Brazilian Journal of Geology (Jul 2020)

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

  • Diego Nery do Amaral,
  • José Roberto Cerqueira,
  • Consuelo Lima Navarro de Andrade,
  • Hélio Jorge Portugal Severiano Ribeiro,
  • Karina Santos Garcia,
  • Flávia Lima e Cima Miranda,
  • Olívia Maria Oliveira,
  • Antônio Fernando Queiroz,
  • Luiz Carlos Lobato dos Santos

DOI
https://doi.org/10.1590/2317-4889202020190058
Journal volume & issue
Vol. 50, no. 3

Abstract

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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.

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