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292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most samples whereas abundant nitrogen in some samples is positively correlated with δ13C1. Although methane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected in most samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, eth-ane and propane are defined as δ13C1: -45.5‰ to -33.5‰, δ13C2: -30.2‰ to -10.5‰, and δ13C3: 27.6‰ to -11.2‰, respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showing normal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormal distribution. Based on the isotopic composition, the disagreement between the relationship of △(δ13C1-δ13C2) and δ13C2 and that of △(δ13C1-δ13C2) and δ13C2, and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms.
292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most of samples abundant abundant nitrogen in some Samples are positively correlated with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, eth-ane and propane are defined as δ13C1: -45.5 ‰ to -33.5 ‰, δ13C2: -30.2 ‰ to -10.5 ‰, and δ13C3: 27.6 ‰ to -11.2 ‰, respectively. According to the distribution of carbon isotope ratios, 2 families of gas based on the isotopic composition of delta (δ13C1-δ13C2) and δ13C2, and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms.