Geochemical Characteristics and Generation Process of Mixed Biogenic and Thermogenic Coalbed Methane in Luling Coalfield, China

The exploration and development of coalbed methane (CBM) is often associated with the thermogenic gas. Because the mixed CBM derived from thermogenic and secondary biogenic gases was discovered in many coal-bearing basins of the world, secondary biogenic CBM which makes a significant contribution to the gas content is becoming a hot topic. In the present study, the origin of the gas in the Luling coalfield of China was first identified through molecular and stable isotope testing. Then, based on the basin evolution history, the generation process of thermogenic CBM and mixed CBM from biogenic and thermogenic gases was analyzed using two calculations. The results show that the carbon isotopic ratios of methane and carbon dioxide in Luling coalfield range from -67.6 parts per thousand to -50.5 parts per thousand and from -12.6 parts per thousand to -8.7 parts per thousand, respectively, and the hydrogen isotopic ratios of methane range from -228 parts per thousand to -206 parts per thousand. The isotope data indicate that the CBM in the Luling coalfield consists of both biogenic and thermogenic gases. Moreover, the generation process of mixed CBM can be divided into three stages: primary biogenic gas, thermogenic gas, and secondary biogenic gas. Cap outburst dissipation was determined to be the main migration mechanisms of hybrid CBM in Luling coalfiled, whereas diffusion was along with the whole process of CBM generation. Various factors, including maturity, temperature, and the time required for allochthonous methanogenic bacteria to move through the coal bed, were discussed in affecting the generation of secondary biogenic gas. These factors control the quantity, rate, and start and end of secondary biogenic CBM generation, respectively.