Abstract: The deep coalbed methane (CBM) is a new research subject in current CBM exploration and development in China. Based on the isothermal adsorption experiments, the adsorption effect of simulated deep coal beds under varying temperature and pressure conditions was analyzed. Then, the critical depth for gas-bearing of the Carboniferous-Permian coal beds in the Jiyang depression was determined, and the adsorption capacity of deep coal beds in this depression was predicted through regression analysis. It is revealed that the gas-bearing property of the coal beds are controlled jointly by temperature, pressure and coal rank in deep environment (>2000 m). The study results indicate that the critical depth for gas-bearing of the deep coal beds in the Jiyang depression ranges from 800 m to 1200 m, below which the adsorption capacity decreases as the depth increases, due to the covariant effect between temperature and pressure. According to the predicted results of the adsorption capacity of deep coal beds by the Langmuir model, the high-rank coals at depth of 4000 m still have higher adsorption capacity (12.29 cm³/g as predicted), while the low-rank coals only have adsorption capacity of 1.83 cm³/g. Due to thermal effect at high temperatures in deep environment, coal rank increases with increasing burial depth. The adsorption capacity of deep coal beds occurs transition at depth of 3000-3500 m due to three effects of temperature, pressure and coal rank, larger than which the adsorption capacity increases with increasing burial depth.

Key words: deep coal bed, adsorption effect, transition depth, Carboniferous-Permian coal beds, gas-bearing property, Jiyang depression