Geochemical characteristics and genesis of coalbed methane in Baode area

In order to find out the geochemical characteristics and origins of coalbed methane(CBM)in Baode area, coal, CBM and water samples were collected, gas component analysis, quality detection of CBM well-produced water and stable isotope analysis were carried out. The results show that CH₄ is the main hydrocarbon gas in CBM composition and its volume fraction ranges from 88.60% to 97.59%; a small amount of ethane is contained, and the volume fraction is only 0.01%-0.14%. The drying coefficient is greater than 0.99, the CBM belongs to the extremely dry CBM. The non-hydrocarbon components mainly contain CO₂ and N₂, of which the CO₂ volume fraction ranges from 1.74% to 7.61% and N₂ volume fraction ranges from 0.04% to 8.18%. CBM δ¹³C(CH₄) value is between –56.8‰ and –47.7‰, the δ¹³C(CO₂) value lies between –6.6‰ and 13.9‰, the δD(CH₄) value is between –252.6‰ and –241.6‰.The water produced from coal seam is weakly alkaline, belonging to NaHCO₃ type water, which is similar to the ion composition, salinity, value of δD(H₂O) and δ¹⁸O(H₂O) of surface water, and the recharge from surface water is conducive to the proliferation of CH₄-producing bacteria and the formation of secondary biogas. It is generally believed that CBM in the study area is a mixture of thermal and biogenic gas, formed mainly through carbon dioxide reduction, and “lightened” due to coal seam desorption, diffusion and migration, water dissolution and secondary biological action. The research results provide guidance for subsequent CBM exploration and development.