A reconnaissance study of 13C- 13C clumping in ethane from natural gas

Clog, M. , Lawson, M., Peterson, B., Ferreira, A. A., Santos Neto, E. V. and Eiler, J. M. (2018) A reconnaissance study of 13C- 13C clumping in ethane from natural gas. Geochimica et Cosmochimica Acta, 223, pp. 229-244. (doi: 10.1016/j.gca.2017.12.004)

153630.pdf - Accepted Version
Available under License Creative Commons Attribution Non-commercial No Derivatives.

153630Suppl.pdf - Supplemental Material



Ethane is the second most abundant alkane in most natural gas reservoirs. Its bulk isotopic compositions (δ13C and δD) are used to understand conditions and progress of cracking reactions that lead to the accumulation of hydrocarbons. Bulk isotopic compositions are dominated by the concentrations of singly-substituted isotopologues (13CH3-12CH3 for δ13C and 12CDH2-12CH3 for δ D). However, multiply-substituted isotopologues can bring additional independent constraints on the origins of natural ethane. The 13C2H6 isotopologue is particularly interesting as it can potentially inform the distribution of 13C atoms in the parent biomolecules whose thermal cracking lead to the production of natural gas. This work presents methods to purify ethane from natural gas samples and quantify the abundance of the rare isotopologue 13C2H6 in ethane at natural abundances to a precision of ±0.12‰ using a high-resolution gas source mass spectrometer. To investigate the natural variability in carbon-carbon clumping, we measured twenty-five samples of thermogenic ethane from a range of geological settings, supported by two hydrous pyrolysis of shales experiments and a dry pyrolysis of ethane experiment. The natural gas samples exhibit a range of ’clumped isotope’ signatures (Δ13C2H6) at least 30 times larger than our analytical precision, and significantly larger than expected for thermodynamic equilibration of the carbon-carbon bonds during or after formation of ethane, inheritance from the distribution of isotopes in organic molecules or different extents of cracking of the source. However we show a relationship between the Δ13C2H6 and the proportion of alkanes in natural gas samples, which we believe can be associated to the extent of secondary ethane cracking. This scenario is consistent with the results of laboratory experiments, where breaking down ethane leaves the residue with a low Δ13C2H6 compared to the initial gas. Carbon-carbon clumping is therefore a new potential tracer suitable for the study of kinetic processes associated with natural gas.

Item Type:Articles
Additional Information:This study was made possible through financial support of the NSF-EAR program, Petrobras and ExxonMobil.
Glasgow Author(s) Enlighten ID:Clog, Dr Matthieu
Authors: Clog, M., Lawson, M., Peterson, B., Ferreira, A. A., Santos Neto, E. V., and Eiler, J. M.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Geochimica et Cosmochimica Acta
ISSN (Online):1872-9533
Published Online:13 December 2017
Copyright Holders:Copyright © 2017 Elsevier Ltd.
First Published:First published in Geochimica et Cosmochimica Acta 223: 229-244
Publisher Policy:Reproduced in accordance with the publisher copyright policy

University Staff: Request a correction | Enlighten Editors: Update this record