The characteristics of bound biomarkers released from asphaltenes in a sequence of naturally biodegraded oils

Abstract

Previous studies have showed that the hydrocarbon biomarkers bound in asphaltenes can preserve original geochemical information during subsequent thermal alteration and biodegradation relative to their free counterparts in crude oil. In the present study, asphaltenes separated from a set of heavy oils which possess an identical source origin and similar thermal maturity, but have experienced different degrees of biodegradation, were analysed to study the influence of biodegradation on the bound biomarkers of asphaltenes. On-line flash pyrolysis directly coupled with gas chromatography–mass spectrometry (Py–GC–MS) was used on these asphaltenes to release the hydrocarbon biomarkers bound within the asphaltene structure. Additionally, the free biomarker profiles in these oils were investigated by GC–MS for a comparison with the bound phase in the corresponding asphaltenes. The results reveal that most of the free biomarkers in the reservoired oils were significantly altered at heavy to severe biodegradation stages, and thus lost much original geochemical information. However, the biomarkers bound in the asphaltenes show very little alteration due to biodegradation, and they also appear to be less mature than their free counterparts in the corresponding oils. This indicates that the bound biomarker profiles in asphaltene flash pyrolysates can represent “original oils” at an early and low thermal maturity stage. Therefore, the biomarker parameters on the basis of asphaltene flash pyrolysates can still be used as indices for correlation studies even though the oils were heavily to severely biodegraded.