Organic-rich shales present significant potential for underground hydrogen storage, yet our understanding of the interactions of H₂ with CH₄ and CO₂ in kerogen-hosted nanopores remain insufficient. This study constructs and validates macromolecular models of high-maturity and overmature kerogens via combining solid-state carbon-13 nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Grand canonical Monte Carlo and molecular dynamics simulations are performed, which reveal that kerogen maturity controls the competitive adsorption and diffusion of methane/hydrogen and carbon dioxide/hydrogen mixtures by regulating nanopore structure and surface chemical heterogeneity. Compared with high-maturity kerogen, overmature kerogen shows stronger confinement and a more pronounced near-surface enrichment of CH₄ and especially CO₂, which reduces the effective storage space available for H₂. Mechanistically, CH₄/H₂ competition is driven by van der Waals interactions, whereas CO₂/H₂ competition is dominated by stronger electrostatic and inductive interactions, establishing a thermodynamic affinity order. The radial distribution functions and interaction energies are measured, which confirm that CH₄ and CO₂ monopolize high-energy surface sites, relegating H₂ to a weakly adsorbed, bulk-like state. Although H₂ exhibits the weakest adsorption affinity, its high mobility suggests a stronger migration tendency and potential leakage risk, which should be considered when evaluating long-term containment security during underground hydrogen storage. Overall, this study reveals that maturity-controlled coupling exists between kerogen structure, competitive adsorption and gas transport, providing molecular-scale insights into hydrogen storage security, injectivity, leakage risk, and recovery in organic-rich shale reservoirs.