Abstract
The Pirambóia Formation tar sands constitute one remarkable occurrence of heavy oil in the eastern border of the Paraná Basin, São Paulo State, Brazil, being characterized as highly degraded oils related to the Irati-Pirambóia oil system. In this context, the aim of the present work was to investigate biomarkers that are more resistant to degradation and use them to establish a more reliable geochemical correlation between the tar sand oils of the Pirambóia Formation (collected from the Fazenda Betumita and Guareí I outcrops) and the organic extracts of black shales of the Irati Formation (facies Irati A and B, collected in the Amaral Machado Quarry, São Paulo State). For this purpose, analysis of stable carbon isotopes and diagnostic ratios of saturated and aromatic biomarkers were investigated, and the most reliable diagnostic ratios were selected based on the principal component statistical analysis. The results pointed to different degradation extent between the tar sand outcrops, being higher for the Guareí I samples. In this scenario, it was verified that most of the diagnostic ratios commonly used in oil-source rock correlation studies were susceptible to degradation processes. It was also observed that samples from Pirambóia and Irati formations are at the beginning of the oil generation window, with the C29 ββ/(αα + ββ) ratio as the most suitable to assess thermal maturity of extremely degraded oils. The multivariate statistical analysis of the results, based on ten more resistant diagnostic ratios, allowed to select six main variables responsible for the similarity between the samples from the Irati and Pirambóia formations: δ13C (‰), TPP ratio (C30 tetracyclic polyprenoids over diasteranes), %C28 and %C29 monoaromatic steranes, (C20 + C21)/(C23 + C24) tricyclic terpanes, and C23 tricyclic terpane/(C23 tricyclic terpane + C29 hopane). These geochemical parameters showed greater reliability in the correlation between the Pirambóia Formation tar sand oils and the Irati samples of the facies A, deposited in a restricted marine environment under anoxic-euxinic conditions.
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