Geochemical Genesis Model And Its Applications Of Natural Gas In High-over Matured Shale

High-over maturity?Ro>2%?is one of the most important characteristics of the lower Paleozoic marine shales from Sichuan,Chongqing and Hubei?SCH?area.It is also an important feature of the lower Permian alternating marine-terrigenous deltaic facies shale in the southern North China basin?SNCB?.In this paper,the geochemical anomalies and the genesis of shale gas in these two areas are analyzed,the source and enrichment mechanism of nitrogen in lower Cambrian shale are discussed,the model of shale gas evolution and the genetic model of nitrogen-rich shale gas are established,a new method for identifying the source and occurrence of shale gas is proposed,mainly achieved the following understanding:?1?It is difficult to distinguish between coal type gas and oil type gas in the natural gas from the high-over mature shale.The main difference of the natural gas in the high-over mature shale from the transitional facies between marine facies is the nonhydrocarbon component in shale gas,the difference of CO₂ content and carbon isotope composition was obvious.The transitional facies shale gas is mainly distributed in the range of CO₂ content greater than 5% and the ?¹³C?CO₂?value is less than-10‰,while the marine shale gas is mainly distributed in the range of CO₂ content less than 5% and the ?¹³C?CO₂?value is greater than-15‰.?2?Based on the analysis of the reason for the carbon isotopic reversal of shale gas in the well ZD-1 from the lower Cambrian in the Zigui area and the lower Permian shale in the SNCB,it is suggested that the isotopic reversal of alkane gases in these two areas was probably due to destructive redox reactions at maximum burial resulting in Rayleigh-type fractionation.Sichuan basin representative longmaxi formation shale gas drilling data show that there is a positive correlation between the degree of isotope reversal and the formation pressure coefficient and the initial gas production,this finding has important implications for the preservation of shale gas reservoirs.?3?Based on the analysis of the geochemical characteristics of shale gas in the study area,a model of shale gas evolution was established.According to the characteristics of the lower Cambrian shale gas geology in SCH area,we established three models for the nitrogen-rich shale gas,they are genetic model of organic matter pyrolysis,genetic model of atmospheric and complex genetic model.In addition,we analyzed the main controlling factors of the above models,which has guiding significance for the lower Cambrian shale gas exploration.?4?A method for identifying the source and occurrence of shale gas has been established.Based on the study of the source composition of marine shale gas in the SCH area,it is found that the ratio of oil cracked gas in shale gas decreases with the increase of shale maturity.We calculate the content of free gas and adsorbed gas in the transitional shale gas from the SNCB,the calculated results are in good agreement with the log interpretation results.In this paper,the geochemical origin and mode of natural gas in high-over mature shale are systematically studied,the geological application of shale gas geochemistry has been extended,which has a certain reference value for the shale gas exploration in china.