Influence of biodegradation on carbazole and benzocarbazole distributions in oil columns from the Liaohe basin, NE China

Abstract

Alkylcarbazoles and benzocarbazoles in petroleum reservoir core extracts isolated from several oil columns within the Lengdong oilfield, Liaohe basin, China were studied to investigate their occurrence and the effect of biodegradation on their concentrations and distributions. Bulk petroleum composition and molecular data indicate the occurrence of systematic biodegradation gradients within the oil columns, the extent of biodegradation ranging from light (level 1) to moderate (level 4-5) in Es3 columns, and from moderate (level 5) to heavy (level 8) in Es1 columns [all ‘levels’ in this paper refer to the Peters and Moldowan biodegradation scale (Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, NJ.)]. Whereas carbazoles and benzocarbazoles can be useful as migration markers in certain geological situations, the results presented here indicate that biodegradation also plays a significant role in controlling the distribution of carbazole compounds in reservoired oils. Alkylcarbazoles are generally regarded as resistant to biodegradation at low to moderate levels of biodegradation but at biodegradation levels greater than level 4 they may be microbially altered in a way similar to that observed for aliphatic and aromatic hydrocarbons. The concentrations of carbazole compounds in the oils increase slightly during the early stages of biodegradation and then sharply decrease after level 4, when preferential depletion of alkylated carbazoles compared to benzocarbazoles and dibenzocarbazoles or naphthocarbazoles is observed. The susceptibility of alkylated carbazole isomers to biodegradation decreases with increasing carbon number of the alkyl substituents. Furthermore, nitrogen (N–H) shielded or partially shielded compounds substituted in the 1 and/or 8 positions seem more susceptible to biodegradation than their nitrogen-exposed counterparts. For example, biodegradation resulted in the preferential removal of 1-methylcarbazole relative to the other methylcarbazole isomers and among the dimethylcarbazoles; 1,8-dimethylcarbazole seems more susceptible to biodegradation than other isomers, even though some exceptions do occur in our sample set. The benzocarbazole ratio [Nature 383 (1996) 593] decreased with increasing degree of biodegradation. Benzo[ b]carbazole has the highest apparent ability to resist biodegradation among the benzocarbazole isomers.