Clumped 13C–13C isotopes of ethane from laboratory pyrolysis of kerogen: Implications for intramolecular 13C distributions

Abstract

The clumped isotope geochemistry of ¹³C–¹³C bonding offers a valuable tool for distinguishing the formation processes of ethane (C₂H₆) and its thermogenic and abiotic origins. Thermogenic ethane is characterized by isotope distributions that are nearly in thermodynamic equilibrium, whereas abiotic ethane is governed by kinetic isotope effects during C–C bond synthesis. ¹³C–¹³C ordering in thermogenic ethane varies with the source organic material, but limited studies on ethane clumped isotopes from natural gases restrict our understanding of these signatures. To address this, we performed pyrolysis experiments at 310–470 °C on various immature kerogens from the Eocene Green River Formation (Type I), Upper Devonian–Lower Mississippian Woodford Shale (Type II), and Pennsylvanian Springfield Coal Member (Type III). The ethane-clumped isotope compositions aligned with those of natural gas samples and varied based on the starting kerogen’s isotope composition. We propose a thermogenic model that describes isotopic systematics, including ¹³C–¹³C variations in the pyrolysis product ethane, driven by (i) combinatorial isotope effect during C–C bond breaking and (ii) intramolecular isotopic heterogeneity in the starting kerogen. Isotopic and clumped isotope variations suggest a zigzag isotopic pattern in kerogen alkyl chains, similar to that seen in biological fatty acids. We could extend the model to position-specific (PS) isotope signatures in propane, showing that intramolecular isotopic heterogeneity in kerogens also affect PS isotopes, alongside structural heterogeneity, such as isoprenoid versus alkyl. Overall, our findings demonstrate that ¹³C–¹³C clumping is a ubiquitous signature for thermogenic ethane, with variations reflecting isotopic information of the original organic matter.