Interstitial water from shales and sandstones shows a contrast in concentration and composition. Sidewall cores of shales were taken every 500 ft between 3,000 and 14,000 ft in a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the total dissolved solids concentrations in waters from normally pressured sandstones (600-180,000 mg/l) and highly pressured sandstone (16,000-26,000 mg/l) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstones, but the concentrations are more similar in the high pressure zone. Shale water generally has a concentration order of SO4 = > HCO3- > Cl-, whe eas water in normally pressured sandstone has a reversed concentration order. Conversion from predominantly expandable to non-expandable clays accelerates near the top of the high pressure zone, which appears correlative with a major temperature gradient change, an increase in shale porosity (decrease in shale density), a lithology change to a massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. The data presented suggest that the clays subjected to diagenetic change release two layers of deionized water and that this released water may be responsible for the lower salinity of the water found in the high pressure section.