Interstitial water from shales and sandstones shows a marked contrast in concentration and composition. Sidewall core shales were taken every 500 ft between 3,000 and 14,000 ft from a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the concentrations of water from normally pressured sandstones (63,000-180,000 mg/l TDS) and high pressured sandstone (16,000-26,000 mg/l TDS) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstone, but the concentrations are more similar in the high pressure zone. Shale pore water generally has a concentration order of SO₄^- > HCO₃^- > CL^-, whereas, water in normally pressured sandstone has a reversed concentration order. Water in high-pressured sandstone has low salinity with the HCO₃^- and SO₄^- concentrations being intermediate between the water from normally pressured sandstone and that from shale.

Conversion from predominantly expandable to nonexpendable 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 lithologic change to massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. These differences and correlations may have a bearing on the processes which alter subsurface waters, cause electric log responses, and could allow an understanding of the diagenesis and migration of petroleum.

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