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The coal system of the Taiyuan Formation in the Dongpu Sag is a good source rock. In this paper, the representative hydrogen-rich coal of the Taiyuan Formation is selected for pyrolysis experiments. We used high-resolution transmission electron microscopy and Fourier transform infrared spectroscopy to characterize the structural evolution of coal during thermal simulation, combined with the characteristics of gas and liquid products from low-maturity coal pyrolysis to comprehensively reveal the process of coal of the Taiyuan Formation to hydrocarbon. The results show that the evolution of a hydrogen-rich coal structure is mainly divided into five stages: (1) Before 350 °C: The content of aromatic hydrogen and the distance between aromatic layers are almost unchanged. The aromatic structure is mainly composed of naphthalene and 2 × 2 aromatic rings, indicating that polycondensation does not clearly occur. Liquid hydrocarbons reach their maximum amount due to liptinite cracking. (2) 350-400 °C: The aromatic layer distance rapidly decreases, indicating that considerable polycondensation occurs. The fatty acid hydrogen in vitrinite is gradually reduced, which is caused by cracking of the side chain of the vitrinite fat to generate gaseous hydrocarbons. The rapid decline in liquid hydrocarbons and total hydrocarbons and the rapid increase in the aromatic hydrogen content of vitrinite may be caused by the cyclization and aromatization of the long-chain fatty acid structure, which converts the liquid hydrocarbons back to a solid. (3) 400-450 °C: The aromatic layer distance is almost unchanged. The fatty acid hydrogen in vitrinite slowly decreases, and the aromatic hydrogen in vitrinite gradually increases. The large amount of gaseous hydrocarbon generated at this stage is partly due to the continued cleavage of the fatty acid side chains in vitrinite and partly from the cleavage of the fatty acid side chains from the solid formed by aromatization. (4) 450-550 °C: The aromatic layer distance rapidly decreases to the minimum, and aromatic rings with sizes above 4 × 4 increase rapidly. Aromatic hydrogen rapidly decreases to the minimum, and the fatty acid hydrogen of vitrinite quickly decreases to 0. Considerable polycondensation and demethylation reactions occur in the vitrinite to produce a large amount of gaseous hydrocarbon. (5) 550-600 °C: The aromatic layer distance and contents of fatty acid hydrogen and aromatic hydrogen in vitrinite change slowly. Additionally, the yields of gas and liquid do not change significantly, indicating that the polycondensation intensity decreases and that the vitrinite and liptinite are cracked without producing hydrocarbons.

Structural Evolution of Hydrocarbon-Rich Coal of the Late Carboniferous Taiyuan Formation in Dongpu Sag