Geological-geochemical models and isotope fractionation laws and control factors of thermogenic coalbed gas in Panxian, China

Coalbed gas (CBG) is a typical self-generated and self-stored gas, and is one of the most ideal materials for studying the geological-geochemical models of natural gas and its source rock. However, studies show CBG is usually a mixed gas consisting of thermogenic gas and secondary biogenic gas, which make its original geochemical compositions change. We collected 9 CBG samples and the corresponding coal samples from a coal core drilled in a CBG well in the Panxian Basin. The gas samples were measured using a MAT-271 trace-gas mass spectrometer for the molecular components and a Delta Plus XP stable isotope mass spectrometer for the carbon and hydrogen isotope values. The coal Rmax values were measured by a LEICA DM2500P & MSP-ⅡMicroscope Photometer. The main geochemical characteristics of the CBG samples are as follows: CH4 concentrations from 97.28% to 95.16%, C1/C1-n from 0.995 to 0.999; δ13C1 from -33.2‰ to -41.9‰, δDCH4 from -170.6‰ to -123.5‰, δ13C2 from -29.7‰ to -23.2‰, and δ13CCO2 from -24.6‰ to -...