Supercritical CO2 fluid is capable of extracting organic matter from rocks. Although the sealing performance of cap rock is essential for CO2 underground storage, typical cap rock such as shale or mudstone usually contains organic matter. To determine how well cap rocks can seal supercritical CO2, cap rock samples from the Nagaoka injection test site were treated with supercritical CO2, and then the porosity and pore radius distribution of the samples were measured by mercury porosimetry. The results showed that the pore radius distributions slightly shifted to a larger size after treatment, while the porosity and permeability changed less than 1%. (c) 2004 Elsevier Ltd. All rights reserved.