Some of the geochemical relationships that exist between subsurface fluids and the associated geologic strata in petroleum-producing areas of Louisiana have been determined. Samples of petroleum- associated water were obtained from 126 wells in Louisiana that produced from sediments of three geologic ages: Tertiary, Cretaceous, and Jurassic. The samples were transported to a central laboratory for analysis of chemical composition, using standard methods, to determine if the compositions varied with the geologic strata of their source and, if so, why. The dissolved constituents were compared with those of a brine that was concentrated by evaporation. The concentrations of bromide, calcium, and strontium were useful in distinguishing the brine produced from sediments of a late age from those of an earlier age. The concentration ratios of total equivalent magnesium and bromide indicated that dolomitization probably had occurred and altered the composion of the brines. A principal-components analysis of the data indicated an inverse relationship between depth and concentrations of brimide, strontium, calcium, and lithium.