Hydrogeology of Barbers Hill Salt Dome, Texas Coastal Plain

Barbers Hill salt dome, located in the Texas Coastal Plain near Houston, has a long history of resource exploitation, including oil and brine production, storage of hydrocarbons in solution-mined caverns within the salt stock, and disposal of brine into the cap rock. Industrial and municipal facilities are concentrated over the dome, and large amounts of ground water are produced within 12 mi (20 km) of the structure. Subsurface data from closely spaced petroleum, brine-disposal, and water wells were used in characterizing near-dome hydrogeology at Barbers Hill. The purpose of this study is to provide a better understanding of ground-water flow and hydrochemistry around a soluble salt dome and to present information that could be helpful in avoiding problems associated with high-density use of surface and subsurface resources under similar hydrogeologic conditions.Barbers Hill Dome is a shallow (<400 ft [120 m] deep), slightly tilted salt diapir about 10,000 ft (3,050 m) wide at it8 crest. It has well-developed salt overhangs and a thick (> 500 ft [150 m]) cap rock composed of anhydrite, gypsum, and calcite. The dome is enclosed in a thick sequence of Cenozoic terrigenous clastic sediments. Fine-grained marine sediments surround the deeper dome flanks, but sand-rich fluvial-deltaic facies dominate shallower strata. Active diapirism has uplifted strata against the dome flanks. Salt flowing into the growing diapir at depth has left peripheral salt-withdrawal basins, which are synclinal features filled with overthickened sedimentary strata. Domal uplift is still active, arching Pliocene to Pleistocene strata and creating the circular hill that overlies the diapir.Permeability distributions, environmental-water heads, pumping-test results, and numerical modeling suggest that hydrologic communication exists between Barbers Hill cap rock, shallow fresh-water sands, and deeper sands containing saline water and hydrocarbons. Every month several million barrels of brine are injected into Barbers Hill cap rock, which contains highly porous and permeable zones. Saline fluids from salt dissolution and deep formations also occur in the cap rock and around the dome flanks. Thick aquifer sand bodies around the dome flanks thin or pinch out toward the crest. Even so, as much as 1,000 ft (310 m) of permeable sands in the Evangeline aquifer are in contact with cap rock. Comparisons of calculated environmental-water and freshwater heads, which normalize fluid-density variations, delineate steep hydraulic gradients directed upward and outward, away from the cap rock. Pumping tests show that the boundary between the cap rock and enclosing sediments is transmitting fluids. During a long-term injection test, brine levels in observation wells in cap rock rose initially but eventually stabilized, matching theoretical solutions for idealized aquifers that are leaking fluids across broad areas vertically and laterally. Numerical modeling indicates that substantial rates of vertical leakage out of the cap rock are possible and are consistent with the injection-test data.Active diapirism, salt dissolution, and cap-rock formation at least partly contribute to heterogeneous vertical and lateral variations in ground-water salinity and composition in Chicot and Evangeline aquifer sands around Barbers Hill. Electric-log interpretation and results available from successive (1958 to 1986) water-well chemical analyses delineate a growing area of abnormally high ground-water salinities in the lower Chicot aquifer south and west of the dome. This high salinity is attributed to natural mixing of dome-related fluids with circulating meteoric waters, although recent dramatic increases in dissolved solids (especially chloride) suggest that brine disposal is accelerating salinization. Continued high-volume cap-rock brine disposal increases the risk of further contamination of ground-water resources.