The molecular and carbon isotopic constrains on origin and storage of Longmaxi Formation shale gas in Changning area, Sichuan Basin, China

摘要

We have developed a vacuum-crushing method for the extraction of gases stored in shale for the detennination of gas yield, chemical composition, and carbon isotopic composition by online gas-chromatography and mass spectrometry. Analysis of the evacuation and parallel experiments showed low standard deviation and good reproducibility. This approach significantly improved the detection limits for trace-gas analysis in shale and measured multiple nonhydrocarbon gas components in a single run. We analyzed the chernical and carbon isotopic compositions of the gases released by one-step vacuum crushing from nineteen samples of organic-rich Longmaxi Formation (Fm.) shale in Changning outcrop, Sichuan Basin, China, and we studied the main controls on gas chemistry of retained gas in the Longmaxi Shale. The results showed that the crushed gas is high in N_2 and CO_2 but low in CH_4. The crushed gas has obviously lower C_1/CO_2 ratios than the gas produced from the Longmaxi Fm. after hydraulic fracturing; it was dominated by CH_4-dry gas while it is normalized to 100% total gaseous hydrocarbons and similar in chemical composition to hydrocarbon gases in the produced gas. Yields of CH_4, CO_2, and H2 in the crushed gas of most samples showed a weak positive correlation with total organic carbon (TOC) contents. The crushed gas had significantly light δ~(13)C_1 (on average -35.8%) values and similar δ~(13)C_2 values when compared with the produced gas in the Changning area. The δ~(13)C values of C_1, C_2, C_3, and CO_2 have similar features to the produced gas from the Weiyuan shale-gas play. Isotopically reversed trends (δ~(13)C_1 > δ~(13)C_2) between ethane and methane were observed for some samples. Our findings regarding the similarity in chemical and carbon isotopic compositions between crushed gas and produced gas from the Weiyuan shale-gas play might provide important evidence of the origin and storage of shale gas in the organic-rich Silurian Longmaxi Fm.