Avaliação do potencial gerador de hidrocarbonetos e do paleoambiente deposicional da Formação Pirara, Bacia do Tacutu, Brasil.

Abstract

The present research focused on the geochemical study of rock samples (siltites, marls and shales) from the Pirara Formation, from the core obtained in the R-B0-SR-01 well drilled in the Tacutu Basin, Roraima State, Northern Brazil. This study is justified by the fact that, despite the volume of subsurface data (geophysical methods) and the few geochemical data (surface geochemistry) about the basin, a broader and more robust geochemical characterization of the organic matter is necessary for a better evaluation of the effective potential of the Pirara Formation for the generation of oil. Following this proposal, the research had as main objective: to geochemically characterize the rocks of the Pirara Formation regarding the potential for the generation of hydrocarbons and to estimate the thermal evolution and the conditions of the paleoenvironment of deposition. The studies involved analyzes of Total Organic Carbon (TOC); Pyrolysis-Rock-Eval (S1, S2, S3, Tmax, HI and OI); gas chromatography of Total Organic Extract (whole oil); saturated biomarkers and stable carbon isotopes. This research showed that: the levels of TOC, in the analyzed samples, are between 0 and 7.78%. The insoluble residue (RI) values ranged between 23.32% and 94.34%. The Rock-Eval pyrolysis data provided distinct characteristics among the analyzed samples, which, in general, showed an oil potential that varied from poor to very good, but only five samples had good oil potential, despite being considered residual values. due to high maturation. The values of IH and IO, despite being also residual, indicate the existence of types I, II and III kerogens. The gas chromatography analyzes of the organic extracts showed results with chromatogram patterns that represent light oil, partially biodegraded petroleum, and biodegraded petroleum. The percentages of the oil fractions (saturated, aromatics and NSO compounds) suggest that the organic matter of the samples from the Pirara Formation has a tendency of high thermal maturity (high percentage of the saturated fraction) and high degree of biodegradation (high percentage of the fraction NSO) for some samples. The results of the analysis of saturated biomarkers in all samples show features that suggest very high thermal maturity. The diagnostic reasons of the saturated biomarkers suggest that the organic matter that gave rise to the organic extracts is mostly from marine planktonic organisms (algae and bacteria) and lesser contribution from terrestrial organic matter. The depositional paleoenvironment is characterized as a transitional-marine environment with variable and variable salinity and suboxic/anoxic condition. The sedimentation was of the mixed type (siliciclastic and carbonate) and the kerogen has thermal maturity referring to the late window of oil generation. All samples present biomarkers indicative of biodegradation, but with evidence of mixing with oil generated by the same source rock at different times.