This paper presents the application of an automated image processing method for detection and analysis of pores in thin section petrographic microscopy (optical porosity). The technique is applied to pore size distribution and pore shape parameter measurements on Spanish sandstone samples before and after their exposure to CO2 under supercritical conditions (SC). The presented optical image analysis (OIA) protocol allows fast, low-cost, effective and reproducible pore size and shape quantification and comparative study in 2D. The images are acquired using a high-resolution scanner (HRS) at 6.35 μm/pixel optical resolution, providing thus preliminary data for further, more detailed studies. After the definition of the pore space, individual pores and pore throats were identified. On the segmented optical pores, parameters such as area, feret diameters, roundness, pore aspect and pore throat diameter were calculated using Fiji software. The results were used for statistical analysis and for the construction of pore size distribution curves. Quantitative analysis with the OIA-HRS method facilitated the characterization of the samples and a preliminary approximation of the CO2-rock interactions at laboratory scale. Unidad de Oviedo, Instituto Geológico y Minero de España, España