End_Page 478------------------------------

Detailed chemical analyses of 120 formation-water samples from 25 oil and gas fields in coastal Texas and Louisiana show that the salinity of water in the geopressured zones ranges from about 10,000 to 270,000 mg/L dissolved solids and may be higher or lower than the salinity of water in the overlying normally pressured zones. All the waters are of the Na-Cl type; Na generally constitutes more than 90% of the total cations and Cl constitutes more than 90% and up to 99.8% of the total anions. Ca concentrations increase with increasing salinity and bicarbonate increases with decreasing Ca concentrations. Magnesium and sulfate concentrations are generally low. The concentrations of copper, lead, and other heavy metals are generally less than 10 µg/L.

Hydraulic fluid potentials and ^dgrD and ^dgr¹⁸O values indicate that the formation waters are most probably modified connate waters representing the original marine water of deposition. The chemistry of these waters, however, is markedly different from that of ocean water. The differences in composition are shown to result from (1) interaction of the waters with evaporites, (2) interaction of the waters with minerals and organic matter present in the enclosing sedimentary rocks, and (3) membrane-squeezing and membrane-filtration properties of shales.

End_of_Article - Last_Page 479------------