Study on Biodegradation of Tricyclic Terpanes in the Subsurface Reservoirs by GC–MS, FT-ICR MS, and Molecular Simulation

Understanding petroleum microbial biodegradation behaviors under in situ conditions in subsurface oil reservoirs is helpful for oil pollution mitigation by microbial methods. In this study, a total of six heavily biodegraded oil samples from the Bongor Basin, Chad are collected to investigate the tricyclic terpane (TT) biodegradation behaviors in combination with GC–MS, FT-ICR MS, and quantum mechanical calculations. A series of 17-nor-tricyclic terpanes (NTTs) including C₁₉ and C₂₀ NTTs rarely identified in previous reports are observed in extremely biodegraded oils where hopanes and steranes are partly or completely removed. This suggests the occurrence of NTTs present in oil with biodegradation scale reaching PM 7–8 levels. The discovery of a low member of NTTs also indicates that the formation of NTTs is likely the removal of the methyl group from the corresponding TT compounds rather than the biological origin. From the molecular simulation results, the carbon atom in the C-10 position of C₂₃TT is enriched with a positive charge indicating a strongly electrophilic reaction ability. It is evidenced by a much higher electrophilic index at the site suggesting that the C-10 methyl group is easiest for oxidation and removal rather than that at the end of the branch, which is also supported by a lower activation energy for its removal. Based on the FT-ICR MS data, it shows a linear relation between the relative contents of hopanoic and tricyclic terpanoic acids, which implies a likely similar biodegradation behavior between hopane and TTs. In combination with the biodegradation of hopanes and computational calculation results, we hypothesized that NTTs likely are formed by the oxidation and removal of the methyl group at C-10 of TTs. Although tricyclic terpanoic acids likely are the major intermediates, they are not essential and may not be yielded during the formation of NTTs. This study provides a multimethod for the TT biodegradation mechanism investigation.