Analysis Of Wall Rock Conditions And Tectonic Environment Of Gas Hydrate In The Muli Coalfield Of Qinghai Province

Muli coalfield is located in the Qilian coal-bearing area of the northern tibetan plateau.Its tectonic unit is part of the Qinling-Qilian-Kulun orogenic belt,wich belongs to the western depression zone of the middle Qilian folded belt.The natural gas hydrate of Qinghai Muli area as the research object,the surrounding rock and its hidden geological structure background,such as occurrence horizon,tectonic environment,mineral and chemical composition and the formation mechanism,etc were studied.Through to rock-mineral identification of the surrounding rocks of gas hydrate in Muli area,their rock types are including fine sandstone,argillaceous siltstone,mudstone,limestone,oil shale,and so on.All samples of the surrounding rocks were tested by XRD,which showed that the main mineral composition as follows:primarily quartz,followed by feldspar,and containing a small amount of muscovite,chlorite,etc.And the mineral composition of the limestone are primarily calcite,followed by dolomite,and containing a small amount of cristobalite,magnetite and bioclastic,etc.Oil shale is given priority to with quartz,time for kaolinite,and then white mica,organic matter,etc.XRF analysis on main chemical composition of 13groups of rock samples are as follows:all samples containing SiO₂,among them 6samples of SiO₂ content of 46.40 to 68.57 per centum,SiO₂ content in a sample even reached 99.32 per centum,and another three samples of CaCO₃ content is 95.04 to97.73 per centum,shows that this kind of natural gas hydrate occurrence of surrounding rock with high maturity.Geotechnical tests overall moisture content range of surrounding rock is 0.12 to 1.04,bibulous rate is 0.67 to 3.29;At the same time,the porosity and permeability of research,found that sandstone porosity is 0.2 to3.2 per centum,the average is 2.13 per centum,according to the occurrence of surrounding rocks have good permeability,which are good for gas hydrate’s accumulated.Affected by the late tectonic transformation in the study area,the surrounding rock reservoir performance is improved,the migration of natural gas hydrate for the area provides a favorable condition of surrounding rock and reservoir.Summarized predecessors’test data ofδ¹³C and log interpretation in Muli,the material source of the gas hydrate are preliminary clear.The coal seam of the early to middle Jurassic Yaojie group was given priority to gas source,followed by late Triassic Galedeshi group was also done contribution to the gas hydrate in the region.The faulted structure were extremely development in Muli coalfield.The early stage fracture provided the channels for gas migration.The main body of natural gas hydrate was made up of coalbed methane migrating along fault fracture zone.When the coalbed methane was transferred to fracture to crack water-bearing rocks or fissured waterbearing stratum,the natural gas hydrate was formed in the lower temperature and higher pressure.Late faults were adverse for natural gas hydrate,which influenced the distribution of the position and scale.