Geochemistry and petrography of shales from Brazilian Paleozoic basins: an example of the Parnaíba Basin, Devonian Pimenteiras Formation – A review

Abstract

The Brazilian Paleozoic sedimentary basins present an atypical petroleum system where subsidence was not sufficient for the maturation of organic matter, and igneous intrusions were decisive for this process. One of these basins is the Parnaíba Basin where the Pimenteiras Formation displays potential for hydrocarbon generation and constitutes the focus of this study. This study has used geochemical and petrographic data, 1D and 2D thermal modeling to determine organic matter sources, thermal maturation, depositional environment and the potential for hydrocarbon generation of the Pimenteiras Formation. This formation is rich in organic matter, especially in the basin center compared to the margins with low TOC content, probably due to poor preservation of organic matter, oxidizing conditions and weathering processes. The organic matter results from a mixture of marine oil-prone organic matter (type II kerogen), transitional organic matter (type II/III kerogen) with mixed of oil and gas potential, and a dominant contribution of terrestrial gas-prone organic matter (type III kerogen). The transgressive-regressive cycles and flooding in the basin during the Devonian contributed to the lithological heterogeneity, organic matter source variation, and varying depositional environments. The maturation of organic matter in the Pimenteiras Formation is strongly influenced by igneous intrusions. On the margins of the basin and outcrop samples, the organic matter is mostly immature, and may reach the oil generation window. On the other hand, in wells located in the central portion of the basin, the samples range from immature to mature, and reach the overmature stage mainly in samples close to the igneous intrusions. The depositional conditions at the borders and outcrop samples are mostly oxic and dysoxic, while in well samples located in the basin center, the depositional conditions vary from oxic, dysoxic to anoxic. The Pimenteiras Formation has excellent potential for gas and condensate generation due to the significant input of terrestrial organic matter in relation to marine organic matter.