Abstract
In this study, gas contents, geochemical features, and origins of coalbed methane (CBM) and their influence factors were investigated on nos. 7 and 8 CBM reservoirs from the Suzhou mining area of the Huaibei coalfield. Results have shown that the selected CBM reservoirs are characterized by various thickness (0.50-9.19 m) and buried depth (619-1226 m), but have relatively better lithology of surrounding rocks. Coal samples have similar maturity (R(o,max) = 0.71-1.05%), but show differences in chemical composition and macerals. Gas content of nos. 7 and 8 CBM reservoirs ranges from 6.13-12.25 m(3)/t, but the value of former is lower than that of later one overall. In addition, CH(4) is a predominantly component with a value of 88.23-99.00% (avg. 96.69%), and the heavy hydrocarbon gas (C(2+)) is 0.00-1.93% (avg. 0.41%). The δ(13)C(CH4) value ranges from -64.54 to -46.36‰ (avg. -53.92‰), and the δ(13)D(CH4) value is -224.36 to -211.75‰ (avg. -219.09‰). Based on the analysis of components and isotopic values, the CBM samples are thermogenic (20.92-71.30%; avg 50.09%) and secondary biogenic gases (28.70-74.49%; avg. 49.91%). Gas content shows changeable characteristics at a buried depth of 300-1300 m, which is affected by buried depth, reservoir temperature and pressure, M(ad) and vitrinite. However, the CH(4) concentration shows no correlation with buried depth. Moreover, the buried depth is significantly positively correlated to δ(13)C(CH(4)) and δ(13)D(CH(4)). Based on the relationship between gas content and isotope values, it suggests that δ(13)C(CH(4)) or δ(13)D(CH(4)) may have a relationship with the main controlling factors of gas content.