An Integrated Assessment of Different Depositional Paleoenvironment Using Nitrogen Markers and Biomarkers after Chromatographic Methods Optimization

Abstract

In this study, chromatographic methods were first optimized to ensure the robustness of the identification and quantification of nitrogen markers and biomarkers. Then, the optimal conditions were applied to 14 crude oil samples deposited in distinct paleoenvironments from Brazil, Venezuela, and Colombia to perform an integrated geochemical assessment. Analytical standards, certified reference material, and retention indices were used to confirm the identification of biomarkers and N-markers. The results of geochemical interpretations based on unresolved complex mixture (UCM), pristane/n-heptadecane (Pr/n-C17), and phytane/n-octadecane (Ph/n-C18) ratios and concentrations of carbazole and benzo[b]carbazole indicated that all oils are not biodegraded. The Pr/n-C17, Ph/n-C18, and Pr/Ph ratios showed that the organic matter that generated the oils from Brazil and Venezuela was deposited under anoxic conditions and Colombia oil reached dysoxic conditions. Some samples present a greater abundance of low- to high-molecular-mass n-alkanes, indicating freshwater lakes’ organic matter (Brazil oils). In contrast, other samples showed a lower abundance of high-mass n-alkanes, suggesting marine and saline lake origins (Colombia and Venezuela oils). The tricyclic/hopane ratio, the ternary diagrams using 1-methylcarbazole, 2-methylcarbazole, and 4-methylcarbazole, and regular steranes C27, C28, and C29 suggested a contribution of algae to the formation of kerogen present in the source rocks of all petroleum samples. The high concentrations of carbazole in oils generated by marine organic matter confirm the more positive δ13C values compared with those generated by lake organic matter (Brazil samples). The use of chemometric tools as principal component analysis exhibited a grouping of samples according to the depositional environment using carbazole and tricyclic/hopane ratio. The integration of all parameters analyzed provides a guide for refined interpretations and differentiation of oils according to their depositional environments.