In main oil and gas fields of Japan, an extensive investigation of subsurface water including interstital water in mudstone has been in progress for over ten years for the explration of oil and gas deposits. In a result of geochemical consideration of analytical deta, the writer suggests as follows. 1. Cl- and SO42- concentrations in brine are usually lower than those in Recent sea water, especially SO42- concentration is extremely low in case of brine. It is suggested that these evidences are resulted from microbiological activites in the initial sedimentary environment, reaction between swelled clay-mineral and original interstitial water in the process of the geological genesis, and dilution by rain water. 2. Dissolved hydrocarbons and naphthenic acids in brine are direct indices, I- and HCO3- are indirect indices for oil and gas deposits. The above facts are useful for exploration for oil and gas deposits.