The geochemical features of natural gas in the Ordovician vary obviously from east to west in Tahe oilfield. After years of exploration

a large amount of testing data for natural gas accumulated. In combination with analyses of the geologic conditions and stages for natural gas accumulations in Tahe oilfield

studies of natural gas compositions and C/H isotopic features

led to the following conclusions. In the western part of Tahe oilfield

the natural gas in the Ordovician reservoirs was mainly dissolved or associated gas

featuring in lighter methane carbon isotopes

and mainly originated from the initial cracking of crude oils. In the central part of Tahe oilfield

the natural gas in the Ordovician reservoirs was mainly associated gas and partially condensate gas

and was mature/high-mature gas which was charged at the same time as crude oils. In the eastern part of Tahe oilfield

the natural gas in the Ordovician reservoirs feature in highly dry coefficient and heavy methane C/H isotopes

with higher maturity than those samples from both the western and the central Tahe oilfield. For some natural gas samples

δ

13

C

1

>

δ

13

C

2

indicating that the natural gas were charged and accumulated in stages. The first stage of natural gas charging and accumulation took place when hydrocarbon accumulated in the central part of Tahe oilfield. In the second stage of natural gas charging

the cracking of over-mature hydrocarbon played an important role. The nature gas mainly was sourced from the cracking of crude oil and kerogen

and charged during the late Himalayan period. The different origins of natural gas and the mixing of natural gas during different stages explained the obviously different features of natural gas among different regions in the Tahe oilfield.