(338c) Molecular and Detailed Isotopic Structures of Petroleum: Kinetic Monte Carlo Analysis of Alkane Cracking

The rapidly growing field of “clumped isotopes” chemistry provides analytical detail that goes beyond identifying the abundance of a single rare isotope within a molecule. Rare isotopes may prefer certain non-equivalent positions in a molecule and multiple rare isotopes within a single molecule may have a tendency to clump together. Accurate and precise measurement of these features has enabled applications such as new geochemical thermometers, providing temperatures of past environments, and new abilities to trace the origin of species found in the atmosphere. Developing additional applications depends on our being able to link the structure of target species with the structure of their source molecules and with all the current and past processes influencing the detailed isotopic structure of the molecules.

In the specific example of interest here, developments in clumped isotope geochemistry have given rise to a suite of measurements that provide new signatures related to the thermal history of low molecular weight hydrocarbon gases found in sedimentary basins. However, to date no study has linked these clumped isotope features, along with traditional compositional and bulk isotopic measurements, to any mechanistic understanding of how these signatures develop and evolve. In this study, we developed a kinetic Monte Carlo method to predict consistent and simultaneous molecular distributions, bulk isotopic content, and detailed (multiply substituted and site-specific) isotopic structures of hydrocarbons from a hydrocarbon cracking model. The detailed isotopic structure of the source hydrocarbons (initially modeled as long alkanes), the intermediates, and the product molecules is followed as a function of the level of conversion due to cracking reactions. The bulk 13C content of gaseous alkane products generated via the model is shown to follow the linear natural gas “Chung” plot at low conversion, but deviates at higher degrees of conversion. As examples of the information generated, the populations of center vs. terminal 13C-substituted propane are reported as a function of the starting alkane chain length and level of conversion and the population of doubly-13C-substituted ethane is described as a function of the level of conversion and for different 13C substitution patterns in the source hydrocarbons. The results are compared to experimental data where possible and highlight the possibility of constraining hydrocarbon source isotopic structure and the nature of the generation processes leading to the formation of natural hydrocarbon deposits.