Coalbed Methane Enrichment and Production Potential in Multiple Coal Seams in Yanjiao Composite Syncline, Western Guizhou Province, South China

Coalbed methane (CBM) hosted by multiple (>20) thin (<2 m) seams in South China represents an important unconventional gas supplement. In the Yanjiao composite syncline, high-frequency sea-level fluctuations produced widely distributed thin seams (20–60 layers), with tidal-flat coal groups I (No. 2–No. 9) and III (No. 20–No. 35) as primary targets. Variable magma intrusion drives the present coal-rank partitioning (1.8–4.3% Ro) and pronounced reservoir heterogeneity. A basalt floor (>200 m) and the Lower Triassic Feixianguan caprock (~100 m) confine the Longtan strata into an independent hydrodynamic unit. Groundwater migrates from syncline wings to the axial domain and seals CBM in stagnant zones, resulting in higher gas contents toward the axis. Deep CBM is constrained by high in situ stress and low permeability (typically <0.1 mD below 600 m) and a relatively uniform and low-abnormal pressure system. The syncline is divided into four parts: Part I is the most favorable, where staged fracturing of closely spaced (<60 m) coal group III achieved a maximum production rate of 2400 m3/d and a stabilized rate of 2100 m3/d, whereas Part IV (depth > 1000 m) records a peak daily gas rate of 512–654 m3/d and shows no stabilized-production stage.