Author: ChunhuiCao, Mingjie Zhang, Liwu Li, Yuhui Wang, Zhongping Li, Li Du, GregHolland, Zheng Zhou
Abstract
Thesource and thermal evolution history of organic matter for theLongmaxi shale are still debated. This study analyzed the molecularand stable carbon isotopic compositions of hydrocarbons (CH4,C2H6,and C3H8)and CO2as well as the stable hydrogen isotopic compositions of methane,ethane, and noble gases (He, Ne, Ar, Kr, and Xe). Shale gases in theWY and CN areas show an extremely-low-wetness with CH4concentrations range from 93.41% to 99.01%. Non-hydrocarbon gases aremainly N2(0.22%–2.81%)and CO2(0.03%–1.35%).H2Shave not been detected. Different δ13C1andδ13C2values in WY and CN shale gases (WY:-37.3‰to-35.0‰and-40.3‰to-38.3‰,CN:-29.8‰to-26.3‰and-35.3‰to-32.7‰)and various carbon isotope-composition distribution patterns(δ13C1>δ13C2<δ13C3 and δ13C1>δ13C2>δ13C3)of hydrocarbons indicate a complex evolution process. WY shale gasesinclude more oil-cracking gas than CN shale gases, suggesting WYshale gases more like come from Type I-II organic matter. In shalegas systems, methane content and δ13C1ratiosvary with the degree of thermal evolution, so the origin of shale gascannot be determined using carbon isotope data alone. The wide rangeof δ13CCO2values (-8.9‰to-0.8‰)and N2/40Arratios (20.8–165.1)suggests multiple origins of the gases. Emeishan mantle plumeprovides the source of heat for some thermo-genic gas. Noble gasisotopic compositions (3He/4He:0.001Ra to 0.019Ra) indicate air and crustal origins with nosignificant contribution from the mantle. 40Ar/36Arratios (1194.3–4604.5)are consistent with the age of Longmaxi strata calculated byaccumulative effect of Ar isotope. The shale gas humidity, carbonisotope ratios, and the carbon isotope-composition distributionpatterns may contain information indicating the shale gas sweet spot.
Keywords: Stable isotopic compositions; Shale gas; Noble gas isotopes; Sources; Evolution; Longmaxi formation