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

摘要

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 m~3 and the proved geological reserves were 2485 × 10~9 m~3, 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 CO_2 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 δ~(13)C_2 vs. C_2H_6 plot of a great number of oil-derived and coal-derived gases in China, it is concluded that gases with δ~(13)C_2 > -28.5‰ are coal-derived and those with δ~(13)C_2 < -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 C_1-C_4 alkanes (i.e. δ~(13)C_1 < δ~(13)C_2 < δ~(13)C_3 < δ~(13)C_4) 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) CO_2 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 CO_2 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.