Enrichment model and optimization of favorable areas for coalbed methane in the upper permian of southern Sichuan Basin—a case study of Gaoxian-Gongxian area

The Upper Permian coalbed methane (CBM) resources in the Gaoxian–Gongxian area of southern Sichuan Basin are enormous. However, complex geological conditions—including intense tectonic deformation, numerous but thin coal seams, low permeability, and high in-situ stress—have obscured the patterns of CBM enrichment and created difficulties for exploration and development. To reveal the main controlling factors of CBM enrichment in this area and to optimize favorable zones, this study systematically analyzed CBM accumulation conditions based on core desorption data and geological logging from key exploration wells such as Well “Chuangaocan 1” and Well “Chuanggaodi 1”. Six main controlling factors for CBM enrichment were identified: coal seam burial depth, cumulative coal seam thickness, coal body structure, gas content, lithology of roof and floor strata, and coal seam concentration. On this basis, a comprehensive evaluation index system was established, covering resource potential, preservation conditions, and reservoir fracability. The weighted sum method was used to calculate the comprehensive evaluation value, and the Analytic Hierarchy Process (AHP) was applied for consistency checking. The results show that the target coal seams C 6 , C 7 , and C 8 have a favorable burial depth of 600–800 m, a cumulative thickness of greater than 4 m, a coal body structure dominated by primary-cataclastic coal, a gas content generally exceeding 10 m³/t, roof and floor strata consisting mainly of tight sealing layers such as mudstone, and an interlayer spacing of typically 20–40 m. Based on the comprehensive evaluation value S, the favorable areas are classified into three types: Type I most favorable areas (S > 0.8) are mainly distributed in the Luochang Syncline and Jianwu Syncline; Type II favorable areas (0.7 < S ≤ 0.8) are distributed in the Baijiao–Furong, Tengda–Yuhe–Jiusicheng blocks; Type III areas (0.6 < S ≤ 0.7) are located in the shallow parts of structural margins. This study proposes a CBM enrichment model suitable for the complex structural area of southern Sichuan, termed the “planar gentle syncline hydraulic sealing – vertical multi-seam superimposed enrichment” model. This model emphasizes that the syncline core features high formation pressure and high gas content, and that hydraulically stagnant to weak runoff areas provide effective sealing. Vertically, multiple thin coal seams such as C 6 , C 7 , and C 6 are stacked in a “mud–coal–mud” configuration, ensuring gas supply and reservoir sealing. The quantitative evaluation method based on the six main controlling factors established in this study can provide scientific basis and technical reference for CBM exploration decisions undegeological conditions.