Mineralogical and geochemical changes in conglomerate reservoir rocks induced by CO 2 influx at Mihályi-Répcelak natural analogue, NW-Hungary

A temporary solution to massive anthropogenic CO 2 emissions can be the capture of industrial CO 2 from flue gas and sequestering it in geological formations. For safe and effective storage of CO 2 , interaction processes in the rock-pore fluid–CO 2 system should be known. Investigation of natural CO 2 accumulations provides valuable examples to what physical and chemical effects could be expected during CO 2 influx at future CO 2 storage sites. One of the key controlling factors of the processes occurring in natural CO 2 reservoirs is the lithology of the storage rocks, which is primarily determined by the formation conditions of these rocks. In this respect, the lithologies of individual CO 2 accumulation areas influence the processes between the host rock, the pore fluid, and the CO 2 in different ways. In the current study, we focus on a well-studied natural CO 2 storage reservoir, namely the Mihályi-Répcelak area, NW Hungary. We provide insight into the so far unstudied conglomerate reservoirs that represent a stratigraphically deeper reservoir unit with significantly different lithology and pore water compositions compared to the sandstone reservoirs. Our results indicate that dawsonite /NaAlCO 3 (OH) 2 / formation also affected the conglomerate reservoirs, which indicates that at least part of the CO 2 could be trapped in mineral form. An important role of salinity in reducing the CO 2 mineral trapping capacity of the storage system is also demonstrated. Furthermore, H isotope analysis of diagenetic kaolinite was applied to trace the origin of the pore water that was present during the rock formation. Based on the data, dawsonite formation was induced by the flux of meteoric water that infiltrated during a warm and humid period and mixed with ascending CO 2 .