Abstract:
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 CH
4 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 CO
2 and N
2, of which the CO
2 volume fraction ranges from 1.74% to 7.61% and N
2 volume fraction ranges from 0.04% to 8.18%. CBM
δ13C(CH
4) value is between –56.8‰ and –47.7‰, the
δ13C(CO
2) value lies between –6.6‰ and 13.9‰, the
δD(CH
4) value is between –252.6‰ and –241.6‰.The water produced from coal seam is weakly alkaline, belonging to NaHCO
3 type water, which is similar to the ion composition, salinity, value of
δD(H
2O) and
δ18O(H
2O) of surface water, and the recharge from surface water is conducive to the proliferation of CH
4-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.