Most water produced from Cenozoic oil and gas reservoirs in the Gulf of Mexico sedimentary basin is derived from dewatering of the clastic sediments themselves. Locally, both water and solutes enter the Cenozoic section from underlying Mesozoic strata. Aqueous solutes, as distinct from the water, are derived almost entirely from extensive water/rock reactions, including dissolution/recrystallization of the Jurassic Louann Salt. Except for Cl/Br ratios in some samples, no element or isotopic ratio preserves a recognizable connate signature because water/rock interactions have been so pervasive.

Three types of water dominate Cenozoic clastic reservoirs. (1) Water more dilute than seawater (Na-acetate type) is typical of many shale-rich sections not associated with diapiric salt. The water originated as seawater, but was modified by microbial reactions (sulfate reduction and methanogenesis) and by Mg loss during shallow burial. Subsequent thermally driven water/rock interactions during deeper burial include the loss of interlayer water from clay minerals, resulting in lowered salinities, and increasing concentrations of elements such as Li, Sr, and B. Hydrocarbons produced along with this type of water are presumably also derived from shales. (2) NaCl-rich water (NaCl type) is formed in areas of extensive diapiric salt when Na-acetate water or meteoric water dissolves halite. 3) Ca-rich water (Ca-rich type), the least abundant type, originates both within the Cenozoic section by extensive albitization of plagioclase and by injection from underlying Mesozoic strata.

Most reservoirs produce water consisting of mixtures of these three types, explaining why some reservoirs are characterized by highly variable water chemistry. Some Ca-rich brines are allochthonous to the Cenozoic section. Injection of these relatively acid Ca-rich brines from deeper in the Gulf Coast section also may be accompanied by hydrocarbon migration.