Coal Facies and Coal-Forming Environment Evolution of Deep Coals in Shanxi Formation Member 2, Daning–Jixian Block, Southeastern Ordos Basin

The Daning–Jixian Block in the southeastern Ordos Basin hosts abundant deep CBM resources. Characterizing coal facies types and peat-forming environments provides critical evidence for analyzing deep coalbed methane genesis and optimizing prospective favorable areas. This study focuses on the No. 5 coal seam within the Shan 2 Member (S2) of the Permian Shanxi Formation in the Daning–Jixian Block and conducts a high-resolution spatiotemporal analysis of the relationships among depositional environments, peat-forming environments, and coal facies. Multiple technical approaches, including INPEFA filtering based on maximum entropy spectral analysis, wavelet transform (WT) for the analysis of high-frequency well-log cycles, quantitative identification of coal macerals, and coal quality geochemical analyses, are comprehensively adopted to establish a high-resolution sequence stratigraphic framework. Combined with two coal facies discrimination diagrams (GI-TPI and GWI-VI), the interdistributary bay paleoenvironment for coal accumulation is reconstructed in detail. The research results show that: (1) The Shan 2 Member (S2) develops a typical meandering river delta front–shallow lagoon sedimentary system constrained by paleogeomorphology, which can be divided into four fourth-order sequences (S24,S23,S22,S21), recording the full evolutionary sequence of the distributary-channel system: progradational erosion, channel abandonment and bay formation, bay shallowing and mire development, and secondary progradation.(2) The No. 5 coal seam is characterized by ultra-low sulfur and medium-low ash content, and the coal facies parameters follow a distribution pattern of high Gelification Index (GI), extremely low Groundwater Influence Index (GWI), and widely varying Tissue Preservation Index (TPI). (3) The coal facies are dominated by an assemblage of wet forest mires and herbaceous mires, indicating that peat-forming mires likely developed under weakly reducing, stagnant, freshwater-dominated conditions sheltered by paleogeomorphic highs. This study indicates that the large-scale lateral abandonment of subaqueous distributary channels on the meandering delta front and the shallowing of lagoon–interdistributary-bay depressions constitute the key dynamic mechanisms controlling the regional, continuous development of high-quality, thick coal seams characterized by high GI and low GWI. This understanding of the depositional processes and environments provides a theoretical basis for optimizing and geologically evaluating favorable reservoir zones for deep coalbed methane.