Coarse–Grained Simulation of Kerogen: From Realistic Molecular Models to Complex Cross–Linked Structures

Kerogen plays a crucial role in the exploitation of shale fields as the main component of the organic content of sedimentary rocks and is directly related to hydrocarbon storage and recovery. Modeling its complex, heterogeneous, and highly cross-linked structure remains a significant challenge. This work aims to develop a coarse-grained (CG) model of overmature type II-D kerogen based on the MARTINI-3 force field, combined with a systematic cross-linking procedure to construct macromolecular networks with realistic molecular weights that approach the experimentally observed insolubility limit in common organic solvents. In this respect, we adopted the MARTINI approach to describe representative models in combination with a cross-linking process, which increases the molecular weight of the molecular entities, addresses related limitations of molecular models, and incorporates their statistical nature. The derived MARTINI-based CG model was validated against all-atom molecular simulations and experimental data. The CG model reproduces key properties, including bulk density, structural properties, mechanical characteristics, and methane adsorption, while achieving an approximately 15-fold increase in computational efficiency compared to all-atom simulations. Cross-linking significantly enhanced the model’s ability to represent kerogen with a molecular weight near the insolubility limit in common organic solvents, as well as its characteristic rigidity, with optimal results achieving a Young’s modulus of approximately 3 GPa, within the experimentally reported range. The results further indicate that the CG representation captures the main trends in adsorption behavior, although deviations arise due to differences in accessible volume. This study highlights the potential of CG models for simulating large-scale kerogen systems and provides a framework for constructing CG models that enable improved understanding of shale gas storage and transport.