Geochemical characterization of selected organic-rich shales from the Devonian Pimenteiras Formation, Parnaíba Basin, Brazil – Implications for methane adsorption capacity

The Parnaíba Basin in northern Brazil is one of the largest Paleozoic basins of South America and is believed to have a high potential for unconventional hydrocarbon generation and storage (shale gas). It comprises an extensive volcano-sedimentary sequence from which the Pimenteiras Formation strata (Devonian) is considered to have the highest potential as a petroleum/gas source rock. The present paper evaluates the methane adsorption capacity of the organic-rich shales of this formation using 12 samples collected from the vertical profiles of three wells. The samples were analyzed by geochemical and petrographic techniques as well as by methane adsorption capacity experiments. The total organic carbon (TOC) content varies from 1.57 to 16.6 wt% in organic richness and positively correlates with gas adsorption capacity. The gas adsorption capacity of the samples increases with a pressure between 10 and 15 MPa, with a slight decrease of the adsorbed gas up to the higher and final experimental pressures around 20 MPa. Results of Rock-Eval pyrolysis analysis indicate kerogen types II and III resulting from a mixture of marine and terrestrially derived organic matter. The type of organic matter (mainly kerogen type III) contributes to aromaticity and provides higher gas adsorption capacity. The thermal maturity of the shales varies from immature to mature according to vitrinite reflectance (0.41–0.63% Rrandom) and Tmax (373–442 °C) data. However, in many samples no notable correlation was found between adsorption capacity and thermal parameters in this study. The clay minerals can contribute to high gas adsorption capacity in Devonian shales while the high ash contents had a negative influence on the gas adsorption capacity. Even though some anomalously low values were recorded in some samples, the Pimenteiras Formation shales analyzed in this study suggest a good gas adsorption capacity.