Features and origin of oil degraded gas of Santai field in Junggar Basin, NW China

Based on comprehensive analysis of the natural gas composition, carbon isotopes and associated oil carbon isotopes, saturated hydrocarbon chromatography, and gas chromatography-mass spectrography, the features and formation mechanisms of natural gas in Santai area, Junggar Basin are studied. The Jurassic natural gas in Santai area is characterized by light ethane carbon isotopes, and the features of associated oil are light carbon isotopes, low Pr/Ph ratio, high C29 and C28 sterane value and low C27 sterane value, which shows that the hydrocarbons were mainly derived from the sapropelic source rock of the Permian Pingdiquan Formation in Fukang sag. The natural gas composition is dominated by methane. The methane carbon isotope value of the natural gas is higher than that of biogenic gas, but it is lower than that of thermogenic gas. Natural gas reservoirs were buried shallowly and always associated with or close to heavy oil which was subjected to serious biodegradation, with occurrence of 25-norhopane. All above indicate that the natural gas in Santai area is typical oil degraded gas by bacteria. Biodegradation was a process of water-hydrocarbon reaction which was affected by the bacteria and thermodynamics. The methane bacteria made use of CO2 and H2 to reduce light carbon isotopes preferentially to generate methane of light carbon isotopic compositions. Biodegraded heavy oil and oil sand are widely distributed in the petroliferous basin, therefore, reservoir oil degradation gas has good exploration prospects.