A novel biodegradation parameter derived from bicyclic sesquiterpanes for assessing moderate levels of petroleum biodegradation

Abstract

Gas chromatography–mass spectrometry (GC–MS) and Fourier transform ion cyclotron resonance–mass spectrometry (FTICR–MS) analyses were performed on 39 heavy oils from an onshore basin in southeastern Mexico to investigate the influence of biodegradation on whole oil compositions. Among the biodegradation resistant C14–C16 bicyclic sesquiterpanes (bicyclanes), a rearranged C15 isomer (compound X) becomes relatively enriched with increasing biodegradation levels, contrasting with a reduction in the summed C14–C16 bicyclanes concentration but consistent with an enrichment of the bicyclic naphthenic acids revealed by FTICR–MS data. The concentration ratio of compound X over all the identified C15 bicyclane isomers (DMHD ratio) is proposed as a new biodegradation parameter. This ratio correlates well with the biodegradation ratios of 3-methylchrysene/methylchrysenes, benzonaphtho[1,2-d]thiophene/benzonaphtho[2,1-d]thiophene and those based on NSO compounds. Compound X has an additional ethyl group but one less methyl group compared to the drimane structure. A lessening of steric effects through demethylation can explain the refractory nature of compound X, and this forms the basis on which possible origins of compound X are discussed. Extensive biodegradation of the studied oil samples is implied by the alteration of polyaromatic hydrocarbons and NSO compounds. This suggests the relative enrichment of compound X can be used as a new indicator for heavy oil biodegradation at Peters and Moldowan biodegradation level (PM level) 4–5, when hopanes and steranes are still intact. Changes in the distribution patterns of some NSO compound species also suggest PM level 4–5.