CaltechAUTHORS
  A Caltech Library Service

Equilibrium and non-equilibrium controls on the abundances of clumped isotopologues of methane during thermogenic formation in laboratory experiments: Implications for the chemistry of pyrolysis and the origins of natural gases

Shuai, Yanhua and Douglas, Peter M. J. and Zhang, Shuichang and Stolper, Daniel A. and Ellis, Geoffrey S. and Lawson, Michael and Lewan, Michael D. and Formolo, Michael and Mi, Jingkui and He, Kun and Hu, Guoyi and Eiler, John M. (2018) Equilibrium and non-equilibrium controls on the abundances of clumped isotopologues of methane during thermogenic formation in laboratory experiments: Implications for the chemistry of pyrolysis and the origins of natural gases. Geochimica et Cosmochimica Acta, 223 . pp. 159-174. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20180301-075710981

Full text is not posted in this repository. Consult Related URLs below.

Use this Persistent URL to link to this item: http://resolver.caltech.edu/CaltechAUTHORS:20180301-075710981

Abstract

Multiply isotopically substituted molecules (‘clumped’ isotopologues) can be used as geothermometers because their proportions at isotopic equilibrium relative to a random distribution of isotopes amongst all isotopologues are functions of temperature. This has allowed measurements of clumped-isotope abundances to be used to constrain formation temperatures of several natural materials. However, kinetic processes during generation, modification, or transport of natural materials can also affect their clumped-isotope compositions. Herein, we show that methane generated experimentally by closed-system hydrous pyrolysis of shale or nonhydrous pyrolysis of coal yields clumped-isotope compositions consistent with an equilibrium distribution of isotopologues under some experimental conditions (temperature–time conditions corresponding to ‘low,’ ‘mature,’ and ‘over-mature’ stages of catagenesis), but can have non-equilibrium (i.e., kinetically controlled) distributions under other experimental conditions (‘high’ to ‘over-mature’ stages), particularly for pyrolysis of coal. Non-equilibrium compositions, when present, lead the measured proportions of clumped species to be lower than expected for equilibrium at the experimental temperature, and in some cases to be lower than a random distribution of isotopes (i.e., negative Δ_(18) values). We propose that the consistency with equilibrium for methane formed by relatively low temperature pyrolysis reflects local reversibility of isotope exchange reactions involving a reactant or transition state species during demethylation of one or more components of kerogen. Non-equilibrium clumped-isotope compositions occur under conditions where ‘secondary’ cracking of retained oil in shale or wet gas hydrocarbons (C_(2-5), especially ethane) in coal is prominent. We suggest these non-equilibrium isotopic compositions are the result of the expression of kinetic isotope effects during the irreversible generation of methane from an alkyl precursor. Other interpretations are also explored. These findings provide new insights into the chemistry of thermogenic methane generation, and may provide an explanation of the elevated apparent temperatures recorded by the methane clumped-isotope thermometer in some natural gases. However, it remains unknown if the laboratory experiments capture the processes that occur at the longer time and lower temperatures of natural gas formation.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2017.11.024DOIArticle
https://www.sciencedirect.com/science/article/pii/S001670371730741XPublisherArticle
ORCID:
AuthorORCID
Stolper, Daniel A.0000-0003-3299-3177
Additional Information:© 2017 Elsevier Ltd. Received 28 February 2017; accepted in revised form 18 November 2017; available online 1 December 2017. This work was cosponsored by the NSFC (41772135), NSTMP (2016ZX05007-01) and PetroChina Foundation (2017ycp014); analyses of Woodford Shale gases are either taken from Stolper et al. (2014b) or newly measured for this study as part of a continuing research collaboration supported by ExxonMobil. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government. We thank Dr. Hagit Affek, Dr. Shuhei Ono, Dr. Robert Dias, and two anonymous reviewers for their valuable comments and suggestions.
Funders:
Funding AgencyGrant Number
National Natural Science Foundation of China41772135
National Science and Technology Major Project (China)2016ZX05007-01
PetroChina Foundation2017ycp014
ExxonMobilUNSPECIFIED
Subject Keywords:Thermogenic methane; Clumped isotopologues; Non-equilibrium; Coal; Shale
Record Number:CaltechAUTHORS:20180301-075710981
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180301-075710981
Official Citation:Yanhua Shuai, Peter M.J. Douglas, Shuichang Zhang, Daniel A. Stolper, Geoffrey S. Ellis, Michael Lawson, Michael D. Lewan, Michael Formolo, Jingkui Mi, Kun He, Guoyi Hu, John M. Eiler, Equilibrium and non-equilibrium controls on the abundances of clumped isotopologues of methane during thermogenic formation in laboratory experiments: Implications for the chemistry of pyrolysis and the origins of natural gases, Geochimica et Cosmochimica Acta, Volume 223, 2018, Pages 159-174, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2017.11.024.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85023
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:01 Mar 2018 17:48
Last Modified:01 Mar 2018 17:48

Repository Staff Only: item control page