Geological Controls and Symbiotic Assemblage Models of Coal Measure Gas in the Shanxi Formation, Eastern Pingdingshan Mining Area, China

Abstract Coal measure gas (CMG) is an important unconventional energy resource, and a clear understanding of its accumulation mechanisms is essential for effective exploration and development. This study investigates the coal measure strata of the Shanxi Formation in the eastern Pingdingshan mining area. An integrated approach combining geostatistical analysis, geochemical characterization, X-ray diffraction, and high-pressure mercury intrusion measurements, together with a Markov chain model, was employed to elucidate the accumulation framework and spatial organization of CMG systems. The results show that (1) the study area is characterized by a coal–mudstone–sandstone tripartite assemblage. The thickness of the main coal seam (No. 2-1) ranges from 0.65 to 12.42 m (average 5.50 m), thickening toward the central area and thinning toward the margins, and serves as the primary source rock. Mudstonethickness ranges from 5.66 to 89.91 m (average 39.67 m) in thickness and acts as an effective seal, whereas sandstone (ranging from 2.15 to 68.28 m, average 34.72 m) is preferentially developed in the central area and constitutes the main reservoir. (2) Source rocks exhibit high organic abundance and thermal maturity, whereas reservoirs typically exhibit low porosity and extremely low permeability. The coal seam exhibits a maximum vitrinite reflectance (Ro,max) ranging from 1.106% to 1.651% (average 1.461%) and high organic matter abundance. Mudstone exhibits a total organic carbon (TOC) content ranging from 2.61% to 28.00% (average 9.34%) and is dominated by type III kerogen, indicating a mature to highly mature thermal evolution stage. Coal, mudstone, and sandstone reservoirs all display low porosity and extremely low permeability characteristics. (3) Based on the Markov chain analysis, five dominant lithofacies sequences were quantitatively identified, and three CMG accumulation patterns were recognized: single-source dual-reservoir, dual-source dual-reservoir, and dual-source multi-reservoir types. These results suggest that depositional environment exerts the primary control on the spatial differentiation of CMG accumulation patterns. This study provides a geological basis for integrated source–reservoir–seal assessment and sweet-spot prediction in CMG exploration.