Effect of biodegradation on biomarkers released from asphaltenes

The structural features of asphaltenes enable them to occlude other molecules such as biomarkers. This could hinder the secondary alteration of biomarkers that occurs in oil reservoirs; these biomarkers could, therefore, be considered to be remnants of the “original oil”. Asphaltenes from Brazilian crude oils were obtained and submitted to oxidative treatment to disrupt their structure, releasing the trapped oil. The released hydrocarbons were compared with those from the original crude oil, and used to evaluate the alteration of the oils, especially as a result of biodegradation. The crude oils are depleted in n-alkanes, an effect usually related to biodegradation. However, the released products from the corresponding asphaltenes have n-alkane distributions from C15 to C35, suggesting protection from biodegradation that is probably related to encapsulation by the asphaltene network. The m/z 191 chromatograms showed different relative intensities for tricyclic terpanes in the associated crude and released oils. They are less abundant than the hopanes in m/z 191 mass chromatograms of the hydrocarbons released from the asphaltenes. The occluded saturated hydrocarbons are suggested to represent the original oil derived from kerogen, retained and protected against alteration inside asphaltenes over geological time.