Applying geochemical parameters to verify oil mixtures in the occluded biomarkers from asphaltene structures

Asphaltenes, which can be considered as fragments of kerogen, can occlude, and protect hydrocarbon compounds from secondary alterations that occurs in petroleum reservoirs. The objective of this study was to apply the analysis of occluded biomarkers to address the question of how to understand the mixture of oils in the Potiguar Basin, since the oils produced therein inherited compositional differences from two petroleum systems. Asphaltenes from seven Brazilian crude oils of Potiguar Basin were obtained and submitted to mild oxidative treatment to disrupt their structure, releasing occluded hydrocarbons. The study of the geochemical composition of the occluded hydrocarbons showed that the oils from Areia Branca Trend registered a signature lacustrine, saline lacustrine and marine-evaporitic oils. On the other hand, occluded oils from Carnaubais Trend, suggest contributions from marine-evaporitic and lacustrine oils. As for the maturity parameters, it was observed that the saturated biomarkers occluded from the studied oils are less thermally evolved, the same was observed in the analysis of aromatic compounds. The results of occluded hydrocarbons showed great similarity with free hydrocarbon oils, which confirms the postulated hypotheses of initial filling of lacustrine oil reservoirs generated by the Upanema Member followed by a mixture with marine-evaporitic oil generated by the Galinhos Member and Ponta do Tubarão (PT) Layer. In this case study, it was evident that a geochemical understanding of occluded hydrocarbons can provide a better knowledge of petroleum systems. Mainly, when there is a secondary alteration due to biodegradation, as in the Carnaubais Trend, the biomarkers occluded in the asphaltenes register well the more marine-evaporitic oil. The study of biomarkers occluded in asphaltene structures proved to be a valuable tool in the interpretation of geochemical parameters of origin, maturation, and mixture of different sources, as they are the closest to the original conditions.