The coal measures in the Upper Triassic Xujiahe Formation in Sichuan Basin are coal-bearing continental strata.Its first

third

and fifth members are dominated by dark mudstone and coal and are gas source rocks.The kerogen of the mudstone are mainly of type Ⅱand Ⅲ with an average TOC of 1.96%.They are mature in the central Sichuan Basin and are highly mature in the western Sichuan Basin.Since the organic matter of source rocks are dominated by huminite

the source rocks are gas prone and the gas generated only contains minor light oil or condensate.The second

fourth and sixth members consist mainly of sandstone.There are three sets of source-reservoir-caprock combinations

resulting in many indigenous coal-derived gas fields.The gas reserves discovered in the Xujiahe Formation is only next to that in the Lower Triassic Feixianguan Formation.The Guangan gas field

the second largest gas field in the basin

produce gas from the Xujiahe Formation.The carbon isotope of the coal-derived gas have the following features:1) most of them are positive carbon isotopes(i.e.

δ

13

C

1

<

δ

13

C

2

<

δ

13

C

3

<

δ

13

C

4

)

2) its δ

13

C

2

value is between-20.7‰--28.3‰

the heaviest among that of the 9 gas pays in the whole basin

3) there are lighter δ

13

C

1

in central Sichuan Basin with the lowest value of-43.0‰.A few gas reservoirs sourced from oil-prone kerogens had been discovered in eastern and southern Sichuan Basin

where the Xujiahe Formation thins out.These reservoirs have light carbon isotopes(typically lighter than-30‰

and in extreme cases as light as-36.3‰) and can be easily distinguished from coal-derived gases.