USING BOREHOLE GEOPHYSICS TO IDENTIFY POTENTIAL BRACKISH PRODUCTION AREAS IN THE RUSTLER AQUIFER, WEST TEXAS

Groundwater is a major source of water in Texas, providing about 60 percent of the water used in the state. House Bill 30, passed by the 84^thTexas Legislative Session, requires the Texas Water Development Board (TWDB) to identify and designate brackish groundwater production zones in the aquifers of the state. For this study, the INTERA Team evaluated the brackish groundwater resources of the Rustler Aquifer.

The Rustler Formation is a Permian (Ochoan) Age deposit made up of a series of alternating dolomites, anhydrites, shales and limestones. It was anticipated that the majority of the water resources were within the dolomites and limestones as opposed to the entire formation, as the aquifer was previously characterized. In addition, given the brackish nature of the Rustler Aquifer, sampled water quality data was sparse and poorly distributed. However, since the project area was within the Permian Basin of West Texas, geophysical data from oil and gas logs would be available to evaluate structure and make calculations of water quality.

A total of 589 natural gamma logs were analyzed, making approximately 5,000 stratigraphic picks to gain further insight into the specific depositional and post depositional regime of the Rustler Formation and how this knowledge relates to the Rustler Aquifer. To augment sampled water quality data, we used state-of-the art petrophysical analysis techniques developed in the oil and gas industry to analyze old geophysical logs for both porosity (neutron and sonic) and water quality (resistivity, induction and spontaneous potential). Calculations of Rustler Aquifer water quality (total dissolved solids) using geophysical logs were both novel and provided the additional data needed to better define the groundwater salinity zones within the Rustler Aquifer.

This project represents a successful integration of the specific expertise possessed by hydrogeologists, stratigraphers and petrophysicists in anticipation of evaluating brackish groundwater resources.