Thermal effect of igneous intrusions on organic-rich Irati Formation and the implications for petroleum systems: a case study in the Parana Basin, Brazil

The organic geochemistry of organic-rich facies including shales, marls, and carbonates (Assistencia Member) of the Irati Formation, Parana Basin, Brazil, were analyzed to evaluate the thermal effects of igneous intrusions upon the kerogen present in these facies. Total organic carbon (TOC) content and hydrocarbon source potential (S2) of the Irati source rocks range from 0.03 to 20.4% and 0.01 to 112.1 mg HC/g rock, respectively, indicating excellent potential as a source for hydrocarbon generation. Hydrogen index (HI) values reveal that the kerogen is pre-dominantly type I (HI: up to 892.6 mg HC/g TOC) and, therefore, an oil source, except for samples having low TOC content due to severe maturation caused by the heat from diabase intrusions. The thickness of igneous intrusions in the 64 wells investigated in this study ranged from 2 to 231 m. They clearly had a major impact on TOC, HI, and S2 values, which decrease in the vicinity of intrusions, indicating a gradual increase in maturation toward the igneous body. In wells without the influence of igneous intrusions, Tmax values of Rock-Eval pyrolysis and %Ro indicate that the organic matter is immature for the generation of hydrocarbons.