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

Abstract

Ethane is the second most abundant alkane in most natural gas reservoirs. Its bulk isotopic compositions (δ^(13)C 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 (^(13(CH_3-^(12)CH_3 for δ^(13)C and ^(12)CDH_2-^(12)CH_3 for δD). However, multiply-substituted isotopologues can bring additional independent constraints on the origins of natural ethane. The ^(13)C_2H_6 isotopologue is particularly interesting as it can potentially inform the distribution of ^(13)C 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 ^(13)C_2H_6 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 (Δ^(13)C_2H_6) 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 Δ^(13)C_2H_6 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 Δ^(13)C_2H_6 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.

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