Biodegradation of tricyclic terpanes in crude oils from the Bohai Bay Basin

A suite of 33 biodegraded crude oils from the Miaoxi Depression and Liaodong Bay, Bohai Bay Basin, China were analyzed to investigate the effect of biodegradation on biomarkers. Among these samples, sequential biodegradation of C19-C26 tricyclic terpanes was observed in the most degraded crude oils, in which C27-C29 regular steranes and hopanes were almost completely removed. This suggests that alteration of tricyclic terpanes occurs when almost all hopanes and steranes have been removed. The C21-C22 steranes and diasteranes were unaffected in the crude oil samples, and so they can be used as conserved “internal standards” to evaluate the biodegradation of tricyclic terpanes. The tricyclic terpane distributions, absolute concentrations and their relative abundance compared to the biodegradation-resistant components show systematic changes with increasing biodegradation. The results illustrate that the relative susceptibility of the tricyclic terpanes to biodegradation decreases with increasing carbon number, with the exception of the C20 tricyclic terpane. No demethylated tricyclic terpanes were detected, indicating that tricyclic terpanes were biodegraded without microbial demethylation to generate demethylated counterparts (17-nor-tricyclic terpanes). Tentatively, we predict that the biodegradation pathways of tricyclic terpanes are: (1) microbial oxidation of the methyl group of the side chain to a carboxyl group with formation of tricyclic terpanoic acids, followed by (2) demethylation of the tricyclic terpanoic acids at C-10 and generation of demethylated tricyclic terpanoic acids. Verification of this predicted biodegradation pathway will require analytical data for the related biodegradation products.