Material Composition, Pore-Fracture Structure, and Methane Adsorption–Desorption Characteristics of Coal Reservoirs in the Yingcheng Formation, Longjiapu Mining Area, Southeastern Songliao Basin

Low-rank coalbed methane (CBM) resources are abundant in China, but their commercial development is limited by insufficient understanding of the coupled controls of coal material composition, pore-fracture structure, and desorption behavior in deeper low- to medium-rank bituminous coals. This study focused on the No. II coal seam in the Longjiapu Mining Area, southeastern uplift zone of the Songliao Basin. Fifteen samples were collected from three working faces. All 15 samples were subjected to material composition analysis (proximate analysis, vitrinite reflectance, and maceral composition). Eleven representative samples were selected for reservoir physical property characterization (thin-section microfracture observation, FE-SEM, mercury intrusion porosimetry, and low-temperature N2 adsorption), and nine of them were further used for high-pressure CH4 adsorption experiments. The coals are low- to medium-rank bituminous coals (Ro,max = 0.63–0.80%). The 620 working face has the most favorable pore-fracture structure, with the highest total pore volume and specific surface area, the lowest fractal dimensions, and the strongest microfracture development; the 615 working face is the poorest, and b4 is intermediate. The average Langmuir volume (VL) of the three working faces ranges from 12.40 to 16.94 m3/t, with an overall average of 15.47 m3/t; b4 has the highest value (16.94 m3/t), followed by 620 (15.91 m3/t), and 615 has the lowest value (12.40 m3/t). Partial correlation and ash-free normalization indicate that VL is positively correlated with volatile matter, and the negative correlation between ash and VL mainly reflects mineral dilution, suggesting that VL is controlled by organic matter abundance and micropore development. Among the three characteristic pressures, the 620 working face has the highest sensitive pressure (1.49 MPa); b4 has the highest starting pressure and transition pressure (8.69 and 4.12 MPa); and the 615 working face has the lowest values (6.37, 3.02, and 1.05 MPa). VL positively controls all three characteristic pressures, while coal rank negatively controls the sensitive pressure. Integrating pore-fracture development characteristics with desorption characteristic pressure parameters, the 620 working face has the best desorption performance, followed by b4, while 615 is the least favorable. This study reveals the controlling mechanism of pore-fracture structure on low-rank coal desorption behavior and provides a quantitative basis for favorable target selection of low-rank CBM in northeastern China.