Biomarkers of photosynthetic sulfur bacteria in Lower Cretaceous crude oils, East Brazilian marginal basin

Abstract

Diagenetic aromatic carotenoids and degradation products, which are derived from the carotenoids of photosynthetic sulfur bacteria, were characterized in the aromatic fraction of Brazilian Cretaceous crude oils (NE Brazil, Sergipe-Alagoas Basin) by high-temperature comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (HT-GC × GC–TOFMS). Unresolved complex mixtures (UCM), coeluting compounds, and limited mass spectrometric identification were noted by traditional one-dimensional (1D) GC–MS screening of aromatic compounds. Group-type separation on a two-dimensional chromatographic plane, first- and second-dimension retention times (1tR and 2tR, respectively), and mass spectra enabled the assignment of alkylated mono-, di-, tri-, and tetra-aryl hydrocarbons with confidence. A series of diagenetic C40 carotenoids and degraded products, which are likely indicators for green-brown sulfur bacteria, were detected. This finding corroborates previous results, and isorenieratane was relatively more abundant than 2,3,6-trimethylaryl isoprenoids. Moreover, atropisomeric biphenyls (m/z 237), which coeluted using 1D GC–MS analyses, were resolved by GC × GC and identified by mass spectrometry. The identification of minor abundances of chlorobactane, diagenetic biphenyl chlorobactane, and related compounds, for the first time in these Brazilian Cretaceous oils, may indicate a contribution of green sulfur bacteria (Chlorobiaceae) and photic zone euxinia in shallow waters.