Bulk and position-specific isotope geochemistry of light hydrocarbons from natural gases
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The formation of natural gas and subsequent processes in the subsurface are of great interest for scientific and economic purposes. Conventional, bulk carbon and hydrogen isotopes of natural gases have been widely used to address these processes, but our understanding is still limited. Recently, new dimensions of information on the stable isotopes of natural gases have been explored, including clumped isotopes and position-specific isotopes. A new method was developed to determine both hydrogen and carbon position-specific isotope compositions of propane from natural gas based on quantitative nuclear magnetic resonance (NMR) spectrometry. The new technique is capable of analyzing intact molecules of propane with precision of < ±1‰ and < ±10‰ for carbon and hydrogen isotope compositions, respectively. The accuracy of the method was tested by 13C-labeled compounds as well as an inter-laboratory standard of a light alkane. Thirty-eight natural gas samples were collected from several oil-gas fields from both conventional and unconventional reservoirs in Texas and Oklahoma. The fraction of propane in selected samples was separated and purified chemically and cryogenically, and their purity and isotopic integrity during purification were tested. Our results show that the differences in hydrogen and carbon isotope compositions between the two sites (center and terminal C’s and H’s) of propane range from -205 to +38‰, and from -4.2 to +7.8‰, respectively. Most of position-specific isotope compositions of natural propane do not follow the trajectories predicted by theoretical calculations both at isotope equilibrium and by cracking of longer hydrocarbons. The relationship between the position-specific hydrogen isotope compositions of the Woodford propane and the corresponding maturity can be explained by hydrogen isotope exchange between water and propane during geological time. Very negative position-specific hydrogen isotope fractionations were observed from two gas samples from conventional reservoirs in the Midland Basin, and it is speculated to be related to microbial activities. The new technique for position-specific isotope compositions can provide deeper insights into the origins and history of natural gases.