Geochemical characteristics and genesis of natural gas in the Yan'an gas field, Ordos Basin, China

The Yan'an gas field in the Ordos Basin, China, was discovered in 2014 and contains reserves of up to 100 billion m3. Its occurrence in the basin contradicts the distribution model of “gas in the north, oil in the south”. Upper Paleozoic natural gas in the Yan'an area is dominated by methane, with an average dryness coefficient (C1/C1-4, %) of 99.5% (δ13C1 = −30.8 to −27.5‰), with minor amounts of CO2 = 3.8%, N2 = 0.5%, and ethane (δ13C2 = −37.2 to −30.5‰) and traces of higher hydrocarbons. The gas is predominantly coal-derived originating from Carboniferous-Permian humic hydrocarbon source rocks and minor oil-type gas from sapropelic source rocks. The carbon isotope series includes reversals, with δ13C2 < δ13C1 and rare δ13C3 < δ13C2 < δ13C1. The enriched δ13C1 value is a result of the extremely high thermal maturity, while the depleted δ13C2 value cannot be explained solely by the simple addition of oil-type gas. The complex carbon isotope reversals result from the combined effects of several secondary reactions under extremely high thermal maturity.