Reservoir Characteristics and Peat-Forming Environment of No. 8 Coal Seam in Benxi Formation, Yichuan Area, Southeastern Ordos Basin

The No. 8 coal seam in the Benxi Formation of the Yichuan Area on the southeastern margin of the Ordos Basin is extensively developed, serving as the primary target layer for coalbed methane exploration and development. To clarify the reservoir characteristics and peat-forming environment of this coal seam, experimental analyses were conducted based on core data from five wells, focusing on coal composition, pore structure characteristics, and elemental geochemistry. The macerals of the No. 8 coal are predominantly vitrinite, accounting for 80.5%. Proximate analysis indicates low moisture (0.94%), medium-low ash (13.29%), and low volatile matter content. The average random vitrinite reflectance is 2.99%, classifying it as high-rank anthracite. Among pore types, secondary organic gas pores are the most abundant, followed by intergranular pores and cellular pores. Well-developed micro-fractures significantly enhance the permeability of the coal matrix. Pore structure analysis reveals that the No. 8 coal is rich in micropores, which contribute 64.53% of the total pore volume, providing nearly all the storage space for adsorbed gas. Mesopores are underdeveloped, while macropores and micro-fractures are relatively well-developed, offering storage space for free gas. The main types of peat-forming swamps for the No. 8 coal were flooded forest swamps and deep-water reed swamps. The warm, humid paleoclimate favored the growth and proliferation of peat-forming plants, and the closed, reducing water environment promoted gelification, providing favorable conditions for the formation of high-quality coal.