Abstract: The systematic research is made on the geochemical characteristics of natural gas in Jiuxi depression. The result shows thatcomposition of the gas is dominated by hydrocarbon gas which accounts for 91 .82%~99.22%, averaging 75.81%, composed of methane(68.85%~85. l3%, 75.81% in average) and heavy hydrocarbons (20.34% in average) and C₁/C_1-5 ratio ranges from 0.75 to 0.87, belongingto typical wet gas. The non-hydrocarbon gas is dominated by CO, accounting for 0.78%~8.18%, averaging 3.85%. The δ¹³C₁value rangesfrom-45.3%o to-40.2%o, δ¹³C₂ value from-31. 1%o to-28.4%.(-30.27%0 in average) andδ¹³C₂value from-28.6%o to-26.9%o (-27.6%o inaverage), i.e.δ¹³C₃, which belongs to the ty pical petroliferous gas.

Key words: Yumen field, Jiuxi depression, natural gas, composition, carbon isotope, petroliferous gas