Sequence Stratigraphy and Coal Petrology Applied to the Early Permian Coal-bearing Rio Bonito Formation, Paraná Basin, Brazil

Abstract

The coal-bearing Early Permian succession of the Paran Basin in southernmost Brazil is linked to a third-order depositional sequence, where the most important coals occur in the initial transgressive systems tract. In the Candiota area, the main coal zone consists of 17 seams, which were analyzed for petrographic properties (macerals, gelification index, tissue-preservation index, vitrinite reflectance). These results are compared to the high-resolution sequence-stratigraphic framework to enhance our understanding of the stratigraphic controls on coal formation, coal distribution, and coal quality, providing guidelines for optimal exploitation.The results show that local changes in accommodation trends and high sediment influx practically preclude coal formation in the lowstand and highstand systems tracts, whereas major coal development occurred in the transgressive systems tract. Seam distribution and thickness are controlled directly by flooding events, as depicted by the parasequences mapped in the study area. The main variations in thickness and extent occur at, or close to, the parasequence bounding surfaces. The most important coals, which are as much as 2.50 m in thickness, occur in the initial transgressive systems tract.Coal petrographic parameters suggest an overall drying-upward trend in the coal seams developed in the upper part of third-order sequence 2, with significant differences of coal properties relative to their stratigraphic position between and within the parasequences. Detailed petrographic analysis of seam subsections indicates a transgressive nature for the thick coal seams occurring in parasequence 4 (CCI and CCS seams). These are characterized by decreased vitrinite reflectance at the base and top of the coal seams. The high inertinite content of the overlying BL seam at the top of PS 4 suggest accumulation of the precursor peat in a regressive phase of the parasequence.