The geological process for gas hydrate formation in the Qilian Mountain permafrost

The Qilian Mountain permafrost is the only place where gas hydrate occurs onshore China at present and its gas hydrate distribution is very complex and irregular. What patterns affecting the accumulation of gas hydrate or what process controlling the formation of gas hydrate are not clear in the study area. Aiming at a gas hydrate geological system, the geological process of gas hydrate formation was studied, based on geological data and analytical results obtained from drilling wells in the Qilian Mountain permafrost. As a result, three stages for the geological process of gas hydrate formation are put forward in the study area. During the late Mid-Jurassic, the upper Triassic generated and provided a major gas source for gas hydrate, secondarily in combination with gas associated with oil generated from the middle Jurassic. The main gas source migrated upward via faults of F1 and F2, partly and occasionally mixed with the coal-bed methane and the microbial methane produced in the shallow strata. It was blocked jointly by thrust faults and thick mudstone or oil shale to be initially accumulated in gas reservoir. From Cretaceous to Pleistocene, the sedimentary strata experienced erosion and the initial gas accumulation turned into residual gas after series of the Qinghai-Tibet plateau uplift. Since the early middle Pleistocene, glaciations formed a gas hydrate stability zone (GHSZ) and the residual gas was coupled with GHSZ to form gas hydrate subsequently. Hence three patterns for the coupling of the residual gas with GHSZ are summarized in the study area. When the residual gas happened to lie within GHSZ, the residual gas directly formed gas hydrate, which was indicated by the drilling results that gas anomalies were encountered within GHSZ as well as occurrences of gas hydrate in the field. When the residual gas was below GHSZ, the residual gas would continually migrate into GHSZ to form gas hydrate, which was indicated by the drilling results that gas anomalies had ever been encountered even if below GHSZ as well as occurrences of gas hydrate within GHSZ in the field. When the residual gas was above GHSZ, the residual gas remained or escaped, which was indicated by the drilling results that gas anomalies even with a high pressure abnormity were encountered in the shallower strata above GHSZ without occurrences of gas hydrate within GHSZ in the field.