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A suite of oils from the Bongor Basin, South Western Chad was characterized geochemically to investigate the behavior of carbazoles and benzocarbazoles during biodegradation. The oils show similar maturity levels and organic facies type with biodegradation as the primary control on molecular compositions. According to compositional variation, the biodegradation levels of oils fell within PM level 0–8 [PM biodegradation scale (Peters, K.E., Moldowan, J.M., 1993)]. Absolute concentrations of carbazole, methylcarbazoles, C 2 -carbazoles, and benzocarbazoles in the oils show an increase from PM level 1–3, fluctuation at PM level 4–7, and sharp decrease above PM level 7. Relative abundances of the carbazole series indicate that C 0 –C 2 -carbazoles are more susceptible to biodegradation than benzocarbazoles, 2- and 3-methylcarbazoles are more susceptible than 1- and 4-methylcarbazoles, partially shielded dimethylcarbazoles are more susceptible than the exposed ones and both of these are more susceptible than shielded dimethylcarbazoles. Benzo[c]carbazole seems to show a higher ability to resist biodegradation than benzo[a]carbazole in heavily biodegraded oils. Therefore, ratios of 1-/(2-+3-) methylcarbazole, 4-/(2-+3-) methylcarbazole, 1,8-dimethylcarbazole/total dimethylcarbazoles, 1,8-/1,3-dimethylcarbazole, and 1,8-/2,4-dimethylcarbazole may be used to assess the degree of biodegradation in this area. [ABSTRACT FROM AUTHOR]