The Geochemical Characteristics Of China's Typical Shale Gas

China's shale gas has made siginificant achievements in exploration and development,but the study of geochemical characteristics of shale gas is almost blank.It is important to research on the causes of shale gas,gas source identification and enrichment regularity.In this paper,through collecting 67 domestic shale gas data from mainly shale gas reservoir of the SiChuan basin and Ordos basin,at the same time,collecting 315 US shale gas data reported in literatures,then sort,map and analysize these data to study China's typical shale gas geochemical characteristics,combined with a large number of North American shale gas research report.Longmaxi shale gas is dry gas,containing a small amount of CO₂ and N₂ which both are derived from high temperature decomposition of inorganic mineral.With the increase of maturity,the carbon isotope sequence will experience normal ordinary,reversal and negative.Longmaxi shale gas shows negative carbon isotope sequence,Chang 7 for normal ordinary sequence and Shanxi formation and Longtan formation for negative sequence.The reverse of shale gas in high mature phase is that oil and condensate crack to produce ethane and propane of ¹²C rich.And in overmature phase,the carbon isotope values of methane,ethane and propane are approaching.When Longmaxi shale gas has run out of oil cracking and enter the stage of pure moisture cracking,ethane fractional distillation rate is greater than propane,resulting in?¹³C₂>?¹³C₃.At the same time,because of tectonic uplift,shale gas desorption,loss and other secondary effect,increase the carbon isotope,eventually forming negative carbon isotope sequence.The light hydrocarbon data and the hydrocarbon isotopic composition of the marine shale gas in China show the characteristics of coal derived gas.Because of the high gas maturity of the marine shale gas,light hydrocarbon begins to cracking destruction,which preserved are stable compounds of thermodynamics properties.As for showing abnormal high value of methane carbon isotope,Rayleigh fractionation or physical action such as desorption,diffusion and dissipation in the high evolution stage may be the reason.These anomalies should be paid attention to in the gas source identification at the overmature stage.Isotopic reversals have nothing to do with shale gas production,only with humidity maturity index.The maturity of high-yield wells of Longmaxi formation is mostly between 2.4%³.4%,the exploration risk of higher maturity is high and the lower maturity has poor resource prospect.