The article describes the changes taking place in the rock matrix (reservoir and cap rock) due to the effect of carbon dioxide in the presence of brine. The experiment was carried out on samples of Lower Jurassic sandstones (reservoir rocks) and claystones (cap rocks) from Chabowo 1 and Chabowo 3 boreholes, Chabowo Anticline (NW Poland) a potential structure for underground storage of carbon dioxide. Rock samples were placed for a period of 18 months in a designed apparatus system and the experiment was performed under the conditions of T = 25 degrees C and P = 6 MPa, by soaking the rock samples in a brine of simplified chemical composition similar to the brine present in the rock formation analysed. Mineral composition was determine by observation in transmitted light, XRD and SEM-EDS. In order to perform the changes in the individual mineral phases, using the SEM-EDS method, the results of mineralogical and petrographic observations of the same rock samples before and after the experiment are presented. Dissolution and precipitation processes of minerals were observed as the result of the experiment. Corrosion-prone minerals were feldspars, etchings were observed on pyrite and mica grains. The dominant mineral phase formed after the experiment was halite (which caused the precipitation of this mineral after the evaporation of brine). Precipitation of poorly developed kaolinite crystals was also observed. The long-term experiment conducted at low temperature and pressure showed similar changes that are obtained in the experiments carried out for the actual reservoir conditions, but on a smaller scale. (C) 2014 Elsevier B.V. All rights reserved.