Depositional paleoenvironment of Brazilian crude oils from unusual biomarkers revealed using comprehensive two dimensional gas chromatography coupled to time of flight mass spectrometry

Abstract

Comprehensive two dimensional gas chromatography (GC×GC) coupled to time of flight mass spectrometry (TOFMS), currently one of the most powerful analytical tools that can separate mixtures at a high resolution, was applied to a suite of Brazilian crude oils. The oils were selected based on their different sources, similar maturity levels and little to no biodegradation in the reservoir, in order to study the molecular composition changes for different depositional paleoenvironments. Unusual compounds that are typically at trace levels in petroleum samples were investigated. Besides the usual biomarkers, the technique allowed for the identification of 3β- and 2α-methylhopanes as well as 8,14-seco-hopanes, onocerane isomers, monoaromatic, triaromatic and methyl-triaromatic steroids, benzohopanes and D-ring monoaromatic 8,14-seco-hopanoids. The biomarker parameters that are typically used to predict the organic matter input, paleoenvironmental conditions, thermal maturity and biodegradation of the oils were calculated using the GC×GC-TOFMS data and chemometric techniques were applied. The 3β-methylhopanes and onoceranes were only predominant in the lacustrine oil samples. In contrast, the relative abundance of 2α-methylhopane with an extended side chain was high, and the onocerane levels were low only in the marine oil samples. A new index is proposed based on methylhopanes. The percentage of C31 3β-methylhopane (3βMH31) relative to C30 hopane (H30), 100× (3βMH31/H30) is > 1 for the oils with a lacustrine origin and < 1 for the oils with a marine origin.