Avaliação do paleoambiente deposicional dos folhelhos da formação Barreirinha, Bacia do Amazonas, município de Rurópolis, Brasil

Abstract

The Barreirinha Formation consists of black to dark-gray shales with high content of organic matter and is considered to be the main rock source of the Amazon Basin. In this context, the present work proposed to carry out the palynological and geochemical study from three samples collected at the outcrop along the Tapajós River, located in the city of Rurópolis, PA. The results of total organic carbono (TOC), total sulfur and insoluble residue (IR) indicate siliciclastic sedimentation, high amount of organic matter deposited under low oxidation conditions, reflecting good and excellent conditions of production of organic matter. According to the Rock-Eval pyrolysis data, the potential generator (S2) ranged from 4,23 to 27,74 mg of HC/g of rock indicating a good to excellent hydrocarbon generating potential. On the other hand, the data of Tmax got values < 440°C, associated with the production index (PI) with values < 0,1, indicated thermally immature rocks. About the type of matter organic, according to the Van Krevelen diagram, all samples were defined as type II kerogen, derived from marine source appropriate for the generation of gas and condensate. On the other hand, in the analysis of the kerogen (palynofacies), a high concentration of amorphous organic matter was observed, followed by palynomorphs and subordinate phytoclasts. Taking these aspects into account and the compilation of organic and microscopic geochemistry techniques, the depositional paleoenvironment of the shales of the Barreirinha Formation was defined as euxinic distal marine and from the Tyson diagram were deposited in a basin in suboxic condition.