Geochemical characterization of the organic matter in the Devonian Pimenteiras Formation, Parnaiba Basin, Brazil – Implications for depositional environment and the potential of hydrocarbon generation

The Paleozoic-Mesozoic Parnaiba Basin is an intraplate volcano-sedimentary basin from which the Pimenteiras Formation (Devonian) has potential for hydrocarbon generation, mainly natural gas. The thermal influence of igneous intrusions contributes to hydrocarbon generation in the basin. This present paper provides a geochemical characterization of the organic matter in the Devonian Pimenteiras Formation, Parnaiba Basin, Brazil – Implications for depositional environment and the potential of hydrocarbon generation using geochemical and petrographic parameters. A total of 60 shale samples of the Pimenteiras Formation from three wells underwent analysis for TOC content, Rock-Eval pyrolysis, x-ray diffraction, organic petrology and palynofacies. The TOC content varies from 1.54 to 16.60 wt%, with the variations being related to different conditions of accumulation and preservation of the organic matter. The S1 values range between 4.20 and 51.43 mg HC/g rock, with an average of 14.9 mg HC/g rock, and S2 varies between 3.41 and 16.28 mg HC/g rock, with an average of 7.3 mg HC/g rock, showing very good to excellent source rock potential. The low Tmax values with an average of 430 °C indicate immaturity despite the high S1 and S2 values. The high S1 values can be related to natural impregnation or contamination by drilling fluids. The HI vs Tmax plot showing the presence of kerogen type II and III, with potential for oil and gas generation. The mineral assemblage is associated with diagenetic or thermal processes (illite) and oxidation conditions (hematite and pyrite). The talc and chlorite can be related to metamorphism and provenance, while the calcite and gypsum indicate evaporitic conditions. The vitrinite reflectance varies between 0.31 and 0.67% Rrandom (average of 0.55% Rrandom). Thus, the samples can be classified as immature and mature and are consistent with Tmax values. The palynological records show a significant presence of phytoclasts and variations of palynomorphs along with the sequences from top to bottom, suggesting a transitional depositional environment (marine and continental) associated with the Devonian transgressive-regressive cycles. The data reinforce that the Pimenteiras Formation has the potential for hydrocarbon generation, mainly gas.