Thermal Simulation Experimental Study On Gas Source Conditions Of Gas Hydrate In The Qilian Mountain Permafrost

Qilian Mountain permafrost,Qinghai Province,China,is one of the important potential areas of gas hydrate.The estimate amount of gas hydrate resource is about 20.2×1012m3?30.3×1012m3,showed the huge value of economic development.But gas hydrate has complex gas composition,and the gas source is unknown.That gases are from gas hydrate are related to oil-typed gases or coal-typed gases,and gases are from organic matter of mudstone,organic matter of oil shale or organic matter of coal,has been a practical problem puzzling exploration research.In the dissertation,geochemistry features of different organic matters from mudstone,oil shale and coal at gas hydrate-bearing intervals in drills,Qilian Mountain permafrost were carried out.For the first time,thermal simulation experiments of a closed system and a constant temperature heating method was systematically analysis applied,and gas chromatography-mass spectrometry technology was used in combination,to explore different generated hydrocarbon processes of different organic matter types,and features of gas composition and carbon isotopic.By comparing gas composition and carbon isotopes of hydrocarbon gases from gas hydrate with cores(mudstone,oil shale and coal),the purpose is to investigate possible links between these potential gas source rocks and gas source of gas hydrate,and then to analyze possible gas source rocks of gas hydrate.Combined with the geological background and relationships of the thermal simulation temperature,the vitrinite reflectance well as the depth,present burial depths and generated hydrocarbon potential of gas source rocks were discussed.By analyzing element compositions of C,H and O,maceral compositions as well as pyrolysis parameters,organic matter evaluation was carried out.The results showed that,the organic abundances of mudstone samples are high,and are mainly good source rocks,the organic matters are mainly ?1 type to ?2 type,and reach low to medium mature stage;the organic abundances of oil shale samples are same high,and belong to good source rocks,the organic matters are mainly ? type,and reach low to medium mature stage;the organic abundances of mudstone are high,and are mainly good source rocks,the organic matters of coal samples are mainly ? type,and reach medium to high maturity.Characteristics of gas component from samples by thermal simulation experiment indicated that,gases generated by samples in the thermal simulation experiment are mainly composed of CO2 and a small amount of hydrocarbon gases on the low temperature condition.As the temperature increased,contents of hydrocarbon gases increased,but amounts of CO2 are little changed.In the whole thermal simulation experiment,yields of hydrocarbon gases generated from mudstone,oil shale and coal also increase,and reach the peak at 500?,being 10.44ml/g,17.17ml/g,and 130.18ml/g respectively;yields of methane generated from samples has been increasing,whereas,yields of heavy hydrocarbon gases is rising rapidly when the temperature is below 450?,and shows an obvious downward trend after 450?.The hydrocarbon gases are mainly methane,and contain quite a few of heavy hydrocarbon gases.The dry coefficients are less than 95%,which shows characteristics of wet gas.Characteristics of carbon isotope indicated that,Carbon isotope values of ?13C1,?13C2 and ?13C3 of hydrocarbon gases from samples in thermal simulation experiments present a trend of becoming lighter firstly and then weight with thermal simulation temperature increasing.On the whole,carbon isotope values of hydrocarbon gases from mudstone and oil shale are similar,and lighter than them from coal.Gas-source correlations showed that,hydrocarbon gases from mudstone on 350? to 400? and oil shale on 380? to 400? have similar features of gas compositions and carbon isotope with gas hydrate.Therefore,it is speculated that the gases of gas hydrate has geochemical genetic relation with deep mudstone and oil shale.In contrast,gas compositions of hydrocarbon gases from coal and gas hydrate are closer,but carbon isotope values are far away,so it is speculated that the relationship between coal and gas source of gas hydrate is not close.According to relationships of the vitrinite reflectance,the thermal simulation temperature,the palaeogeothermal gradient as well as the burial depth,presumed that present burial depths of mudstone and oil shale provided gases for gas hydrate maybe range 1500m to 2000m.