Geochemical characteristics and their origin of CBM in Xingxian area, Ordos basin

The distribution of carbon and hydrogen isotope of methane is of signification in revealing the origin, the distribution of CBM reservoir, and the source evaluation of CBM. To study the origins of coalbed methane and carbon dioxide, the data of the components、carbon and hydrogen isotopes of CBM from the desorption gas of coal core from Xingxian area were analyzed. With the CBM origins chart board, and the geological environment, the geological effect of the formation and the evolution of CBM were analyzed. the ratios of CBM with different origins were estimated. The results show that the δ¹³C, values of CH₄ in coal seams 8 are between -55.1‰ and -44.2‰, with an average of -49.2‰; the δ¹³C₁ values of CH₄ in coal seams 13 are between -65.7‰ and -55.7‰, with an average of -59.8‰. The carbon isotopes of CH₄ of the same coal seam were characterized by increase with the buried depth and decrease with the hydrodynamic conditions. The methane carbon isotope values were obviously lighter and the heavy hydrocarbon component were less, which show the gas was influenced by some factors and secondary transformation. Tectonic-thermal history and geological conditions of coal seams show that the forming conditions of secondary biogenic gas were found, and the biogas was generated by the reduction of CO₂. The δ¹³C values of CO₂ in coal seams 8 are between -17.3‰ and -4.8‰, the δ¹³C values of CO₂ in coal seams 13 are between -26.3‰ and -6.9‰, the CO₂ could be generated from thermal degration or thermal cracking or both at the early stage of coalification or after the latest coal seam uplift. This study provides a theoretical basis for the direction of coalbed methane exploration and development in this area.