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Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases

Thiagarajan, Nivedita and Xie, Hao and Ponton, Camilo and Kitchen, Nami and Peterson, Brian and Lawson, Michael and Formolo, Michael and Xiao, Yitian and Eiler, John (2020) Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases. Proceedings of the National Academy of Sciences of the United States of America, 117 (8). pp. 3989-3995. ISSN 0027-8424. PMCID PMC7049135 . https://resolver.caltech.edu/CaltechAUTHORS:20200211-133518831

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Abstract

Natural gas is a key energy resource, and understanding how it forms is important for predicting where it forms in economically important volumes. However, the origin of dry thermogenic natural gas is one of the most controversial topics in petroleum geochemistry, with several differing hypotheses proposed, including kinetic processes (such as thermal cleavage, phase partitioning during migration, and demethylation of aromatic rings) and equilibrium processes (such as transition metal catalysis). The dominant paradigm is that it is a product of kinetically controlled cracking of long-chain hydrocarbons. Here we show that C₂₊ n-alkane gases (ethane, propane, butane, and pentane) are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the isotopic distribution of these species approaches thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage, becoming indistinguishable from equilibrium in the most thermally mature gases. We also find that the pair of CO₂ and C₁ (methane) exhibit a separate pattern of mutual isotopic equilibrium (generally at reservoir conditions), suggesting that they form a second, quasi-equilibrated population, separate from the C₂ to C₅ compounds. This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature, and source substrate. Additionally, an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1073/pnas.1906507117DOIArticle
https://www.pnas.org/content/suppl/2020/02/10/1906507117.DCSupplementalPublisherSupporting Information
http://www.ncbi.nlm.nih.gov/pmc/articles/pmc7049135/PubMed CentralArticle
Additional Information:© 2020 National Academy of Sciences. Published under the PNAS license. Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved January 17, 2020 (received for review April 25, 2019). PNAS first published February 11, 2020. All of the data reported in this manuscript are available in the SI Appendix. Author contributions: N.T., H.X., C.P., and J.E. designed research; N.T., H.X., C.P., N.K., B.P., M.L., M.F., and Y.X. performed research; N.T., H.X., C.P., and J.E. analyzed data; and N.T., H.X., C.P., B.P., M.L., M.F., and J.E. wrote the paper. The authors declare no competing interest. This article is a PNAS Direct Submission. This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1906507117/-/DCSupplemental.
Subject Keywords:stable isotopes; compound-specific isotope analysis; clumped isotopes; natural gas; methane
Issue or Number:8
PubMed Central ID:PMC7049135
Record Number:CaltechAUTHORS:20200211-133518831
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200211-133518831
Official Citation:Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases. Nivedita Thiagarajan, Hao Xie, Camilo Ponton, Nami Kitchen, Brian Peterson, Michael Lawson, Michael Formolo, Yitian Xiao, John Eiler. Proceedings of the National Academy of Sciences Feb 2020, 117 (8) 3989-3995; DOI: 10.1073/pnas.1906507117
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101227
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:11 Feb 2020 23:27
Last Modified:02 Apr 2020 17:14

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