STRATIGRAPHY AND WELL LOG ANALYSIS IN AN ACTIVE RIFT BASIN, HUECO BOLSON-WEST TEXAS

The Hueco Bolson aquifer is located on Rio Grande rift basin in west Texas and provides 40% of El Paso’s water. El Paso Water Utilities (EPWU) has drilled wells up to 350 m into this aquifer. These wells provide a unique opportunity to study the interaction of tectonics and sedimentation in an actively subsiding rift basin. Stratigraphy and sedimentation studies in active extensional basins have received little study because they are usually buried. EPWU drilled sixteen wells in 2005 along the western-central portion of the Bolson to provide brackish water to a desalination plant and create a trough in the ground water to prevent incursion of saline water aquifer into the fresh water aquifer. There is a northwest to southeast increase in total dissolved solids (from 976 to 1979 ppm) across the well field. Four wells were chosen to document the stratigraphic and structural features across this gradient. The wells penetrate thin (0.2 to 3m) clay, silt and gravel beds, as well as several faults. These beds can be correlated between the wells and help vertically subdivide the reservoir. The clay beds adjacent to one of the faults obstruct the incursion of the saline water into the fresh water aquifer. In addition, this fault offsets strata by 20 m and offsets the water table elevation by 8.5 m. Grain size analysis has been performed using sieves for particles larger than 1 mm and a Malvern particle analyzer for particles smaller than 1 mm. Correlations of the means and percentages of gravel, coarse to medium grained sand, medium to fine grained sand, silt, and clay has been done using the Petra software package. Log correlations and sediment properties have been used to model the depositional environments and stratigraphy. Five major environments can be correlated across the westernmost 10 wells: 1) desert floor sands and playa clays; 2) playa margin clays interbedded with gravel and coarse sand; 3) playa margin delta coarsening upward from clay to sand; 4) alluvial fan toe units and muddy gravel; 5) alluvial fan gravel. The depositional environments for wells located to the southeast of the initially examined wells will be determined with grain size analysis and identification of local analogous outcrops. In addition, gravity data will be collected to locate the faults identified through well log correlations.