Carbon isotopic characteristics of CH4 and its significance to the gas performance of coal reservoirs in the Zhengzhuang area, Southern Qinshui Basin, North China

To better understand the law of gas generation, migration and accumulation of the No. 3 coal seam in the Zhengzhuang (ZZ) area, Southern Qinshui Basin, North China, isotopic geochemical experiments, including determination of the molecular and isotopic compositions of coalbed gas and the hydrogen and oxygen isotope decompositions of the coalbed methane (CBM) co-produced water, as well as the geological and hydrogeological surveys, were conducted. The results reveal that CBM from the No. 3 coal seam is dominated by CH4 (46.5-99.54 vol%) and the carbon isotope ratio of the produced CH4 (δ13C1) ranges from −28.7‰ to −57.2‰. Secondary thermogenic CH4 caused by a tectonic thermal event in the Yanshan Epoch is the main gas source, with a small amount of secondary biogenic gas generated from methyl-type fermentation. The carbon isotope is fractionated due to the terrain change of the No. 3 coal seam, a discrepancy in internal structures of basin and the flushing of the groundwater. Geological and hydrogeological information shows that fluid pressure traps for gas preservation will easily form in the northern and eastern deep stagnation areas, or local syncline in the central part. Based on the experimental data and CBM well data, the gas content and gas saturation have a logarithmic correlation with δ13C1. These investigations may serve the exploration of favorable zones for gas accumulation and exploitation of CBM with the geochemistry and carbon isotope composition of CH4 in the study area.