Universidade Federal do Piauí

1. Evidence for euphotic zone anoxia during the deposition of Aptian source rocks based on aryl isoprenoids in petroleum, Sergipe–Alagoas Basin, northeastern Brazil

^a Organic Geochemistry Laboratory, Chemistry Department, College of Natural Sciences, Federal University of Piauí, Campus Minister Petrônio Portella, Teresina, PI 64049-550, Brazil

^b Chemistry Institute, University of Campinas, POB 6154, Campinas, SP 13083-970, Brazil

• ^c Petrobrás/Cenpes/PDGEO, Av. Horácio Macedo 950, Cidade Universitária-Ilha do Fundão, Rio de Janeiro, RJ 21941-915, Brazil
• ^d UNESP-IGCE-DGA, Av. 24A, 1515, Rio Claro, SP 13506-900, Brazil

Abstract

Four crude oil samples from the Sergipe–Alagoas Basin, northeastern Brazil, were analyzed using full scan gas chromatography–quadrupole mass spectrometry (GC–qMS) for biomarkers, in order to correlate them using aromatic carotenoids thereby enhancing knowledge about the depositional environment of their source rocks. The geochemical parameters derived from saturated fractions of the oils show evidence of little or no biodegradation and similar thermal maturation (Ts/(Ts + Tm) for terpanes, C₂₉αββ/(αββ + ααα), C₂₇, and C₂₉ 20S/(20S + 20R) for steranes). Low pristane/phytane ratios and the abundance of gammacerane and β-carotane are indicative of an anoxic and saline depositional environment for the source rocks. Moreover, we identified a large range of diagenetic and catagenetic products of the aromatic carotenoid isorenieratene, including C₄₀, C₃₃, and C₃₂ diaryl isoprenoids and aryl isoprenoid derivatives with short side chains and/or additional rings. These results indicate anoxia in the photic zone during the deposition of the source rocks.