A Caltech Library Service

The evolution of intra- and inter-molecular isotope equilibria in natural gases with thermal maturation

Xie, Hao and Dong, Guannan and Formolo, Michael and Lawson, Michael and Liu, Jianzhang and Cong, Fuyun and Mangenot, Xavier and Shuai, Yanhua and Ponton, Camilo and Eiler, John (2021) The evolution of intra- and inter-molecular isotope equilibria in natural gases with thermal maturation. Geochimica et Cosmochimica Acta, 307 . pp. 22-41. ISSN 0016-7037. doi:10.1016/j.gca.2021.05.012.

[img] MS Word (Supplementary data) - Supplemental Material
See Usage Policy.


Use this Persistent URL to link to this item:


Naturally occurring hydrocarbon fluids in sedimentary basins have economic, geological and environmental significance. Connecting sedimentary basin temperature-time evolution with petroleum generation and transformation is a long-studied problem. In this study, we investigate the use of a novel tool – multiply substituted isotopologues of methane, for distinguishing between different chemical mechanisms in catagenesis and for characterizing the extent of thermal maturation of thermogenic natural gases. We analyze the stable isotope compositions of a suite of thermogenic gas samples that are globally distributed and cover a wide range in composition and thermal maturation, from dominantly unconventional shale gas formations and a few conventional gas plays. Our data show that methane generated at early thermal maturity has a stable isotope composition governed by chemical kinetics, characterized by a pronounced deficit in Δ¹²CH₂D₂; this signature can be explained by its formation chemistry that combines a more D-rich methyl radical pool and more D-poor H radical pool. Methane from higher thermal maturity fluids increases in Δ¹²CH₂D₂, reaching equilibrium at vitrinite reflectance maturity (R₀) of approximately 1.5% (equivalent to 170–210 °C peak burial temperature) and higher, which is interpreted to be the result of isotope exchange erasing the disequilibrium signature of catagenetic chemistry, mediated by C H activation during either radical chain reactions or organic-inorganic interactions on mineral surfaces. We further examined hydrogen isotope fractionations among methane, ethane and propane for a compiled global dataset and found that the intermolecular fractionation exhibits a trend similar to that seen for the Δ¹²CH₂D₂ value of methane, departing from equilibrium at low thermal maturities and moving towards equilibrium as maturity increases. These findings indicate that the inter- and intra-molecular hydrogen isotope structures of components of thermogenic natural gas transition from chemical-kinetic control at low thermal maturities toward thermodynamic control at higher thermal maturities, mediated by hydrogen exchange reactions. We propose that these systematic relationships could be used to identify the exact thermal maturation stages for natural gases and their associated fluids, especially for oil-associated gas at early maturation.

Item Type:Article
Related URLs:
URLURL TypeDescription
Xie, Hao0000-0001-5656-2035
Dong, Guannan0000-0002-6844-2492
Cong, Fuyun0000-0002-3618-3924
Mangenot, Xavier0000-0001-8770-6767
Ponton, Camilo0000-0001-7608-1116
Eiler, John0000-0001-5768-7593
Additional Information:© 2021 Elsevier Ltd. Received 18 January 2021, Accepted 7 May 2021, Available online 19 May 2021. This work was supported by Caltech, ExxonMobil Upstream Integrated Solutions, the Caltech Joint Industry Partnership for Petroleum Geochemistry, Thermo Fischer Scientific, the Department of Energy BES program and Natural Science Foundation of China (Grant No. 41690134 and 42072179). We thank the ExxonMobil Corporation, Sinopec, PetroChina and Lundin companies for collecting natural gas samples. We thank Nami Kitchen for laboratory assistance. We thank Nivedita Thiagarajan, Alex Sessions and David Wang for helpful discussions. We thank Nina Albrecht, Ulrike Wacker, Darren Tollstrup and Timothy Piles for advices on mass spectrometry. 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.
Funding AgencyGrant Number
ExxonMobil Upstream Integrated SolutionsUNSPECIFIED
Thermo Fisher ScientificUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Natural Science Foundation of China41690134
Natural Science Foundation of China42072179
Subject Keywords:Methane; Natural gas; Clumped isotopes; Basin thermal history
Record Number:CaltechAUTHORS:20210821-160205003
Persistent URL:
Official Citation:Hao Xie, Guannan Dong, Michael Formolo, Michael Lawson, Jianzhang Liu, Fuyun Cong, Xavier Mangenot, Yanhua Shuai, Camilo Ponton, John Eiler, The evolution of intra- and inter-molecular isotope equilibria in natural gases with thermal maturation, Geochimica et Cosmochimica Acta, Volume 307, 2021, Pages 22-41, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:110360
Deposited By: Tony Diaz
Deposited On:21 Aug 2021 16:16
Last Modified:21 Aug 2021 16:16

Repository Staff Only: item control page