Degradation-resistant biomarkers in the Piramboia Formation tar sands (Triassic) and their correlation with organic facies of the Irati Formation source rocks (Permian), Parana Basin (Brazil)

摘要

The Piramboia Formation tar sands constitute one remarkable occurrence of heavy oil in the eastern border of the Parana Basin, Sao Paulo State, Brazil, being characterized as highly degraded oils related to the Irati-Piramboia 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 Piramboia Formation (collected from the Fazenda Betumita and Guard: I outcrops) and the organic extracts of black shales of the Irati Formation (facies Irati A and B, collected in the Amaral Machado Quarry, Sao 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 Guard: 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 Piramboia and Irati formations are at the beginning of the oil generation window, with the C29 beta beta/(alpha alpha + beta beta) 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 Piramboia formations: delta C-13 (parts per thousand), TPP ratio (C-30 tetracyclic polyprenoids over diasteranes), %C-28 and %C-29 monoaromatic steranes, (C-20 + C-21)/(C-23 + C-24) tricyclic terpanes, and C-23 tricyclic terpane/(C-23 tricyclic terpane + C-29 hopane). These geochemical parameters showed greater reliability in the correlation between the Piramboia Formation tar sand oils and the Irati samples of the facies A, deposited in a restricted marine environment under anoxic-euxinic conditions.