Convergence Between Hydropressured and Geopressured Zones in the Wilcox Group,

Central Texas Gulf Coast

By

DUTTON, ALAN R.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

HARDEN, ROBERT

R. W. Harden and Associates, Inc., Austin, TX,

KIER, KATHERINE S.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Near its outcrop in Central Texas, the Wilcox Group and overlying Carrizo Formation make up a major aquifer. Farther downdip the Wilcox Group contains hydrocarbons and saltwater having fluid pressures that increase to geopressured levels. Hydraulic head decreases from ~500 ft at the aquifer outcrop to ~200 ft at the downdip limit of freshwater then increases to >3,000 ft at the updip limit of geopressure. Salinity increases from <400 mg/L near the outcrop to ~3,000 mg/L at the downdip limit of freshwater and varies with the transmissivity and interconnectedness of sandstone. Salinity increases farther downdip by almost 2,000 mg/L per mile to >100,000 mg/L.

A convergence zone occurs where meteoric freshwater moving downdip from the aquifer outcrop encounters modified-connate saltwater from the geopressured zone. Water recharged at the outcrop discharges updip of the hydraulic-head minimum. Further evidence of the convergence zone is the accumulation of hydrocarbons near the hydraulic-head minimum.

Although most groundwater-flow models of the Carrizo-Wilcox aquifer have a downdip boundary near the freshwater limit, they allow groundwater exchange with the deeper subsurface. Setting the downdip boundary at the convergence zone, where the horizontal gradient should be near zero, more closely reflects a hydrologic boundary and may improve model results. A steady-state model boundary within the transition zone may be used to evaluate hydrogeologic controls on the convergence zone.