Interactions of Iron and Associated Minerals with CO₂ in Paleozoic Sandstones of Saudi Arabia

Geological sequestration of CO2 has become a promising approach for mitigating anthropogenic carbon dioxide emissions to the atmosphere, and some of the primary storage sites consist of sedimentary basins, depleted hydrocarbon reservoirs, and saline aquifers. A key factor in the long-term stability and effectiveness of CO2 storage is the study of the geochemical interaction between injected CO2, the minerals associated in the reservoir, and the present brine in the formation. These geochemical interactions could influence the porosity, permeability, mineralogy, and overall reservoir integrity. Although iron-bearing minerals are often present as associated minerals in sandstone reservoirs, their reactivity remains poorly understood. This study investigates the interaction between carbonated water (formed by the dissolution of CO2 into brine during injection and CO2 plume migration) and iron-bearing minerals within the cement of the Silurian–Devonian Tawil Formation (Samra Member) in Saudi Arabia. Two core samples from this formation were characterized and then matured with carbonated water for 30 days to assess the reactivity of iron under the acidic conditions typical of carbonated water. Laboratory analyses included micro-CT scanning, X-ray diffraction (XRD), X-ray fluorescence (XRF), and thin-section petrography to evaluate the mineralogical and porosity changes after the maturation process. Micro-CT scan results show that the iron content and distribution remained stable in the before–after exposure. However, a significant increase in porosity, which almost doubled the preexposure values, was observed due to the dissolution of calcite and clay minerals that are also associated with the sandstone cement. These findings suggest that while iron cement may not be significantly affected during CO2 sequestration, the dissolution of other cementing phases could enhance reservoir quality in similar storage formations. Finally, this study provides insights into the geochemical behavior of the Samra Member of Tawil Formation under CO2 injection and offers an analogue to similar subsurface storage elsewhere worldwide.