Biodegradation of Hopanes, Steranes and Tricyclic Terpanes in Heavy Oils

Abstract

Nine heavy oils were analyzed to investigate the biodegradation of biomarkers. According to the characteristics of the presence of a fully developed series of 25-norhopanes and to the conspicuous depletion of hopanes, regular steranes and even tricyclic terpanes, the level of biodegradation of the samples were determined as ranging from PM 6 to PM 9. Among the samples, hopanes and regular steranes were biodegraded severely and the depletion of hopanes and of regular steranes increased with the increase of the biodegradation level. When the level of biodegradation reaches PM 8, the biodegradation of tricyclic terpanes begins. With the increase of biodegradation, the C_21–22 steranes and diasteranes remain unaffected under the biodegradation level of PM 8, indicating that they can be used as conserved “internal standards” to evaluate the biodegradation of hopanes, regular steranes and tricyclic terpanes. However, the C_21–22 steranes and diasteranes are degraded when the biodegradation level is over PM 9. Among the C_19–26 tricyclic terpanes family, C₂₃ and C₂₁ tricyclic terpanes are the most readily degraded members, followed by C₂₀ tricyclic terpane, while the C₁₉, C₂₂ and C_24–26 tricyclic terpanes seem more resistant to biodegradation.