An Assay on Alkyl Aromatic Hydrocarbons: Unexpected Group-Type Separation of Diaromatic Hydrocarbons in Cretaceous Crude Oils from Brazilian Marginal Basin

Numerous aromatic biomarkers such as aromatic carotenoids and aryl isoprenoids have been reported in Brazilian Cretaceous oils. The n-alkyl aromatics such as n-alkyl benzenes (ϕ-Cn), toluenes (ϕT-Cn), xylenes, and mesitylenes have also been detected in geological samples and can be valuable tools in geochemical and environmental investigations. In this work, comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC–TOFMS) and GC–tandem MS (GC–MS/MS) were applied to identify the alkyl aromatic series in two crude oils from the Sergipe-Alagoas Basin, Brazil. Geochemical characteristics indicate a different depositional paleoenvironment for each sample, marine and lacustrine end-members. Aromatic compounds were identified using full mass spectrum information, diagnostic ions (i.e., m/z 92 for alkylbenzenes and m/z 106 for alkyltoluenes), selected reaction monitoring transitions, and coinjection with an authentic C21 n-pentadecylbenzene standard. The Sergipe-Alagoas oils presented a series of n-alkylbenzenes and n-alkyltoluenes ranging from ϕ-C7 to ϕ-C33 and ϕT-C7 to ϕT-C30, respectively. In addition, phytanyl arenes and a new series of α,ω-bisphenylalkanes were identified for the first time in Sergipe-Alagoas oils. The n-alkylbenzene profile for marine crude oil resembles that of n-alkanes and n-alkylcyclohexanes, which suggests that they may have the same precursors. The investigation of aromatic fractions and detection of n-alkyl aromatic hydrocarbons in Brazilian Cretaceous oils provide new assignments to the contribution of organic matter and its geochemical significance