The geochemical characteristics of deep groundwater, or formation water, are essential in all processes of geochemical trapping in an open aquifer CO2 storage. We have been constructing a database of groundwater chemical compositions in deep aquifers in Japan (“Formation-water database”). The database have two major objectives; (1) to be a dataset on groundwater of reservoir depths for evaluating CO2 solubility; and, (2) providing model water compositions for geochemical modeling and experiments in our study of underground CO2 storage. More than 2600 datasets are collected from literature on geochemistry of groundwater reported from 10 selected areas in Japan; the areas of investigation include populated cities in which large point sources of CO2 are located. The accumulated data indicate that groungwater of reservoir depth (>800m) is generally dilute in composition compared to average seawater, suggesting a high potential of CO2 solubility. Systematic geochemical differences are also observed between groundwater hosted in marine and freshwater sediments.