USING ELECTRIC LOGS TO ESTIMATE SALINITY AND MAP RESOURCES OF FRESH AND BRACKISH GROUNDWATER

Electrical surveys conducted in wells (electric logs) can be used to estimate groundwater salinity where chemical analyses are scarce. This technique works well in areas that have been extensively drilled for oil and gas exploration. We used electric logs to map fresh and brackish groundwater resources in three separate parts of the Carrizo-Wilcox aquifer along the Texas coastal plain. Electric logs include several different curves that graphically display variations in electrical properties with depth. Both induced currents and natural currents are measured. Primary controls on electric log response are pore-fluid composition, lithology, permeability, porosity, and temperature. Within limited areas where lithology and temperature are relatively constant, electrical log resistivity in permeable layers (aquifers) is related to concentration and composition of total dissolved solids (TDS) in groundwater. We used empirical relationships as well as theoretical principles to estimate groundwater salinity from electric logs.

We also used electric logs to correlate stratigraphic boundaries, map lithologies, and interpret depositional environments. Electric log correlation is an established procedure where recurring curve patterns reflect bedding continuity between wells. The Carrizo-Wilcox aquifer is composed of interbedded sandstones and shales, which can be distinguished with confidence by electric log response. Shales have uniformly low resistivities, whereas sandstone resistivities vary with groundwater salinity. Depositional environments, which include nonmarine fluvial and marine shoreline facies, are useful for interpolating sandstone geometries between wells.

We estimated volume and distribution of fresh (<1,000 mg/L TDS), slightly saline (1,000 – 3,000 mg/L), moderately saline (3,000 – 10,000 mg/L), and very saline (>10,000 mg/L) groundwater. Commonly, groundwater salinity is inversely proportional to sandstone thickness and lateral continuity. Salinities display distinct variations between sandstones separated by shale interbeds. Brackish groundwater overlies fresh groundwater locally. Downdip salinity interfaces can be identified within thick sandstones. Relative hydraulic connectivity between fresh and brackish groundwater systems can be evaluated.