主要影响因素,冻土层特征和气体组分对冻土区天然气水合物形成的影响较大,富含重烃等气体组分更有利于该地区天然气水合物的成藏,而纯甲烷气体在该地区不易形成天然气水合物。天然气水合物储集层段主要以粉砂岩、油页岩、泥岩和细砂岩为主,并且泥岩和油页岩占90%以上,岩石裂缝较发育,以裂隙型天然气水合物为主要赋存类型。初步认为木里冻土区天然气水合物为下部热解气在断层等构造通道作用下运移而形成的一种重烃气体组分热解气-低温冷冻-地层型为主的动态成藏。

Formation Conditions and Characteristics of Natural Gas Hydrate in Muli Permafrost

Natural gas hydrate has been found in Qilian Mountain Mull permafrost. It is the first dis- covery of gas hydrate in mid--low latitude permafrost zone. Compared with foreign permafrost gas hy- drate, Muli permafrost gas hydrate has some characteristic, including shallow buried depth, thin per- mafrost thickness, and high heavy hydrocarbon gas components. As the isotope analysis of gas hydrocar- bon, 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 Mull 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 accumula- tion in Mull region, but pure methane gas is hard to form gas hydrate in the region. The formation litholo- gy of gas hydrate reservoir intervals are mainly siltstone, oil shale, mudstone and fine sandstone, further- more, 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 Mull permafrost natural gas hy- drate reservoir belongs to heavy hydrocarbon thermal gas--low temperature freezing--strata dominant dy- namic 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.