Devonian strata of the Amazonas Basin: Organic geochemistry and palynofacies of outcrops of the Barreirinha Formation’s shales - Potential for generation of hydrocarbons

Abstract

During the Upper Devonian, a thick package of strata rich in organic matter belonging to the Barreirinha Formation developed in the Amazon Basin. Twenty-six samples collected from two outcrops on the southern edge of the basin, were studied using Rock-Eval pyrolysis and palynofacies, and interpreted using inferential and multivariate statistics. The total organic carbon (TOC) values vary from 1.47 to 9.50%, indicating good to excellent production conditions of organic matter. The values of the generating potential (S2) range from 3.92 to 33.98 mg HC/g, denoting a generating potential that falls between moderate to excellent. These samples present high hydrogen index (HI) and low oxygen index (OI) values, which indicates that type II kerogen is predominate. The low free hydrocarbon values (S1) and the maximum pyrolysis temperature (<440 °C) may be associated with their thermal immaturity. The organic component analysis shows a predominance of amorphous organic matter (AOM) and palynomorphs and marine components predominate over terrestrial components, indicating deposition within a marine environment.

Cluster analysis established two palynofacies correlated to stratigraphic intervals associated with sea-level variability. The ternary diagrams of organic components indicate that the depositional paleoenvironment was that of a distal platform that varied between oxic and anoxic conditions.