Kinetics of hydrocarbon generation as a function of the molecular structure of kerogen as revealed by pyrolysis-gas chromatography

The timing and extent of hydrocarbon generation depends on both the thermal/burial history of a source rock interval and the reaction kinetics of hydrocarbon generation from the associated kerogen. The present study is focused on understanding reaction kinetics of source rocks as a function of kerogen composition. We have compared the calculated kinetic parameters of a global set of immature kerogens to their bulk chemical composition and molecular characterization using analytical pyrolysis techniques. A large variation of calculated kinetic parameters was observed, which could be explained as a function of the molecular structure of kerogen as revealed by pyrolysis-GC. The molecular structure is related to: (i) the types and mixture of biomacromolecules selectively preserved during diagenesis; and (ii) diagnetic incorporation of sulfur. The first factor provides the initial kinetic diversity, whereas the latter causes a shift in magnitude of kinetic parameters, resulting in lower temperatures for kerogen decomposition. In contrast, a poor correlation was observed between bulk chemical properties, such as hydrogen index, and kinetic parameters. This is because bulk chemical properties do not adequately reflect the molecular structure of kerogen. Pyrolysis-GC fingerprints provide a much better indicator of kerogen composition. Using a diagnostic set of Py-GC attributes (wax index, thiophene ratio, phenol/aromatic content and others), we have defined a series of end member kerogen types, each with its own characteristic kinetic parameters. The end member kinetics can be used to simulate the kinetic behavior of mixtures, which has been validated with both numerical and physical mixing experiments. Interpretation of pyrolysis-GC data in light of these observations provides an improved means of arriving at representative hydrocarbon generation kinetics for potential source rocks that have not been subjected to direct kinetic analysis