The δ

13

C values change in a regular manner with the increase of hydrocarbon gas molecular weight

i.e. change in normal order or reverse order.The normal order is δ

13

C

1

<

δ

13

C

2

<

δ

13

C

3

<

δ

13

C

4

and the reverse order is δ

13

C

1

>

δ

13

C

2

>

δ

13

C

3

>

δ

13

C

4

.Partly reversed order is also common in gas samples from different basins in China

such phenomenon can be attributed to one or several of the following reasons:(a)Mixing of biogenic gas and abiogenic gas;(b)Mixing of gases from sapropelic sources and humic sources;(c)Mixing of gases from two source rock intervals of the same type but of different maturity;(d)Mixing of gases from one source rock interval of varying maturity.