Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation

Abstract

Intramolecular isotope distributions, including isotope clumping and position specific fractionation, can provide proxies for the formation temperature and formation and destruction pathways of molecules. In this study, we explore the position-specific hydrogen isotope distribution in propane. We analyzed propane samples from 10 different petroleum systems with high-resolution molecular mass spectrometry. Our results show that the hydrogen isotope fractionation between central and terminal positions of natural propanes ranges from −102‰ to +205‰, a much larger range than that expected for thermodynamic equilibrium at their source and reservoir temperatures (36–63‰). Based on these findings, we propose that the hydrogen isotope structure of catagenic propane is largely controlled by irreversible processes, expressing kinetic isotope effects (KIEs). Kinetic control on hydrogen isotope composition of the products of thermal cracking is supported by a hydrous pyrolysis experiment using the Woodford Shale as substrate, in which we observed isotopic disequilibrium in the early stage of pyrolysis. We make a more general prediction of KIE signatures associated with kerogen cracking by simulating this chemistry in a kinetic Monte Carlo model for different types of kerogens. In contrast, unconventional shale fluids or hot conventional reservoirs contain propane with an isotopic structure close to equilibrium, presumably reflecting internal and/or heterogeneous exchange during high temperature storage (ca. 100–150 °C). In relatively cold (<100 °C) conventional gas accumulations, propane can discharge from its source to a colder reservoir, rapidly enough to preserve disequilibrium signatures even if the source rock thermal maturity is high. These findings imply that long times at elevated temperatures are required to equilibrate the hydrogen isotopic structure of propane in natural gas host rocks and reservoirs. We further defined the kinetics of propane equilibration through hydrogen isotope exchange experiments under hydrous conditions; these experiments show that hydrogen in propane is exchangeable over laboratory timescales when exposed to clay minerals such as kaolinite. This implies rather rapid transfer of propane from sources to cold reservoirs in some of the conventional petroleum systems. Propane is also susceptible to microbial degradation in both oxic and anoxic environments. Biodegradation of propane in the Hadrian and Diana Hoover oil fields (Gulf of Mexico) results in strong increases in central-terminal hydrogen isotope fractionation. This reflects preferential attack on the central position, consistent with previous studies.

Introduction

Natural propane and other volatile hydrocarbons in the subsurface are of great economic value and environmental significance. Compositional and stable isotope properties of these gases have been widely used to help trace their origins and fates (e.g., Berner and Faber, 1996, Whiticar, 1999). Recent studies of the intramolecular isotope structures of these gaseous compounds bring novel constraints to these processes (Stolper et al., 2014a, Wang et al., 2015, Young et al., 2017, Eiler et al., 2018, Piasecki et al., 2018, Clog et al., 2018, Xia and Gao, 2019). These new methods are revealing fundamental geochemical processes that control the geological distributions of hydrocarbons.

Propane (C₃H₈, or CH₃—CH₂—CH₃) has two chemically nonequivalent sets of atomic sites: the central CH₂ group and the terminal CH₃ groups. The carbon and/or hydrogen isotope differences between these two positions have been analyzed by GC-pyrolysis-GC-IRMS (gas chromatography isotope ratio mass spectrometry) (Gilbert et al., 2016, Li et al., 2018), biochemical degradation with GC-IRMS (Gao et al., 2016b), high-resolution direct molecular mass spectrometry (Piasecki et al., 2016a, Xie et al., 2018) and nuclear magnetic resonance (Liu et al., 2018). It has been shown that site-specific isotopic measurements are able to differentiate abiotic propane sources from common thermogenic propane (Suda et al., 2017), track thermal maturation (Piasecki et al., 2018, Liu et al., 2019, Julien et al., 2020), and identify residues of subsurface microbial degradation (Gilbert et al., 2019). Position-specific hydrogen isotopes are especially interesting because hydrogen may undergo isotopic exchange at the conditions of some gas reservoirs, potentially driving propane to intramolecular hydrogen isotope equilibrium. The temperature dependence of equilibrium isotope fractionation between the central and terminal hydrogen positions has been theoretically predicted (Webb and Miller, 2014, Piasecki et al., 2016b) and experimentally calibrated (Xie et al., 2018). Therefore, position-specific hydrogen isotope distribution in propane can potentially work as a ‘geothermometer’ that could track the equilibration temperature at which propane has been generated and/or stored. And, because the approach to equilibrium may be time dependent, it is possible that site specific hydrogen isotope fractionation may serve as a kind of ‘geospeedometer’ for evaluating gas reservoir storage times. This kinetic property in the carbonate geothermometer has been shown to have significant value for constraining thermal histories of rock samples (e.g., Passey and Henkes, 2012, Shenton et al., 2015, Stolper and Eiler, 2015, Lawson et al., 2018, Mangenot et al., 2019, Ingalls, 2019). If such a property were demonstrated in propane, it would provide an opportunity to assess the thermal histories of fluids that migrate within sedimentary systems.

In this study, we explore what controls the position-specific hydrogen isotope distribution in propane via natural observations and laboratory experiments. We present a dataset of measurements of propane from produced natural gases in 10 different, globally distributed petroleum systems. In addition, we report isotope exchange experiments and hydrous pyrolysis experiments designed to investigate the timescales and mechanisms of hydrogen isotope exchange and the position-specific isotope effects of thermal cracking. Finally, we construct a model of the position-specific isotopic fractionations associated with kerogen cracking as a means of interpreting and extrapolating from laboratory cracking experiments. We show that the geochemistry of the source rock determines the primary position-specific hydrogen isotope signature in propane immediately after formation by thermal cracking, that exchange in relatively hot reservoirs brings the position-specific hydrogen isotope structure of propane close to equilibrium, and that biodegradation in shallow reservoirs leads to distinctive central-terminal hydrogen isotope fractionations.