Geochemical characterization of coal deposits of Candiota coalfield Rio Bonito Formation (Eo-Permian) of Paraná Basin, South Brazil.

• Main Authors: Hernani Aquini Fernandes Chaves, René Rodrigues, Marcus Vinicius Berao Ade
• Format: Article
• Language: English
• Published: Universidade do Estado do Rio de Janeiro 2018-03-01
• Series: Journal of Sedimentary Environments
• Subjects: Coal. Organic Geochemistry. Organic Petrography. Rio Bonito Formation. Eo-Permian interval. Paraná Basin. Brazil.

This work integrates organic geochemistry (total organic carbon - TOC, total Sulphur - S and Rock Eval pyrolysis data) and detailed organic petrography results of the Eo-Permian interval (Rio Bonito Formation, Paraná Basin) in the Grande Candiota area, Rio Grande do Sul State (RS), where the largest Brazilian coalfield is located. Samples were collected along the SVH-07 well and in front of the open-pit mine at Seival. The organic petrography analysis allowed to characterize the coals composition and to classify the stage of coalification. The results reveal that the Grande Candiota coals were deposited in a stormy coastal environment. As the samples present high percentage of inorganic material, the Seival coals must be considered more appropriately as coaly shales. The highest percentage of TOC was observed at the base of the coal Seams. The Upper Seam 7(US7) presents the highest average organic matter content and the Lower Candiota Seam (LCS) the lowest one. The sampled coals can be classified as humic coal due to the predominance of vitrinite, followed by liptinite and inertinite maceral groups. Analyzing each coal Seam individually, it is observed that vitrinite and organic carbon contents are higher in the base of Rio Bonito Formation, whereas inertinite and liptinite contents increase upwards. These variations encompassed a parasequence scale whose base is represented by the carbon Seam deposited at higher humidity conditions, whereas the top paleo soils are associated to longer dry periods. Considering the coal Seams as a whole, it is observed an upward decreasing of the vitrinite content and an increase of inertinite and liptinite, as well as the Hydrogen Index values. These data should be related to a retro gradational system in a transgressive context of the Rio Bonito Formation. The significant increase of sulfur content in Banco Louco Seam also suggests a greater influx of seawater into the depositional system. The Hydrogen Index values show that the coal of US7 has the highest volatile ratio. This behavior is related to the higher liptinite group content in these strata, diminishing the calorific power of this coal. Data from the organic petrography analyzes of the Seival coals were compared with those of the Candiota coalfields, for Lower Candiota and Upper Candiota Seams; it is observed that they have a certain similarity. In addition, the Seival coal organic composition was compared with other South Hemisphere Gondwanan coals, for which they have significant differences related to the higher proportions of liptinite and smaller of inertinite. The coalification stage (rank) for the Seival coals, on the basis of the vitrinite reflectance, ranged from peat to a sub-bituminous B type.