Replica Exchange Molecular Dynamics Simulation of Organic Matter Evolution: From Lignin to Overmature Type III Kerogen

Much more than any other form of carbon, kerogen undergoes considerable chemical and structural evolution during natural maturation. Some properties, such as gas adsorption and transport capacity, which are very important for key energy and environmental issues in the context of shale hydrocarbon recovery or CO2 sequestration, may therefore depend significantly on maturity. Here, we report on an extension of a recent work by Atmani et al. [Energy Fuels2020, 34, 1537-1547] in which we use the replica exchange molecular dynamics method to investigate the natural evolution of a softwood lignin model up to a largely overmature state with H/C = 0.13. We discuss in details the production of a fluid during the process and the evolution of the kerogen texture, structure, and properties via a detailed analysis of nine kerogen models of increasing maturity. We show that the kerogen is composed of small aromatics, branched by aliphatic chains, and is mostly nonporous in its immature state. It becomes increasingly aromatic during evolution, up to creating a percolating ring network at the overmature stage, while progressively increasing in porosity. In the final state, the overmature kerogen is highly porous, and the results suggest the presence of mesopores, even though the latter could not be captured due to the limited size of the simulation cell. The series of type III kerogen models produced in this work, with maturity ranging from immature to overmature, can be used in future work to investigate gas adsorption and transport properties.