Abstract: Devonian-Carboniferous Petroleum Systems From Brazilian lntracratonic Basins

Gonzaga, F. G. and Gonçalves, F. T. T. - Petrobras/E&P

A multidisciplinary study, involving geochemical, geological and geophysical data allowed a better understanding of the petroleum systems from Solimões, Amazonas, Parnaíba and Paraná basins (Fig. 1). These basins are large intracratonic sags, filled with Paleozoic and Mesozoic sediments and Mesozoic basic igneous rocks. In Solimões, Amazonas and Parnaíba basins the igneous event is represented by diabase dykes and sills of Triassic-Jurassic age (200M.y.). In Paraná Basin the magmatism event comprises diabase intrusions (dykes and sills) and mainly, thick layers of basalts of Early Cretaceous age (1 32M.y.).

Four petroleum systems were characterized: Jandiatuba-Juruá (!) (Solimões), Barreirinha-Curiri (!) (Amazonas), Pimenteiras-Cabecas (!) (Parnaíba) and Ponta Grossa-Itararõ (!) (Paraná). In the Solimões, Amazonas and Parnaíba basins the source rocks are Late Devonian (Frasnian/Fammenian) black shales with high TOC contents and marine type II organic matter. In Paraná Basin the source rock comprises Early to Late Devonian shales with middle to high TOC contents and type II/III organic matter.

The main reservoirs comprise Late Devonian (Parnaíba) and Late Pennsylvanian to Early Permian (Paraná) shallow marine sandstones, Early Pennsylvanian eolian sandstones (Solimões), and marine sandstones deposited within incised valleys under glacial conditions of Late Devonian age (Amazonas). The seals are mainly evaporites (Solimões), shales and diamictites (Amazonas), and shales and diabase sills (Parnaíba and Paraná). In Solimões, Parnaíba and Paraná basins the traps are of structural nature and Jurassic to Cretaceous age. In Amazonas Basin the petroleum is accumulated in stratigraphic traps of Late Devonian age.

Basin Modeling studies revealed that the burial history in Solimões, Parnaíba and Paraná basins, although sufficient to start hydrocarbon generation during the Permian, was not enough to account for the observed maturity stage. Hence, the additional heat associated with the igneous intrusions was responsible for the generation and expulsion of an expressive amount of hydrocarbons from the source rock and has cracked the previously trapped petroleum to condensate and gas. In Amazonas Basin, kinetic modeling shows that source rock maturity is mainly controlled by the subsidence history. Barreirinha Formation source rock started to generate oil during the Late Carboniferous/Permian and most of the petroleum expulsion was completed by the Early Triassic, with maximum transformation ratios and expulsion efficiencies of 100% and 55% respectively, in basin depocenter. Heat effect of igneous intrusions played an important role only in areas were the dykes and seals are close to the source rock.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil