Paleoenvironmental characterization of a Lower Cretaceous section of the Recôncavo Basin, Bahia, Brazil

Abstract

The objective of this work was to evaluate the paleodepositional conditions and potential for the hydrocarbons generation of the outcropping shales in the Southern Compartment of the Recôncavo Basin, indicative of the Candeias and Maracangalha Formations (Lower Cretaceous), from the organic geochemical characterization and analysis of palinofacies. For this, 23 samples of outcrops were collected near the cities of Santo Amaro, São Francisco do Conde and Simões Filho, State of Bahia, Brazil. All samples were analyzed for total organic carbon (TOC), total sulfur, insoluble residue, Rock-Eval pyrolysis, whole oil chromatography, stable isotopes of oxygen, organic and inorganic carbon, saturated biomarkers and palynofacies, performed at the Center of Excellence in Geochemistry of the Institute of Geosciences of the Universidade Federal da Bahia (Lepetro), Brazil. The parameters provided by the geochemical analyzes indicate significant differences between the outcrops under study, mainly regarding the quantity and the quality of the organic matter, thus configuring a difference in the conditions of production and preservation of the organic matter during the time of deposition of the shales. There are layers rich in organic carbon, containing well-preserved type I kerogen, indicating anoxic depositional conditions. In contrast, poor layers of organic content, containing residual kerogen (type IV), occur, reflecting oxidizing conditions of the depositional environment. The observed differences suggest that the variability of the geochemical properties of the organic matter reflects the climatic variations occurred during the deposition of the sediments of the Candeias and Maracangalha Formations. As a consequence, there are levels with potential for generation and levels with no potential for hydrocarbon generation.

Geoquímica Orgânica de Folhelhos da Formação Pojuca, Bacia do Recôncavo, Brasil

“…Já as amostras do Grupo B marcariam a transição entre as duas condições, ocorrendo mais influxo de terrígenos e consequente maior oxigenação no lago, em comparação com o Grupo A, porém ainda com boas condições para preservação da matéria (Figura 7). Tal interpretação está compatível com o proposto por Amaral et al (2020), onde o autor relata que, ao analisar amostras da Formação Candeias, os folhelhos negros enriquecidos em matéria orgânica estão associados a deposições em clima árido (reduzido influxo de água, com condições propícias à estratificação do corpo d'água, ao desenvolvimento de anoxia de fundo e à consequente preservação da matéria orgânica), enquanto as camadas menos enriquecidas em matéria orgânica estariam relacionadas a deposições em clima úmido (que possibilita maiores descargas fluviais, proporcionando a circulação e consequente oxigenação da massa d'água).…”

Lake level oscillations recorded in Buracica Stage (Barremian, Early Cretaceous) of Recôncavo Basin, Bahia State (N.E. Brazil)

“…According to these authors, the sediments of the Candeias Formation, represented by a fluvial-deltaic-lacustrine system and making part of the rift sequence, developed during the Lower Cretaceous. According to Amaral et al (2020), Candeias Formation corresponds to sediments deposited in a deep and stratified lake environment under the prevalence of arid climate. During this phase, an anoxic lacustrine environment should have developed (Balbinot and Kalkreuth 2010).…”