Classification of coal-forming environments and the formation and evolution of coal

There is a lack of systematic understanding of coal-forming environment classification and its influences on coal petrological characteristics, a coal-forming mire classification scheme applicable to the petroleum industry is proposed based on modern ecological peatland frameworks. The formation, evolutionary processes, and diagnostic criteria of coal-forming environments are systematically clarified. The results show that: (1) modern peatlands can be classified according to hydrological conditions, vegetation types, and geomorphic settings, and coal-forming mires can be divided into low moor, transitional, and high moor peat mires based on geomorphology; (2) the development of coal-forming environments includes three modes: subaqueous peat infilling, autochthonous peat accumulation in wetlands, and mire development in arid regions; (3) peat accumulation is jointly controlled by plant production and decomposition, hydrological disturbances, and sediment input, and the peat-to-coal thickness ratio varies with coalification; (4) diagnostic criteria for low moor, transitional, and high moor peat mires are established based on ash yield, gamma-ray log responses, and vitrinite-to-inertinite ratios; and (5) transgression–regression processes exert a key control on peat mire evolution, directly influencing peat thickness and continuity, while the evolution of low moor, transitional, and high moor mires governs coal maceral assemblages and thereby affects hydrocarbon generation potential and reservoir properties of coals. The coal-forming environment classification and identification system developed in this study effectively reveals the vertical heterogeneity of coals in the Ordos Basin, providing theoretical and practical guidance for efficient exploration and development of coal-rock gas.