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Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation

Xie, Hao and Ponton, Camilo and Formolo, Michael J. and Lawson, Michael and Ellis, Geoffrey S. and Lewan, Michael D. and Ferreira, Alexandre A. and Morais, Erica T. and Spigolon, Andre L. D. and Sessions, Alex L. and Eiler, John M. (2020) Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation. Geochimica et Cosmochimica Acta, 290 . pp. 235-256. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20200928-091727864

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Abstract

Intramolecular isotope distributions, including isotope clumping and position specific fractionation, can provide proxies for the formation temperature and formation and destruction pathways of molecules. In this study, we explore the position-specific hydrogen isotope distribution in propane. We analyzed propane samples from 10 different petroleum systems with high-resolution molecular mass spectrometry. Our results show that the hydrogen isotope fractionation between central and terminal positions of natural propanes ranges from −102‰ to +205‰, a much larger range than that expected for thermodynamic equilibrium at their source and reservoir temperatures (36–63‰). Based on these findings, we propose that the hydrogen isotope structure of catagenic propane is largely controlled by irreversible processes, expressing kinetic isotope effects (KIEs). Kinetic control on hydrogen isotope composition of the products of thermal cracking is supported by a hydrous pyrolysis experiment using the Woodford Shale as substrate, in which we observed isotopic disequilibrium in the early stage of pyrolysis. We make a more general prediction of KIE signatures associated with kerogen cracking by simulating this chemistry in a kinetic Monte Carlo model for different types of kerogens. In contrast, unconventional shale fluids or hot conventional reservoirs contain propane with an isotopic structure close to equilibrium, presumably reflecting internal and/or heterogeneous exchange during high temperature storage (ca. 100–150 °C). In relatively cold (<100 °C) conventional gas accumulations, propane can discharge from its source to a colder reservoir, rapidly enough to preserve disequilibrium signatures even if the source rock thermal maturity is high. These findings imply that long times at elevated temperatures are required to equilibrate the hydrogen isotopic structure of propane in natural gas host rocks and reservoirs. We further defined the kinetics of propane equilibration through hydrogen isotope exchange experiments under hydrous conditions; these experiments show that hydrogen in propane is exchangeable over laboratory timescales when exposed to clay minerals such as kaolinite. This implies rather rapid transfer of propane from sources to cold reservoirs in some of the conventional petroleum systems. Propane is also susceptible to microbial degradation in both oxic and anoxic environments. Biodegradation of propane in the Hadrian and Diana Hoover oil fields (Gulf of Mexico) results in strong increases in central-terminal hydrogen isotope fractionation. This reflects preferential attack on the central position, consistent with previous studies.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2020.09.009DOIArticle
ORCID:
AuthorORCID
Xie, Hao0000-0001-5656-2035
Lawson, Michael0000-0003-2743-3773
Ellis, Geoffrey S.0000-0003-4519-3320
Sessions, Alex L.0000-0001-6120-2763
Additional Information:© 2020 Elsevier Ltd. Received 16 May 2020, Revised 2 September 2020, Accepted 7 September 2020, Available online 16 September 2020. This research is supported by an NSF-EAR instruments and facilities grant and Caltech. Additional funding is provided by Exxon Mobil. The kinetic Monte Carlo computations were conducted on the Caltech High Performance Cluster, partially supported by a grant from the Gordon and Betty Moore Foundation. We thank Nami Kitchen for assistance with the operation of the DFS mass spectrometer. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funders:
Funding AgencyGrant Number
NSFUNSPECIFIED
CaltechUNSPECIFIED
Exxon MobilUNSPECIFIED
Gordon and Betty Moore FoundationUNSPECIFIED
Subject Keywords:Natural gas; Position-specific stable isotope analysis; Propane; Isotope fractionation
Record Number:CaltechAUTHORS:20200928-091727864
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200928-091727864
Official Citation:Hao Xie, Camilo Ponton, Michael J. Formolo, Michael Lawson, Geoffrey S. Ellis, Michael D. Lewan, Alexandre A. Ferreira, Erica T. Morais, Andre L.D. Spigolon, Alex L. Sessions, John M. Eiler, Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation, Geochimica et Cosmochimica Acta, Volume 290, 2020, Pages 235-256, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2020.09.009. (http://www.sciencedirect.com/science/article/pii/S0016703720305603)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105578
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:28 Sep 2020 17:09
Last Modified:30 Sep 2020 19:20

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