THE ORIGIN OF AQUEOUS SULFATE IN FRIO PORE FLUIDS AND ITS IMPLICATIONFOR THE ORIGIN OF OIL-FIELD BRINES

The O and S isotopic composition of aqueous SO4 in saline formation wa ters from the Frio Formation of S Texas can be used to deduce the orig in of the SO4. The average O and S isotopic composition of aqueous SO4 in Frio brines is +13.4 parts per thousand SMOW and +8.2 parts per th ousand 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 oxidati on of pyrite. We conclude, therefore, that some of the chemical compon ents in Frio brines are derived from water-rock interactions that occu rred in much deeper units and that these components have migrated into the Frio. Copyright (C) 1996 Published by Elsevier Science Ltd.