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 Reconcavo 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, Sao Francisco do Conde and Simoes 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.