Dual functionality of late-stage carbonized kerogen in ultra-deep petroleum systems: Geochemical characterization, molecular modeling, and resource implications

The dual role of late-stage carbonized kerogen as both a gas source and a seal is critical for assessing ultra-deep subsurface systems. This study investigates a Maidiping Formation kerogen (R_o-Bitumen ≈ 2.54%), a natural analogue for ultra-deep organic matter in Sichuan Basin, to constrain its structural characteristics and bifunctional behavior. Multiscale characterization reveals a nanoporous carbon framework composed of small fused aromatic domains (typically ~ 3×3 rings), poorly aligned stacking motifs, an expanded interlayer spacing, and dispersed heteroatoms. These empirical constraints guided the construction of a representative 3-D molecular model (C₃₀₆₆H₁₉₆₀N₂₈O₁₆₈S₁₄) featuring short aliphatic chains, oxygen/nitrogen/sulfur functional groups, and microporous aromatic clusters. Reactive molecular dynamics simulations quantified the kerogen’s dual functionality: the peak pyrolysis rate indicates a late-stage methane potential of 25.94 mL/g (STP), while the model exhibits strong sealing capacity reflected in high methane adsorption (33.8 mg/g) and a very low self-diffusion coefficient (0.2243×10^-7 cm²/s). These findings demonstrate that overmature kerogen acts as an active, bifunctional geomaterial, which provides a new molecular-level framework for assessing integrated source-seal systems in ultra-deep exploration.