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Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from -50‰ to -60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ 13C1 ranging from -50‰ to -60‰, δ 13C2 > -35‰, Δ values (δ 13C3 - δ 13C3) < 5‰ and C1/ΣC2+ ratios < 40. Immature to low-mature gases display the characteristics of δ 13C1 ranging from - 50‰ to - 60‰, δ 13C1 < -40‰, Δ values (δ 13C3 - δ 13C3) > 7‰, and C1/ΣC2+ ratios >60. [ABSTRACT FROM AUTHOR]

A low-mature coal (R0 = 0.4%, from the Manjia'er depression, Tarim Basin, China) was subjected to closed system pyrolysis, in sealed gold tubes, under isothermal temperature conditions. The carbon isotopic compositions of the pyrolyst fractions (hydrocarbon, CO2, CO, etc.) at two temperature points (350°C and 550°C) were measured. The results showed that δ 13CCH4 value is generally heavier at 350°C than that at 550°C, because the high abundance of CO generated at low temperature would greatly influence δ 13CCH4 value, and the retention time of CO in gas chromatograph is close to that of CH4. But CO is formed through chemical reaction of the oxygen-containing functional group -C = O, e.g. lactones, ketones, ether, etc. at low temperature, while CO2 comes mainly from decarboxylization. The carbon isotopic composition of coal gas from Lanzhou Coal Gas Works as definitely different from that of thermally pyrolysed products from coal. The δ 13CCH4 value of coal gas was abnormally heavier than δ 13CCO. At the same time, the reversed sequence (δ 13C1 > δ 13C2) of δ 13C1 and δ 13C2 happened. The bond energy of free ions generally decides the sequence of generation of hydrocarbon fractions according to the chemical structure, whereas the stability of pyrolysate fractions and their carbon isotope fractionation are affected by the C-C bond energy. [ABSTRACT FROM AUTHOR]