Saturated biomarkers adsorbed and occluded by the asphaltenes of some Venezuelan crude oils: Limitations in geochemical assessment and interpretations

Abstract

This paper presents the study of saturated biomarkers in maltenes and the compounds adsorbed and occluded by the asphaltenes of six Venezuelan crude oils. The study is intended to know to what extent geochemical information stored in the adsorbed and occluded compounds matches the information in maltenes when it comes to evaluating source input, depositional environment, and thermal maturity. Our findings suggest that geochemical information obtained from the adsorbed and occluded compounds agrees with the information provided by maltenes of unaltered crude oils such as Alturitas, Boscan, Guafita, and Silvestre. We confirmed a clear correlation of Alturitas and Boscan crude oils with a carbonate source rock sedimented under anoxic conditions and containing marine organic matter, while Guafita and Silvestre crude oils resulted from a marl-type source rock characterized by mixtures of marine and terrigenous organic matter inputs preserved under varying redox conditions (anoxic to sub-oxic). Regarding maturity, Alturitas and Silvestre resulted more mature than Boscan and Guafita. Despite the good correspondence between the geochemical information provided by the maltenes and the adsorbed and occluded compounds, we observed compound enrichments that caused limitations in the use of some geochemical indicators. The adsorbed compounds of the six studied crude oils were enriched in linear alkanes higher than n-C24, regular steranes (C27–C28), and pentacyclic terpanes, while in the occluded compounds pregnanes, tricyclic terpanes, and n-alkanes higher than n-C24 were more abundant. Thus, maturity relationships defined by the relative abundances of low and high molecular mass steranes and terpanes yielded values that confused when the adsorbed and occluded compounds were compared with the respective maltenes. However, when these relationships were used to compare the occluded compounds of two or more different crude oils, they yielded a reliable appraisal of the maturity stage of the respective source rock. Our results strongly suggest that the adsorbed compounds still preserve their geochemical information as they probably undergo a very low exchange with the surrounding maltenes. So, these compounds together with the occluded ones become a valuable source of geochemical information to assess altered crude oils such as Ayacucho and Junín that have lost most of their maltene biomarkers due to biodegradation. In these cases, the information provided by the adsorbed and occluded compounds confirmed that Ayacucho crude oil is more mature than Junín and that the former derives from a carbonate, marine-organic-matter source rock sedimented under anoxic conditions while the latter correlates with a marl-type source rock sedimented in anoxic conditions and containing mixtures of terrigenous and marine organic matter. Compounds such as oleanane, C27 25,28,30-trisnorhopane (TNH), C28 17α,21β,28,30-dinorhopane (DNH) and C29 17α(H),21β(H)-25-norhopane (NH) detected in the occluded material of the six studied oils, indicate that the major contributions to the occluded compounds probably came from the original material initially adsorbed and occluded in the parent kerogen.