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 Fuels20203415371547] 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.

Neglected for a long time in molecular simulations of fluid adsorption and transport in microporous carbons, adsorption-induced deformations of the matrix have recently been shown to have important effects on both sorption isotherms and diffusion coefficients. Here we investigate in detail the behavior of a recently proposed 3D-connected mature kerogen model, as a generic model of aromatic microporous carbon with atomic H/C ∼ 0.5, in both chemical and mechanical equilibrium with argon at 243 K over an extended pressure range. We show that under these conditions the material exhibits some viscoelasticity, and simulations of hundreds of nanoseconds are required to accurately determine the equilibrium volumes and sorption loadings. We also show that neglecting matrix internal deformations and swelling can lead to underestimations of the loading by up to 19% (swelling only) and 28% (swelling and internal deformations). The volume of the matrix is shown to increase up to about 8% at the largest pressure considered (210 MPa), which induces an increase of about 33% of both pore volume and specific surface area via the creation of additional pores, yet does not significantly change the normalized pore size distribution. Volume swelling is also rationalized by using a well-known linearized microporomechanical model. Finally, we show that self-diffusivity decreases with applied pressure, following an almost perfectly linear evolution with the free volume. Quantitatively, neglecting swelling and internal deformations tends to reduce the computed self-diffusivities.