AbstractTo make sure that CO2 is safely and securely stored in the reservoir, only way for us to know is to monitor the injected CO2. In most of the injection sites, monitoring is conducted to understand the behavior and distribution of CO2. Also we need to estimate the volume of CO2 quantitatively. In order to estimate the quantitative volume, we need to compute the volume from physical parameter like P-wave velocity or resistivity. For the P-wave velocity, recent studies show the way of computing saturation using Gassmann’s theory. But due to the study of Nagaoka, response of P-wave velocity becomes weak in high saturation. Because of the CO2 saturation more than 40% happening in the reservoir, seismic survey needs to be supplied by another theories. In this study we selected resistivity monitoring to overtake the weakness of seismic monitoring. By conducting series of resistivity measuring experiment of laboratory scale, we considered the saturation computing equation which can be thought as a formula to calculate CO2 saturation in the field. By using these equations computation of CO2 saturation in Nagaoka was conducted. By comparing several equations which can be thought to estimate the CO2 saturation in field data, we suggested a simple equation formed by resistivity and shale volume. By using this suggested equation, computational result showed good match to the estimated saturation computed from neutron porosity.