Regional Variations in Formation Water Chemistry, Frio Formation (Oligocene), Texas Gulf Coast

Abstract

Overpressured Oligocene sandstones in Texas contain water having salinities ranging from approximately 8,000 to more than 250,000 mg/L. Water composition varies with depth and among four hydrochemical subregions. Water dominated by NaCl and having variable but locally high salinity occurs within the salt-dome province of the Houston embayment, whereas NaCl-dominated water with generally lower salinity and high concentrations of organic acids occurs along the San Marcos arch. A unique area of highly saline Ca-rich water in south Texas is separated by abrupt boundaries from the middle coast water to the north and from an area of low salinity NaCl water near the Rio Grande. Boundaries between hydrochemical subregions may reflect deep-seated structural discontinuities that re ionally control fluid movement along faults and within sandstone-rich intervals. Ionic and isotopic analyses indicate that depth-dependent changes are primarily due to the mixing of brines from three sources. High salinities are generated by the dissolution of salt, either by vertical transport of brines that evolved from deep-seated salt dissolution and high temperature rock-water interaction (yielding the Ca-rich type), or by local dissolution of salt that penetrated the Tertiary section as diapirs (yielding the NaCl type). Low salinities indicate pore-water dilution by water released from the transformation of smectite to illite, and by mineral dehydration reactions. Low-salinity, high-alkalinity water, which is associated with the extensive development of secondary porosity, commonly occurs near and immediately below the top of the geopressured zone. Thermal c nvection within the geopressured zone may be responsible for concentrating the buoyant low-salinity water (and oil) near the top of the geopressured zone.