Gas content, geochemical characteristics and implications of coalbed methane from the Deep Area of Qi'Nan Coalmine in Huaibei Coalfield

The objective of this work is to investigate the implications of geological influence factors on gas content and geochemical characteristics of deep-buried (> 800 m) coalbed methane (CBM) reservoirs. Results show that bituminous coal accounts for the majority, which exhibits similar maturity but differ in maceral and chemical constituents. CBM reservoirs show low porosity, low permeability and moderate temperature, with thickness of 0.85-4.15 m. In addition, the total gas content is 4.58-12.33 m³/t (average of 8.83 m³/t). CH₄ is the main component with concentration of 92.83-99.22% (average of 96.68%), and the δ¹³C_CH4 and δ¹³D_CH4 is - 53.78‰--44.62‰ (average of - 48.82‰) and - 223.93‰---4.49‰ (average of - 215.37‰), respectively. All CBM samples are the mixtures of thermogenic gases and secondary biogenic gases with CO₂ reduction. In addition, gas content characteristic at the critical point of burial depth is the result by positive and negative geological effects. CH₄ concentration shows a wide range with the increases of buried depth, while the numerical values of which for the selected samples display complex variation characteristics. Furthermore, the values of δ¹³C_CH4 and δ¹³D_CH4 become heavier with the increases of buried depth. Besides, the above two geochemical parameters are related to R_{o, max} and reservoir temperature.