Stacking types and quantitative gas-trapping evaluation of coalbed methane cap rocks in the Shanxi Coalfield

The gas-trapping mechanisms of coalbed methane (CBM) cap rocks in the Shanxi Coalfield remain unclear, and single-lithology assessment methods have inherent limitations. Based on 273 datasets from 125 coal mines, this study classifies cap rock stacking types and quantitatively evaluates their gas-trapping capacity using Markov chain analysis, analytic hierarchy process (AHP), and COMSOL numerical simulation. Cap rocks are classified into 3 major types and 8 sub-types, including composite (28%), thick (58%), and coal cap rocks (14%). The dominant types (I, V, VIII) display distinct gas pressure evolution characteristics, with Type VIII showing a unique “decrease–increase–decrease” trend. A lithology-weighted and distance-attenuated equivalent thickness coefficient model is established, yielding the following sealing capacity ranking: Type VIII (1.1908) > Type V (1.1506) > Type I (0.6664). Gas-trapping capacity is hierarchically controlled: lithology dominates within 0–5 m, while the equivalent thickness coefficient prevails at 5–10 m. The effective sealing zone is within 10 m above the coal seam. These results provide theoretical support for efficient CBM development and CO₂ geological sequestration.