Geochemistry and accumulation of the Upper Palaeozoic natural gas in the Sulige Gas Field,Ordos Basin

The Sulige Gas Field is a giant litholitic gas reservoir in the Upper Palaeozoic strata in the Ordos Basin.The determinations of gas chromatography indicates a kind of humid gas with lower dryness and dominant methane.The carbon isotopes in n-alkane exhibit a trend of δ¹³C₁ < δ¹³C₃ < δ¹³C₂ < δ¹³C₄.There is a positive correlation between the carbon isotopes and methane contents,and a negative correlation between the carbon isotopes and ethane contents.The variations in the contents of carbon isotopes show that the natural gas from the Sulige Gas Field is a kind of coal-derived gas of organic origin and contains minor amount of CO₂.The Permian gas reservoirs went through the deposition and burial of the source rocks during the Carboniferous-Permian to the Middle Jurassic,formation of the gas during the Late Jurassic to the Cretaceous,and preservation and modification of the gas from the Tertiary onwards.