The aim of this article is to describe and interpret qualitative and quantitative changes at the rock matrix scale of Cretaceous sandstones (northern Spain) exposed to supercritical CO2 and brine. Experimental CO2-rich brine injection was performed in a reactor chamber under realistic conditions of deep saline formations (P ≈ 7.8 MPa, T ≈ 38 °C and 24 h exposure time). SEM and optical microscopy, aided by optical image processing and chemical analyses were used to study the rock samples.