Geochemical analysis of different degrees of biodegradated crude oils show that a slight biodegration affects little to the pyrrolic nitrogen compounds in the oils. The concentrations of cathazole

methylcarbazole and C

2

-carbazole decrease with the increasing of the biodegradation for mid-degree biodegraded oils

the concentrations of pyrrolic nitrogen compounds sharply decrease in the heavily biodegraded oils. The relative concentrations of carbazole

methylcarbazole and C

2

carbazole do not show any apparent changes in the non-heavly biodegraded oils; in the heavilybiedegraded oils

the concentrations of carbazole and methylcarbazole decrease

but that of the C

2

-carbazole increases.As the C

2

-carbazole isomers

the relative Percentage contents of the pyrolic N-H shielded

the prplic N-H semishielded and the pyrolic N-H-exposed carbazoles do not display any obvious variation in the non-heavily biodegraded crude oils

but that of the pyrrolic N-H-exposed carbazoles increase and that of the pyrrolic N-H semi-shielded carbazoles decrease in the heavily biodgraded oils.