Abstract: 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, δ¹³C₁ >δ¹³C₂, 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.

Key words: geochemical characteristics, genetic type, natural gas, Ordovician, Tahe oilfield