COMPLEXITY OF THE DOCKUM AQUIFER IN SCURRY COUNTY, TEXAS

Researchers at the Bureau of Economic Geology (BEG) are studying the Dockum aquifer in the vicinity of the ~90 mi2 Scurry Area Canyon Reef Operators Committee (SACROC) oil field, which lies ~225 mi west of the Dallas-Tarrant county line in Scurry County, Texas. The main objective of the BEG study is to look for impacts to groundwater overlying the SACROC oilfield from carbon dioxide (CO2), which has been injected at depths of 6,000 to 7,000 ft for enhanced oil recovery since 1972. First we defined regional variability in aquifer chemistry using data from the Texas Water Development Board (TWDB) and samples we collected from 60 water wells between June 2006 and July 2008. Results show large variations in Scurry County groundwater chemistry (e.g. pH, total dissolved solids, major cations and anion concentrations).

Sources of complexity of Dockum aquifer chemistry include differences in stratigraphic intervals in which water wells are constructed and oil-field activities. According to TWDB, all water wells in Scurry County are classified as Dockum aquifer wells; however, they range in depth from 100-500 ft. In addition, Ogallala Fm. sediments overlie the Dockum throughout most of Scurry County. In order to understand influence of stratigraphic variations on Dockum chemistry, we constructed structure contour maps of the Santa Rosa subunit of the Dockum Group. The Santa Rosa is the lowest member of the Dockum group (McGowan et al., 1979), which contains sand and gravel layers that produce the largest quantity of Dockum groundwater (Dutton and Simpkins, 1986). Groundwater in Ogallala wells located outside of SACROC is higher quality than in Dockum Santa Rosa wells. For example, median concentrations of sulfate in Ogallala wells outside of SACROC are ~33 mg/L versus ~230 mg/L sulfate in Dockum Santa Rosa wells.

We conclude on the basis of stratigraphy, water chemistry, and limited water level data that the 100 to 150 ft deep wells in Scurry County are completed in Ogallala sediments. The deepest wells are completed in Dockum Santa Rosa; these wells are constructed with open or gravel-packed annuli to take advantage of the larger quantity of groundwater in Dockum Santa Rosa and higher quality groundwater in overlying Ogallala sediments. Perhaps most of the water wells in Scurry County should be classified as Ogallala-Dockum wells?