Assessing basaltic intrusive rocks as caprock candidates for CO₂ geological storage: Insights from the Rio Bonito Formation, Paraná Basin, Brazil

Basaltic rocks have emerged as promising targets for the geological storage of carbon dioxide (CO₂) due to their global distribution and their capacity to promote mineralization reactions that permanently immobilize CO₂ as stable carbonates. The Early Cretaceous Paraná Magmatic Province (PMP), one of the largest continental igneous events on Earth, comprises extensive basaltic flows and intrusive bodies (dikes and sills) known as the Serra Geral Group in Brazil. In addition to lava flows, intrusive rocks emplaced along the sedimentary sequence may act as effective caprocks, playing a crucial role in reactive CO₂ storage systems. In southern Brazil, several studies have identified the Upper Permian Rio Bonito Formation (RBF), in the Paraná Basin, as a suitable siliciclastic saline-aquifer reservoir for CO₂ along the Torres Trough (TT). This formation is intruded by multiple basaltic bodies, whose geological and petrological characterization can provide key insights into their suitability as sealing units within a CO₂ storage system. In this context, this study aims to characterize the petrographic and geochemical features of basaltic intrusive bodies hosted in the RBF using analog outcrops from the Criciúma region. These exposures, when integrated with subsurface data from stratigraphic wells in the TT, provide a robust basis for understanding the geometry and extension of such bodies. The integrated analysis of RBF characteristics together with basaltic intrusions is innovative and seeks to build a holistic understanding of the reservoir–seal system for CO₂ storage. The methodology combines detailed geological mapping of the basaltic rocks, petrographic analysis with whole-rock geochemistry. Major and minor oxides were determined by X-ray fluorescence (XRF), whereas trace and rare earth elements were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). The geological data points to the occurrence of expressive sills and rare dikes. Petrographic descriptions focused on quantifying mineral and alteration phases, textures, and mineral compositions that influence rock reactivity with injected CO₂. Preliminary results reveal a mineral assemblage dominated by calcic plagioclase, augite, opaque minerals, and apatite, with intergranular and micrographic mesostasis. These mineral phases are known for their high reactivity in mineral carbonation processes, particularly in basaltic systems subjected to pressure-temperature conditions compatible with geological CO₂ storage, such as those in the TT. Moreover, the variable degrees of alteration observed among samples suggest heterogeneities in permeability and porosity, critical parameters for understanding fluid-flow dynamics and carbonate precipitation in the subsurface. The results contribute to identifying key features necessary to evaluate the potential of basaltic sills as sealing units. This study thus represents an initial step in the characterization of basaltic rocks intruding the RBF and highlights the importance of integrating geology, petrography, geochemistry, and experimental approach to support carbon capture, utilization, and storage (CCUS) strategies in the context of climate change mitigation.