Main Cntrolling Factors of Gas Enrichment in Thin Coal Seams and Evaluation of Favorable Zones in Yichuan Area of Ordos Basin

Coal seams are laterally stable in the Yichuan area, but generally thinner than those in the main body of the Ordos basin. However, the gas enrichment mechanisms and the geological characteristics of these thin seams are still to be fully elucidated, thereby constraining large-scale development. To bridge this gap, this study integrates core, well log, and laboratory data from the Benxi Formation No.8 coal seam and the Shanxi Formation No.5 coal seam to systematically evaluate the geological characteristics and primary controls on gas accumulation in these atypical thin seams. The results reveal follows. (1) The No.8 and No.5 coal seams exhibit elevated higher thermal maturity than those in the main basin, representing over-mature, high-rank coals with high vitrinite content and robust gas generation potential. The pore system is dominated by micropores, and cleats are highly developed. (2) The No.8 seam deposited in a tidal flat-lagoon environment, displays good continuity and stable thickness. In contrast, the No.5 coal seam formed in a delta front-coastal transitional environment, where its overall thickness is reduced due to the influence of sedimentary cycles and sandbody stacking. (3) The gas content of these coals is higher than the main basin. The average gas contents of the No.8 and No. 5 coal seams are 28.8 m³/t and 28.0 m³/t, respectively. The gas exists primarily in an adsorbed state, with free gas accounting for approximately 22%. (4) Based on geological and engineering parameters, evaluation criteria for Class Ⅰ, Ⅱ, and Ⅲ favorable areas were established. The No.8 coal seam is dominated by Class Ⅱ favorable area, whereas the No.5 coal seam is predominantly characterized by Class Ⅲ favorable area. Pilot production practices demonstrate that under conditions of high coal rank and high gas content, thin coal seams can also achieve high and stable yields through the application of horizontal drilling and stimulated reservoir volume fracturing technologies. The average daily gas production of the pilot horizontal wells exceeds 2×10⁴ m³. These findings demonstrate that coal seam thickness is not the determining constraint for coal seam gas development. The thin coal seams in the Yichuan area possess significant potential for large-scale development, providing an important reference for coal seam gas exploration in the Ordos basin and other analogous regions.