Location and characteristics of the interface between brine and freshwater from geophysical logs of boreholes in the upper Brazos River basin, Texas

The Brazos River flows through the eastern part of the Permian Basin in Texas and is one of ·the largest sources of water in the State. Unfortunately, brine springs and seeps flowing into the Brazos River contribute an average daily load of about 1,650 tons of sodium chloride to the Possum Kingdom Reservoir, and the change in chemical quality of this water affects its usefulness. An understanding of the occurrence, source, geologic controls, and chemical composition of the brine is necessary before engineering methods can be devised to control the discharge of brine into the river. This report describes how borehole geophysics was used during a test-drilling program in the upper Brazos River basin to determine the location, characteristics, and relation to lithology of an interface between brine and overlying fresh to slightly saline water. The analysis of several different types of geophysical well logs enabled the accurate location of the interface, proved that it was quite sharp, and showed that the interface was not related to lithology in most places. Some logging tools responded to the interface in open holes; others detected it through casing. For example, single-point-resistance and spontaneous-potential logs were responsive to the interface in many open holes, whereas the neutron-epithermal-neutron and neutron-gamma log combination could be used to find the interface behind well casing. Caliper logs were used to locate fracture zones, which contained brine in some areas and gas in others. All of these logs along with the natural gamma were used to identify and correlate the lithologic units penetrated by the test holes. Natural-gamma and neutron logs helped to distinguish anhydrite from gypsum. Natural-gamma and neutron logs, thus, were useful for determining the depth of hydration of gypsum, which is related to the position of the brine-fresh-water interface. Neutron logs were calibrated in percent porosity by use of core analyses, and the natural-gamma logs were found to be semiquantitatively related to grain-size distribution and to the hydraulic conductivity of the core samples. Fluid-temperature and fluidresistivi'ty logs provided quantitative data on ~e . chemical composition and movement of fluids in the test holes. Analysis of a large number of commercial electric logs from oil wells in the Brazos River basin showed that many of these logs can also be used to determine the characteristics and location of the interface. From the study, it is concluded that certain suites of geophysical logs can be used to establish the position and character of the interface between brine and overlying fresh water. The logs showed that the change in quality from brine to fresh water occurs within a vertical interval of several feet and that, although in places the interface may be controlled by lithology, regionally it transects lithologic boundaries. Logs were also used to estimate porosity and hydraulic conductivity, to locate brine-transmitting fracture zones, to measure chemical and physical characteristics of the brine, and to identify specific lithologies, such as halite, gypsum, and anhydrite. Logs of the test holes in the study area provided a convenient and inexpensive way of laterally and vertically extrapolating the more costly core ·data. Furthermore, geologic and interface information determined in the study area can be extended to the rest of the upper Brazos River basin by use of geophysical logs.