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Stable Carbon Isotopic Characteristics of Alkane Gases in Tight Sandstone Gas Fields and the Gas Source in China

Significant achievements have been obtained in tight sandstone gas exploration in China. At present, tens of tight sandstone gas pools have been discovered in the Ordos basin, the Sichuan basin, the Bohai Bay basin, the Tarim basin, and the Turpan-Harmi basin in China. This study examines the gas components and the stable carbon isotope data of 150 tight sandstone gas samples collected from the main petroliferous basins in China. Tight sandstone gas in China is dominated in alkane gases, and the contents of methane and its homologues decrease with increasing of the carbon number. Major part of the tight sandstone gas have the characteristic of normal stable carbon isotope series (i.e., δ13C1<δ13C2<δ13C3<δ13C4), whereas the rest show a partial reversal, and the reversals between ethane and propane are in dominant (δ13C2>δ13C3). Most of the sandstone gases are coal-derived gas, and mixing of gases from the same humic source but different maturity or from different humic sources caused the partially reversal of the carbon isotopic values. The major part of the tight sandstone gas pools are self-generation and self-accumulation reservoirs formed in coal measures, and the coal measures are the favorable field in tight sandstone gas exploration in China.