Natural gas in Tahe oilfield is mainly composed of hydrocarbon gas with an average content of(94.09)%.The non-hydrocarbon gas is mainly composed of N

2

and CO

2

with average contents of 4.2% and 1.19%

respectively.Hydrocarbon gas is mainly composed of methane with an average content of 76.23%;while the heavy hydrocarbon has an average content of 17.86%.The drying coefficient(C

1

/C

1-5

) averages(0.81)

indicating typical wet gas as a whole.The δ

13

C

2

values of all samples in this area are less than-30‰

indicating that it is of petroleum gas;and the δ

13

C values are in obvious positive sequence

i.e.δ

13

C

1

<

δ

13

C

2

<

δ

13

C

3

indicating that it is typically of organic origin.The associated gas in Tahe oilfield mostly belongs to typical petroleum gas

without any mixture of gas from other sources.The carbon isotope composition of me-thane reveals that the evolution of its source rocks is in the range of mature to over mature stages

and is mainly in highly over-mature stage.Analysis in combination with geologic setting indicates that the source rocks are of Cambrian-Ordovician.The non-linear distribution of some series of carbon isotopes shows that the gas in Tahe oilfield has been charged in two stages.Associated gas has been charged in the early stage

while pyrolysis gas has been charged in the late stage.