The origin of aqueous sulfate in Frio pore fluids and its implication for the origin of oil field brines

The O and S isotopic composition of aqueous S04 in saline formation waters from the Frio Formation of S Texas can be used to deduce the origin of the S04. The average O and S isotopic composition of aqueous S04 in Frio brines is + 13.4‰ SMOW and + 8.2‰ CDT, respectively. The O isotopic compositions fall within the range of Mesozoic and Cenozoic seawater values and are interpreted to be derived from dissolved Jurassic evaporites. The S values appear to have been modified by the addition of light S derived from the oxidation of pyrite. We conclude, therefore, that some of the chemical components in Frio brines are derived from water-rock interactions that occurred in much deeper units and that these components have migrated into the Frio.