POROSITY EVOLUTION IN CRICIÚMA COAL BEDS OF RIO BONITO FORMATION, PARANÁ BASIN

The largest coal production area in Brazil is located in Criciuma, state of Santa Catarina, has been studied for a long time but large gaps remain in the understanding of evolution of porosity. Understanding the dynamics of porosity is essential for assessing how the coal-bearing unit influences the behaviour of the aquifer in the region. In this context, this work seeks to contribute to understanding these aspects that are still unclear. This study integrates petrographic analyses of 21 thin sheets obtained from samples of the Rio Bonito formation collected in drilling mining areas. We also review the unit's chemical characterization data, based on the analysis of associated groundwater. Petrographic analysis indicated the predominance of quartz sandstones, with an absence of carbonate cement and a marked presence of local lithological alteration, evidenced by intense silicification of the unit. The cement observed is essentially composed of silica, responsible for almost completely filling the intergranular spaces; a few pores remain with estimated values around <10.0 %. Basic intrusions and a few acidic intrusions may contribute to the closing of porosity. As a consequence, the effective porosity observed in the samples is practically zero. This decrease in porosity has large impacts on the storage and mobility of fluids in these rocks. The coal contamination dynamics in the region are directly influenced by secondary porosity developed by mining activities. Acid mine drainage is primarily generated by coal mining and has a greater environmental impact due to the percolation of water through waste deposits and underground mines, where the interaction between water and sulfide minerals promotes the formation of contaminated acidic waters.