Stable carbon isotopes of coal-derived gases sourced from the Mesozoic coal measures in China

Coal-derived, large scale gas fields derived from the Mesozoic coal measures in China are mainly distributed in the Middle-Lower Jurassic coal measures in the Tarim, Junggar and Turpan-Hami basins in northwest China, and the Upper Triassic Xujiahe Formation coal measure in Sichuan Basin, central China. In 2011, the annual production was 21.6 × 109 m3 and the proved geological reserves were 2485 × 109 m3, accounting for 21% and 30% of the total in China, respectively. Based on analyses of gas composition and stable carbon isotopes ratios of 203 samples and stable carbon isotopes of 102 CO2 samples, the following conclusions were made. (a) Based on diagnostic plots using the stable carbon isotopic and molecular composition of gas samples, alkane gas from the Mesozoic coal measures in China is shown to be coal-derived. (b) According to the δ13C2 vs. C2H6 plot of a great number of oil-derived and coal-derived gases in China, it is concluded that gases with δ13C2 > −28.5‰ are coal-derived and those with δ13C2 < −28.5‰ are oil-derived in most cases. (c) Among the natural gases from the Mesozoic coal measures in China, primary coal-derived gases with normal carbon isotopic distribution pattern among the C1-C4 alkanes (i.e. δ13C1 < δ13C2 < δ13C3 < δ13C4) are dominant. (d) Carbon isotopic pattern reversal mainly results from the mixing of coal-derived gases having different maturities but the same source and secondly from microbial oxidation of propane (e.g. Mu 3 and Mu 4 wells in the Gumudi gas field, Junggar Basin). (e) CO2 in the coal-derived gases from the Mesozoic coal measures in China has both biogenic and abiogenic origins. The biogenic origin is dominant and the abiogenic CO2 is mainly found in the Kuqa Depression in the Tarim Basin and western Sichuan Basin. (f) Isotopic differences between heavy hydrocarbon gases and methane become less with increasing maturity.