Road to Equilibrium: Stable Isotope Distribution in Gaseous Alkanes and Thermal History of Geological Hydrocarbons

Citation

Xie, Hao (2021) Road to Equilibrium: Stable Isotope Distribution in Gaseous Alkanes and Thermal History of Geological Hydrocarbons. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/neh6-6c82. https://resolver.caltech.edu/CaltechTHESIS:04012021-045507416

Abstract

Naturally occurring hydrocarbon fluids have economic, geological, and environmental significance. Most of the natural hydrocarbon on Earth is formed by thermal alteration of organic matter in the sedimentary basin. My dissertation study is motivated by this question: can we track generation, transformation, storage, and destruction of these subsurface hydrocarbon fluids with isotopic proxies? The conventional geochemical toolkit includes relative compositional abundances, such as wetness and C1/(C2+C3) ratio, and stable isotope ratios of ¹³C/¹²C and ²H/¹H, on both the bulk (material-average) and compound-specific (molecular-average) levels. However, these signatures often rely on empirical categorizations and calibrations, so they can be prone to ambiguities, errors, and inconsistencies. This thesis presents a series of work that develops and refines stable isotope proxies of gaseous hydrocarbon (C1-C5) molecules. My approaches overcome the problems in mainly two ways. (1) I add new analytical techniques to acquire isotopologue ratios of compounds. I establish two new analytical proxies, multiply substituted isotopologues (clumped isotopes) of methane, and position-specific isotope ratios of propane, using recently advanced high-resolution isotope ratio mass spectrometry. (2) I use rigorous thermodynamic and kinetic constraints of isotope distribution in hydrocarbon molecules to interpret isotopic data in natural samples. These constraints are determined by theories and experiments. For thermodynamic control, I conducted catalytic exchange experiments to calibrate equilibrium isotope effect for propane position-specific hydrogen isotopes (Chapter 2) and compound-specific hydrogen isotope fractionation between alkanes (Chapter 4) and tested quantum chemical calculations. For expression of kinetic isotope effects, I implemented a statistical approach, the kinetic Monte Carlo method, to calculate the intramolecular and intermolecular stable isotope composition of alkanes generated by radical cracking mechanism in catagenesis (Chapter 6). I measured position-specific hydrogen isotopes of propane (Chapter 3) and methane clumped isotopes (Chapter 5) in natural gas samples from global reservoirs, and compiled compound-specific isotope data in the literatures (Chapter 5 and 6). Results show similarities in isotope ordering of these molecules, which is that gas formed at lower temperature/depth expresses kinetic isotope effects, but gas formed or buried at higher temperature for longer times is in equilibrium. The switch from kinetic control to thermodynamic control is likely a result of thermally-activated hydrogen exchange. This trend provides the foundation for tracking generation and thermal evolution of subsurface hydrocarbons with stable isotope proxies.

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