Reactive Molecular Dynamics Simulation of Kerogen Thermal Maturation and Cross-Linking Pathways

Abstract

Molecular dynamics simulations were performed with a ReaxFF reactive force field to investigate bond breaking and bond formation mechanisms during the thermal maturation of three kerogens and potential cross-linking pathways toward the formation of three-dimensional (3D) quasi-infinite molecular networks (cross-linked kerogen macromolecules). Starting with small ensembles of high molecular mass models for immature type I Green River Shale kerogen (kerogen 1-I), top of the oil window type II kerogen (kerogen 2-L), and low maturity type III kerogen (kerogen 3-L), low molecular mass species including H2O, C2H4, and C3H6 were produced as the maturities of the remaining kerogens increased. Highly reactive fragments, which are not detected in pyrolysis experiments, were also produced. Further, the cross-linking mechanisms in the newly developed polymeric kerogen networks appear to be highly complex, and covalent —C–S—, —C–O—, and —C–C— bonds were the primary cross-links that structurally bond kerogen monomers together. The trends observed in the simulated thermochemical transformation of kerogen and the kerogen cross-linking pathways are consistent with theoretical and experimental studies reported in the scientific literature. Reactive force field molecular dynamics simulation provides a potentially valuable approach to the development of realistic three-dimensional models for kerogen molecular networks. However, the conversion of kerogen molecules into a 3D cross-linked molecular network was low (the density of the intermolecular cross-links was low).