For the geological storage in Japan, it is important to evaluate the capillary sealing efficiency of relatively younger sediments. The capillary sealing efficiency is influenced by the interfacial tension between gas and water system and by the representative diameter of the pores. As for the fine-grained sediments in Japan, the porosities are large, while the representative diameters of the pores are relatively small. These gaps are caused by the micro fabric of the fine-grained sediments such as clay that was formed during sedimentation in marine. Also, a case study was done for the storage capacity and leakage probability in Osaka Bay area. The results revealed that multiple sealing layers, formed under glacio-eustatic sea-level fluctuations, would prevent the CO 2 leakage during the millennium.