oa An Assay on Unusual Biomarkers Identification and Diagenesis of Immature Marine Shales from the Pimenteiras Formation, Brazil

Evaluating immature shales may give insights into paleoenvironmental conditions and diagenetic routes of organic matter transformation. This work focuses on the identification of traditional and unusual biomarkers present in immature marine shales from the Pimenteiras Formation (middle to late Devonian), Parnaíba Basin, Brazil, assessed by comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS). Special attention was devoted to hopanoid and steroid diagenetic products to propose a diagenetic route for these classes of biomarkers. The comprehensive assessment of biomarkers by high-resolution gas chromatography allowed the identification of several unsaturated and aromatic hopanoid and steroid derivatives for the first time in the Pimenteiras Formation. Most detected unsaturated hopanes and steranes can be associated with acid clay-catalyzed intramolecular double bound and methylation rearrangement. It was possible to propose a diagenetic route for hopanoid transformation. Our findings also suggest that the immature nature of organic matter combined with a highly anoxic environment influenced the preservation of multiple aromatic intermediates. These compounds are associated with methanotrophic and/or anoxygenic phototrophic bacteria and would suffer intense aromatization should these records reach early stages of thermal maturity, which could hinder their correct attribution.