In order to investigate the origin and figure out the source of natural gas in the Upper Triassic Xujiahe Formation in Yuanba-Tongnanba area of Sichuan Basin

the composition and stable carbon and hydrogen isotopes were used to analyze the geochemical characteristics of the natural gas.The study indicates that the natural gas in the Xujiahe Formation in the Yuanba-Tongnanba area is mainly composed of methane.It appears to be dry gas with a dryness coefficient generally higher than 0.97.The δ

13

C

1

and δ

13

C

2

values are in the range of -34.5‰~-29.3‰ and -35.4‰~-21.5‰

respectively.The δ

13

C

CO

2

values are generally higher than -8‰

and the δD

CH

4

values are in the range of -181‰~-144‰ without significant correlation with δ

13

C

1

values.The identification of gas origin and gas-source correlation indicate that the natural gas in the Xujiahe Formation is mainly coal-derived gas and sourced from the coal-measure source rocks in the Xujiahe Formation.The gas in the 1

st

and 2

nd

Members of the Xujiahe Formation in the Yuanba area and the Xujiahe Formation in the Tongnanba area contain a certain amount of secondary oil-cracking gas

which are mainly the product of the low-degree cracking of oil generated by the Wujiaping Fomation source rocks.Some gas samples from the Xujiahe Formation in the Yuanba-Tongnanba area display partial reversal of alkane carbon isotopic series

and the partial reversal of the gas from the 3

rd

and 4

th

Members in the Xujiahe Formation in the Yuanba area is resulted from the fact that they were generated by the Xujiahe Formation source rocks in the high-mature stage

while that from the 1

st

and 2

nd

Members in the Xujiahe Formation in the Yuanba area and from the Xujiahe Formation in the Tongnanba area are attributed to the mixing of a small amount of oil cracking gas and a large amount of high-mature coal-type dry gas.