Nanoscale carbon structure of high-maturity coal-bearing source rocks and its controls on methane generation and enrichment: A case study of the Daning-Jixian block, Ordos Basin

The enrichment potential of coalbed methane is primarily controlled by the nanoscale pore structure of organic matter; however, the relationships among structural arrangement, pore development, and natural gas generation remain insufficiently understood. In this study, coal and shale samples from the Carboniferous-Permian Taiyuan Formation in the Daning-Jixian block of the Ordos Basin were subjected to Rock-Eval pyrolysis, nitrogen adsorption, and high-resolution transmission electron microscopy (HRTEM) analyses to investigate the evolution mechanism of nanoscale carbon structures and their control over pore networks and hydrocarbon generation potential. The results indicate that the organic matter is predominantly Type III kerogen, with pyrolysis data showing low Hydrogen Index (HI) values and limited residual hydrocarbon generation capacity. Nitrogen adsorption experiments reveal Type Ⅳ isotherms with H3/H4 hysteresis loops, indicating the presence of complex slit-shaped micropore and mesopore networks. Fractal analysis confirms the complexity of micropores. HRTEM reveals the evolutionary process of organic matter nanoscale carbon structures: with increasing thermal maturity, aromatic lamellae transform from short-range disordered arrangements to long-range ordered, parallel-stacked configurations. This process generates regular slit-shaped nanopores, enhancing specific surface area and gas storage capacity. The study concludes that thermal evolution simultaneously drives carbon structure ordering and pore formation, and the synergy between structural ordering and pore evolution provides both the material basis and storage space for self-generation and self-storage coal-bearing natural gas accumulation.