It is universal that both the anaerobic bacterial degradation and oxidative bacterial degradation of crude oil exist in the hydrocarbon evelution.They do not only distroy some hydrocarbons

but also reproduce some new hydrocarbons

and even result in new hydrocarbon pools for exploration.A lot of shallow transitional gas pools actually resulted from anaerobic degradation.The products of the anaerobic bacterial degradation of crude oil possess light methane carbon isotope(δ

13

C

1

<

-5.5%)

very light ethane carbon isotope (δ

13

C

2

<

-4.0%)and the negative anomaly of carbon isotope of separate hydrocarbons (δ

13

C

n

<

-3.7%～-5.3%).Natural gas produced in central Tarim Basin contains at least 20% bacterial degrated methane.