Palynofacies of an outcrop section of shales of the Barreirinha Formation (Devonian), southern border of the Amazon Basin, Brazil: paleoenvironmental implications

摘要

© 2023 Universidade de Sao Paulo. All rights reserved.The present work aimed to investigate the palynofacies and paleoenvironment of the Barreirinha Formation in the southern part of the Amazon Basin and to examine their relationships with the organic geochemistry of previous studies. To this end, palynofacies analysis techniques and organic geochemistry results (total organic carbon and Rock-Eval pyrolysis) were applied to 23 shale samples from an outcrop section of the Barreirinha Formation (Devonian) in the southern part of the Amazon Basin, Brazil. Palynofacies analysis revealed the predominance of organic material of marine origin: amorphous organic matter (AOM) and acritarchs. The palynological associations indicate that the sedimentation of the Barreirinha Formation initially occurred in an oxic-anoxic distal marine environment, which then transitioned to an anoxic distal marine depositional environment. Integration of the results of palynofacies analysis in conjunction with data from previous studies allowed the identification of two levels of kerogen dominance: type II, in the middle to the upper part of the section, and types III and II /III in the lower part of the section. Based on the results of the organic-geochemical analyses, the material was found to have good potential for hydrocarbon production, based on TOC values that reached percentages up to 6.29, S2 reached 23.1 (mg HC/g rock), and hydrogen index (HI) with values ranging from 69 to 377 (mg HC/g rock). Tmax values ranged from 425 to 435°C, and the spore color index (SCI) ranged from 3.5 to 4.5, indicating thermal immaturity for hydrocarbon formation.