Hydrocarbon source potential and paleodepositional environment of the (Devonian) Barreirinha formation on the south edge of the Amazonas basin border, Brazil

Abstract

The present work aims to study the depositional paleoenvironment and the hydrocarbon source potential of the (Devonian) Barreirinha Formation, in the southern portion of the Amazonas basin. To accomplish this task, the geochemical parameters (total organic carbon, Rock-Eval pyrolysis, total sulfur and total nitrogen, whole oil hydrocarbons, and biomarkers) and palynofacies of 22 samples distributed in three outcrops are studied. Total organic carbon (TOC) values in the collected samples vary between 1.42% and 5.56%, indicating great variation in the production and preservation of organic matter. The sulfur content, ranging from 0.05 to 2.90%, indicates deposition in a marine environment. Rock-Eval pyrolysis parameters also reflect variations in the depositional conditions of this formation. The potential for hydrocarbon generation varies from low to excellent, noting that the samples that have the highest values of generation potential (S2) have higher TOC levels. This characteristic is justified by the good quality of the organic matter, since such samples have high hydrogen index (HI) values and low oxygen index (OI) values, indicating type I/II kerogen. The low free hydrocarbon (S1) values present in the samples (<0.5 mg HC/g rock) indicate immaturity, as shown by the Tmax (<440 °C) and by the production index (PI < 0.1). For total oil hydrocarbon chromatographic analysis, the largest contribution of algal organic matter is explicit for terrestrial-aquatic reasons (TAR <1). Furthermore, the pristane and phytane isoprenoid ratios (Pr/Phy) indicate differences between anoxic and oxic environments. Saturated biomarkers allow us to state that the organic matter present in the samples has a marine origin with terrestrial contributions and low thermal maturation. Palynofacies analysis shows that there is a predominance of amorphous organic matter (AOM), with a moderate amount of palynomorphs and subordinate amounts of phytoclasts. In the ternary diagram, these percentages allow us to infer that the depositional paleoenvironment was a distal platform with oxic-anoxic variation.