Geochemical characteristics and genesis of the Yanchang Formation terrestrial shale gas in the Yanchang exploration area

Through analyzing the gas composition and isotopes of carbon and hydrogen of the shale gas samples,the geochemical characteristics of the upper Triassic Yanchang Formation shale from the Yanchang exploration area in the southeastern Ordos basin were studied.The genesis and source of the Yanchang Formation shale gas in the Yanchang exploration area were also discussed.The results show that:①Shale gas of Yanchang Formation in the Yanchang exploration area mainly consist of hydrocarbon gases (87.89%~99.66%),with methane content less than 95% (60.44%~91.68%),high content of heavy hydrocarbon component,relatively lower dry coefficient（64.00%~93.43%）,and low non-hydrocarbon gases content (mainly N₂ and CO₂,0.68% and 1.36%,respectively ),belongs to typical wet gas.②The carbon isotope values of methane,ethane,propane and butane of Yanchang Formation shale gas in the Yanchang exploration area are mainly distributed in -53.4‰~-46.3 ‰,-39.7 ‰~-31.9 ‰,-34.3 ‰~-29.6 ‰ and -33.8 ‰~-25.0 ‰.The carbon isotope of Yanchang Formation shale gas in the Yanchang exploration area is mainly characterized by δ¹³C₁<δ¹³C₂ <δ¹³C₃ <δ¹³C₄.Only a small number of samples of the δ¹³C₃ and δ¹³C₄ are reversal.③Shale gas of Yanchang Formation in the Yanchang exploration area is dominated by thermogenic gas,mainly oil type gas formed by kerogen cracking that from the lowmature and mature stage of the partial sapropel type shale.