Depositional Changes in An Active Rift Basin Reflected in Well Logs and Cuttings, Hueco Bolson, West Texas

Sixteen wells were drilled by El Paso Water Utilities in the Hueco Bolson aquifer to provide brackish water to a desalinization plant and to create a trough in the ground water to prevent incursion of saline water into the fresh water aquifer that provides 40% of El Paso's water. There is a northwest to southeast increase in TDS from 976 to 1979 ppm across the study area. The logs of the sixteen wells were correlated and calibrated to cuttings from three wells. The wells exhibited a 1-3 m scale stratification of both clay beds and calcic soils that horizontally subdivide the aquifer.

Wells were analyzed using sieves and a Malvern particle analyzer. The means and percentages of gravel, coarse to medium grained sand, medium to fine grained sand, silt, and clay were correlated with well logs using the Petra software package. The log correlation and sediment properties were used to model the depositional environments and stratigraphy. The sediment gets finer away from the mountains grading from alluvial fans to inter-bedded desert floor silts and clays. Five major environments could be correlated: 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, and 5) alluvial fan gravel. One interesting feature is the presence of gravel clasts deposited across the flat basin floor. Numerous clay and silt beds, as well as soils, can be correlated between the wells and probably vertically subdivide the reservoir. In addition, several faults offset the strata between several of the wells. There is a fault located between two of the three wells of this study that offset the strata by 0.6 m, but change the water table elevation 8.5 m.