WHERE RECHARGING METEORIC WATER MEETS SEAWATER AND BASINAL BRINE: THE CONVERGENCE OF HYDROPRESSURED AND GEOPRESSURED ZONES BENEATH THE CENTRAL TEXAS GULF COASTAL PLAIN

Near its outcrop in Central Texas, the Wilcox and Claiborne Groups make up a regional aquifer system under hydropressured conditions with hydraulic head (250 to 500 ft msl) decreasing downdip from the aquifer outcrop. Downdip of the freshwater aquifers, these groups contain hydrocarbons and saltwater having fluid pressures that increase to geopressured levels (>3,000 ft msl). Salinity increases from <400 mg/L near the outcrop, to ~3,000 mg/L at the downdip limit of freshwater, to >100,000 mg/L in the geopressured zone. A reversal in hydraulic gradient and the presence of “updip” oil fields mark a broad convergence zone between the hydropressured and geopressured zones. Study of the convergence zone is made possible by pooling data from groundwater and petroleum industries and by modeling.

Chemical composition of groundwater suggests mixing of three waters in the convergence zone: recently recharged meteoric water moving downdip from the outcrop, brine moving updip from the geopressured zone, and connate seawater in the deep part of the hydropressured zone. Extensional faulting near the outcrop in central Texas restricts the net downdip flux of recharge; the convergence zone is closer to the outcrop downdip of the fault zone than in south or northeast Texas. A MODFLOW model, developed for assessing water resources, was assigned a general-head boundary at the updip limit of the geopressured zone. A small simulated updip flux of basinal brine, given the constraints of assuming uniform fluid density, gives a reasonable match of the plan-view hydraulic-head gradient reversal and a rough estimate of vertical cross-formational flow in the convergence zone.