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2018 AAPG International Conference and Exhibition

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Thermal Effects on the Neopaleozoic Reservoirs of the Parnaíba Basin, Brazil

Abstract

In the last few decades, the Parnaiba Basin is being subject of study since it was discovered its high potential on hydrocarbon exploration. On this manner, this work is intended to determine the quality of reservoir rocks thermally influenced by intrusive igneous rocks in this basin. The study area is located on a central part of the Piauí state, northern Brazil, in the eastern edge of the basin. In this area, the sedimentary rocks analyzed are composed of sandstones and siltstones of the Cabeças and Poti Formation, with Devonian and Carboniferous ages, respectively. Subordinately, the intrusive magmatic rocks are divided in two events, the Mosquito and Sardinha formations of Mesozoic ages, Jurassic and Cretaceous, respectively. The first event is related to the breakup between the North America and South America, and the second, between South America and Africa. The samples used are from drilled core and outcrops within the study area and encompasses sedimentary rocks. In these rocks, it was used fission-tracks thermochronology method on apatite and zircon minerals, and standard petrography, with the objective to integrate data of thermal history and physical properties of the rocks. The separation of apatite and zircon is relevant since they have low closure temperatures, concordant with hydrocarbon maturation temperatures. The closing temperatures of apatite and zircon vary in ranges of ca. 60-120 °C and 180-320 °C, respectively and both work as thermochronometers, corresponding with the maturation hydrocarbon window (80-225 °C). The results of fission-tracks in apatite and zircon indicated that the Cretaceous magmatic event of the Sardinha Formation was the major influence in the portion of the basin. In portions near to the intrusions, the paleotemperatures reached more than 300 °C, condition in which natural gas reservoirs are altered and degraded. These high temperatures mobilized hydrothermal fluids which altered the rocks by dissolving minerals and precipitating clay minerals in the pore spaces, reducing permeability. In portions where the thermal influence of the Cretaceous event is attenuated, in terms of maximum paleotemperature, the conditions become favorable for the reservoir rocks. Therefore, this study integrates the thermal history with physical properties of the rocks to help establishing prediction models pointing out the heterogeneities in the reservoirs.