Rights statement: This is the author’s version of a work that was accepted for publication in Journal of Natural Gas Science and Engineering. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Natural Gas Science and Engineering, 78, 2020 DOI: 10.1016/j.jngse.2020.103304
Accepted author manuscript, 2.29 MB, PDF document
Available under license: CC BY-NC-ND
Final published version
Research output: Contribution to Journal/Magazine › Journal article › peer-review
| Article number | 103304 |
|---|---|
| <mark>Journal publication date</mark> | 1/06/2020 |
| <mark>Journal</mark> | Journal of Natural Gas Science and Engineering |
| Volume | 78 |
| Number of pages | 14 |
| Publication Status | Published |
| Early online date | 9/04/20 |
| <mark>Original language</mark> | English |
The source and thermal evolution history of organic matter for the Longmaxi shale are still debated. This study analyzed the molecular and stable carbon isotopic compositions of hydrocarbons (CH 4, C 2H 6, and C 3H 8) and CO 2 as well as the stable hydrogen isotopic compositions of methane, ethane, and noble gases (He, Ne, Ar, Kr, and Xe). Shale gases in the WY and CN areas show an extremely-low-wetness with CH 4 concentrations range from 93.41% to 99.01%. Non-hydrocarbon gases are mainly N 2 (0.22%–2.81%) and CO 2 (0.03%–1.35%). H 2S have not been detected. Different δ 13C 1 and δ 13C 2 values 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 (δ 13C 1>δ 13C 2<δ 13C 3 and δ 13C 1>δ 13C 2>δ 13C 3) of hydrocarbons indicate a complex evolution process. WY shale gases include more oil-cracking gas than CN shale gases, suggesting WY shale gases more like come from Type I-II organic matter. In shale gas systems, methane content and δ 13C 1 ratios vary with the degree of thermal evolution, so the origin of shale gas cannot be determined using carbon isotope data alone. The wide range of δ 13C CO2 values (−8.9‰ to −0.8‰) and N 2/ 40Ar ratios (20.8–165.1) suggests multiple origins of the gases. Emeishan mantle plume provides the source of heat for some thermo-genic gas. Noble gas isotopic compositions ( 3He/ 4He: 0.001Ra to 0.019Ra) indicate air and crustal origins with no significant contribution from the mantle. 40Ar/ 36Ar ratios (1194.3–4604.5) are consistent with the age of Longmaxi strata calculated by accumulative effect of Ar isotope. The shale gas humidity, carbon isotope ratios, and the carbon isotope-composition distribution patterns may contain information indicating the shale gas sweet spot.