Superimposed secondary alteration of oil reservoirs. Part II: The characteristics of biomarkers under the superimposed influences of biodegradation and thermal alteration

Abstract Petroleum biomarkers could be altered to varying extents under the influence of biodegradation and thermal alteration in reservoir, and their respective influence has been well documented. However, the superimposed influence of successive biodegradation and thermal alteration has not been systematically studied so far. In this study, a sequence of crude oils and oil sands that are of identical origin but at different biodegradation levels from the Liaohe Basin of China were artificially matured via closed-system pyrolysis. The distributions of biomarkers such as steranes and hopanes in the variably biodegraded oil samples and in their pyrolyzates at varying artificial thermal maturities were compared. The results showed that the presence of normal (n-) alkanes in crude oil does not necessarily excludes the possibility of previous moderate–severe biodegradation as n-alkanes can be further generated under advanced thermal stress. Besides, biomarkers in the oils that have suffered superimposed alterations could be derived not only from the modifications of free biomarkers in the oils, but also from the release of biomarkers bound in the asphaltenes. The mix of these two sources complicated the biomarker profiles in this type of oils. Most source related parameters based on steranes and tri-, tetra- and penta-cyclic terpanes seem to be reliable only when the oil reservoir has suffered biodegradation not beyond moderate level and the maturity of the crude oil is below 0.91 Easy%Ro. The TT23/(TT23 + TT24) ratio, however, was found to remain constant even after severe biodegradation and thermal alteration, making it applicable in oil-source correlation in superimposed oil reservoirs. In addition, isomerization-based maturity parameters such as methylphenanthrene index of crude oils are reliable if subsequent thermal alteration reached above 0.91 Easy%Ro despite prior moderate–severe biodegradation, because the related isomerization reactions would approach the same level observed for the non-biodegraded oil after subsequent thermal alteration.