Hydrocarbon generation in the Permian Irati organic-rich shales under the influence of the early cretaceous Paraná Large Igneous Province

Abstract

The increasing global demand for natural gas and petroleum byproducts is turning organic-rich shales into an important target for hydrocarbon exploration and production. Current understanding about hydrocarbon generation in unconventional plays, such as oil shales and gas shales, is still evolving, especially in sedimentary basins where hydrocarbon generation is influenced by magmatism. This is the case for the Permian oil shales of the Irati Formation in the Paraná Basin, southern South America. The Irati shales are considered an atypical petroleum system due to thermal effects of the Paraná Large Igneous Province (LIP) emplaced during the Early Cretaceous. In this context, the focus of this study was to characterize the thermal conditions of hydrocarbon generation in the Irati Formation as well as the composition of related aqueous diagenetic fluids. The kerogen from the Irati organic-rich shales has an aromaticity around 30% (ƒ ar ≈ 30%), as determined by carbon nuclear magnetic resonance ( 13 C-NMR), and vitrinite reflectance (R o ) of around 0.6%. Fluid inclusions in calcite and quartz veins present aqueous fluids with salinity of 0–5 wt% of NaCl equivalent and homogenization temperatures greater than 250 °C. Hydrocarbon fluid inclusions were classified as black oil through their vapor fraction volumes (F v = 1.6–9.1%) and homogenization temperatures (Th) in the 60–80 °C range. Methane (CH 4 ) or carbon dioxide (CO 2 ) fluid inclusions were absent within the analyzed calcite and quartz veins. The PVT model for trapped fluid inclusions suggests a shallow hydrocarbon petroleum system (≈2.5 km depth) active during the Early Cretaceous, synchronous with the Paraná LIP. • Low salinity fluids have contributed to the formation of calcite and quartz veins. • Paleotemperatures and composition of hydrocarbons were reported by fluid inclusions. • Vitrinite reflectance and kerogen aromaticity report oil-window maturities. • The maturity of the Irati Formation shales was controlled essentially by burial. • Liquid hydrocarbons were classified as black oil due to their Fv and Th parameters.