Geology and ground-water resources of Winkler County, Texas

Field: Earth and Planetary Sciences (Physical Sciences)

Winkler County, in west Texas, is adjacent to the southeast corner of New Mexico. Most of the county lies in the Pecos Valley; the remainder, in the northeastern part of the county, is part of the Llano Estacado, or the High Plains. Its principal industries are those related to the production and refining of oil, but ranching also is an important occupation. The county has an arid to semiarid climate, an area of about 887 square miles, and a population of about 12,000 in 1957. The principal fresh-water-bearing formations in Winkler County are the Cenozoic alluvium, which mantles the surface of most of the county, and the Santn Rosa sandstone of Late Triassic age. The alluvium is saturated at a depth ranging from a few feet below the land surface to about 150 feet, and the water is under water-table conditions. The Santa Rosa sandstone is fully saturated throughout most of the county, the water occurring under both unconfined (water-table) and confined (artesian) conditions. The underlying older formations contain water too highly mineralized for most uses, but they are important because water from these underlying beds, which is produced with the oil, is a source of pollution to the fresh-water aquifers. The formations also are a source of water to waterflooding projects for the secondary recovery of oil. The alluvium is replenished chiefly by precipitation, throughout the county. The sand-dune area in the eastern part is the most favorable area of natural recharge. The Santa Rosa sandstone is recharged through the alluvium where the two formations are in contact. Infiltration of saline water, which with oil and waste water has been produced from industrial plants, has been taking place through surface earthen pits. About 18 mgd (million gallons per day) of saline water was being produced from the many oil fields throughout the county during August 1957, and almost all this water was placed in surface pits. The Hendrick oil field alone produced about 14.5 mgd during that month, and an estimated 800,000 acre-feet from 1937 to 1957. About 20 million acre-feet of fresh water is stored in the explored groundwater aquifers of Winkler County, of which an estimated 5 to 7 million acrefeet can be practicably recovered. A daily average of about 8.2 million gallons (9,200 acre-feet per year) of fresh water was used in the county during 1956. Public and industrial use accounted for about 6.8 mgd; irrigation, domestic, and stock uses accounted for 1.4 mgd. Projects engaged in the secondary recovery of oil were the largest consumers, using an average of about 3 mgd of fresh water and about 2.7 mgd of saline water during 1956. Water wells drilled in the deep trough of Cenozoic alluvium south of Wink may yield 1,000 gpm (gallons per minute) or more. A pumping test of the alluvium in that area indicated a coefficient of transmissibility of about 25,000 gpd (gallons per day) per foot. Wells, other than windmill wells, drawing water from thinner sections of alluvium in the rest of the county yield between 100 and 300 gpm. Most of the wells in the Santa Rosa sandstone yield 30-400 gpm. Two municipal wells at Kermit that tap the Santa Rosa sandstone were tested at 1,200 and 1,875 gpm. The large yield of these wells probably results from the presence of fractures in the structurally deformed aquifer. Pumping tests in the Kermit area indicate that locally a full section of the Santa Rosa sandstone has an average coefficient of transmissibility of about 25,000 gpd per foot and a coefficient of storage of about 0.0003. The transmissibility in two other tested areas is much less. The chemical quality of the water in the principal aquifers is generally acceptable for industry and for public supply. About two-thirds of the samples collected from fresh-water wells had a dissolved-solids content of less than 1,000 ppm (parts per million) ; however, some samples in a few areas were hard and were high in fluoride and silica. Samples from wells in polluted areas contained dissolved solids ranging from about 1,400 to 71,100 ppm. Two comprehensive analyses of water samples from the Rustler formation showed a dissolved-solids content of 18,400 ppm. and 157,000 ppm. In most of the water produced with the oil in the Hendrick oil field, the content of dissolved solids ranged from about 4,000 to about 10,000 ppm. The water produced with the oil in the rest of the oil fields in Winkler County was mainly brine.