Influence of organic matter carbonization on pore structure and CO₂/CH₄ adsorption in marine kerogen: Insights from experimental observations and molecular simulation

The carbonization of organic matter (OM) in shale is a key factor contributing to low resistivity, poor reservoir quality, and a significant decrease in gas content impacting methane production and CO₂ storage capacity of organic-rich shales. In this study, a combination of experimental methods and molecular dynamics simulations was employed to investigate the effects of organic matter carbonization in shale reservoirs on pore structure and the CO₂/CH₄ adsorption mechanisms. Kerogen samples from the Lower Silurian Longmaxi Formation in the southern Sichuan Basin were analyzed using laser Raman spectroscopy, high-resolution transmission electron microscopy, gas adsorption, X-ray photoelectron spectroscopy, and nuclear magnetic resonance, among others. A model of kerogen with varying degrees of carbonization was established to explore the effect of OM carbonization on reservoir characteristics and its controlling mechanisms by comparing the reservoir properties, chemical compositions, and adsorption behaviors of shale kerogen at different carbonization degrees. The results indicate a strong positive correlation between the maturity of the Longmaxi Formation (high-mature to over mature shale) and the degree of carbonization. Carbonization of OM results in a reduction of pore volume and surface area. As carbonization increased from 6.7 % to 25.8 %, the micropore surface area and mesopore volumes in kerogen decreased by 50.1 % and 43.6 %, respectively. The carbonization process involves the polymerization and rearrangement of aromatic rings, primarily affecting the carbon skeleton, spatial arrangement, aromatic cluster distribution, and the formation of graphite-like crystals. As the degree of carbonization increases, aliphatic side chains gradually detach, and the number of bridging aromatic carbons in the aromatic structure rises significantly, promoting the interconnection of aromatic cluster units. The rearrangement of aromatic clusters, condensation, and formation of graphite-like crystals reduce the irregularity and number of effective adsorption sites in the aromatic layer, leading to pore closure and a reduction in OM pore volume, which weakens gas adsorption capacity. At higher carbonization levels, the reduction of high-energy adsorption sites on the kerogen surface makes it more difficult for CO₂ to displace adsorbed CH₄, thereby lowering the CO₂/CH₄ selectivity coefficient. Additionally, the smaller pore space further limits CO₂ effective occupancy, reducing the adsorption selectivity coefficient. These findings contribute to a deeper understanding of how OM carbonization affects hydrocarbon storage potential.