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Methane clumped isotopes in the Songliao Basin (China): New insights into abiotic vs. biotic hydrocarbon formation

Shuai, Yanhua and Etiope, Giuseppe and Zhang, Shuichang and Douglas, Peter M. J. and Huang, Ling and Eiler, John M. (2018) Methane clumped isotopes in the Songliao Basin (China): New insights into abiotic vs. biotic hydrocarbon formation. Earth and Planetary Science Letters, 482 . pp. 213-221. ISSN 0012-821X.

[img] MS Word (The supplementary material includes two figures. Fig. S1 shows the calculated mixing trends between putative thermogenic methane and abiotic methane. While a plot of He isotopic ratios vs. δ13CCO2 is shown in Fig. S2) - Supplemental Material
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Abiotic hydrocarbon gas, typically generated in serpentinized ultramafic rocks and crystalline shields, has important implications for the deep biosphere, petroleum systems, the carbon cycle and astrobiology. Distinguishing abiotic gas (produced by chemical reactions like Sabatier synthesis) from biotic gas (produced from degradation of organic matter or microbial activity) is sometimes challenging because their isotopic and molecular composition may overlap. Abiotic gas has been recognized in numerous locations on the Earth, although there are no confirmed instances where it is the dominant source of commercially valuable quantities in reservoir rocks. The deep hydrocarbon reservoirs of the Xujiaweizi Depression in the Songliao Basin (China) have been considered to host significant amounts of abiotic methane. Here we report methane clumped-isotope values (Δ_(18)) and the isotopic composition of C_1–C_3 alkanes, CO_2 and helium of five gas samples collected from those Xujiaweizi deep reservoirs. Some geochemical features of these samples resemble previously suggested identifiers of abiotic gas (^(13)C-enriched CH_4; decrease in ^(13)C/^(12)C ratio with increasing carbon number for the C1–C4 alkanes; abundant, apparently non-biogenic CO_2; and mantle-derived helium). However, combining these constraints with new measurements of the clumped-isotope composition of methane and careful consideration of the geological context, suggests that the Xujiaweizi depression gas is dominantly, if not exclusively, thermogenic and derived from over-mature source rocks, i.e., from catagenesis of buried organic matter at high temperatures. Methane formation temperatures suggested by clumped-isotopes (167–213 °C) are lower than magmatic gas generation processes and consistent with the maturity of local source rocks. Also, there are no geological conditions (e.g., serpentinized ultramafic rocks) that may lead to high production of H_2 and thus abiotic production of CH_4 via CO_2 reduction. We propose that the Songliao gas is representative of an atypical type of thermogenic gas that can be mistaken for abiotic gas. Such gases may be encountered more frequently in future exploration of deep or over-mature petroleum systems.

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Additional Information:© 2017 Elsevier B.V. Received 5 April 2017, Revised 7 October 2017, Accepted 27 October 2017, Available online 20 November 2017. This research was cosponsored by the NSFC (41772135), NSTMP (2016ZX05007-01) and PetroChina Funding (2017ycq14). We thank Yirui Xu and Yongqiang Cui for assistance in the field; Jingkui Mi for discussion of abiotic gas experiments. Two anonymous reviewers provided valuable comments and suggestions to improve the manuscript.
Funding AgencyGrant Number
National Natural Science Foundation of China41772135
National Science and Technology Major Project (China)2016ZX05007-01
Subject Keywords:abiotic methane; clumped isotopologue; coal; natural gas; geochemistry
Record Number:CaltechAUTHORS:20180215-094703583
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Official Citation:Yanhua Shuai, Giuseppe Etiope, Shuichang Zhang, Peter M.J. Douglas, Ling Huang, John M. Eiler. Methane clumped isotopes in the Songliao Basin (China): New insights into abiotic vs. biotic hydrocarbon formation, Earth and Planetary Science Letters, Volume 482, 2018, Pages 213-221, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84846
Deposited By: Tony Diaz
Deposited On:15 Feb 2018 23:00
Last Modified:03 Oct 2019 19:23

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