INTEGRATING GEOPHYSICAL AND GEOCHEMICAL MEASUREMENTS TO CHARACTERIZE FRESH/BRACKISH/SALINE GROUNDWATER: A CASE STUDY IN A CARBONATE AQUIFER

The Burgos Basin in Mexico covers most of the northern region of the Tamaulipas state, is the southern extension of the Maverick basin in Texas. Several investigations specify it includes a thick clastic non-marine sequence overlying Mesozoic marine formations, reflecting alternating transgressions and regressions of sea level. This region has been producing (conventional reservoirs) gas and condensate since 1943. With the Energy Reform, one of the PEMEX’s objectives for the near future is to increase rate of extraction of proved reserves from existing fields through optimization of hydraulic fracturing and unconventional drilling. In addition to advance in current technology, infrastructure and investment, these activities will require the use of water obtained from local sources. The Lower Rio Bravo aquifer is not suitable for public water supply; actually people use limited volumes of water for irrigation. Groundwater salinity is highly variable,. total dissolved solids increase with depth. With the transient electromagnetic method (TDEM) is possible to identify vertical and lateral changes in water quality. It has not the detail of electrical resistivity tomography, but it is easier to implement to map the subsurface. In this investigation, we present a case study in a carbonate aquifer, correlating TDEM surveys with specific conductance logs in monitoring wells and water chemistry. The identification of geoelectric units was as follows: i) U₁ (>5 Ohm-m) was located at the top, this unit includes the vadose zone as well as the freshwater zone, ii) U₂ (> 1 and <5 Ohm-m) is correlated with the brackish or mixing water zone, iii) U₃ (<1 Ohm-m) correlates with saline water. Chemical water types were correlated with geoelectric units, HCO₃-Ca, HCO₃-Mix and Mix-Mix water types is associated with U₁ , Cl-Ca, Cl-Mix y Cl-Na water types with U₂ , and Cl-Na water type with U₃. With the high costs for acquisition of fresh water rigths and transportation, treatment of nontraditional water supplies for unconventional drilling may be economically feasible. The TDEM could be a practical and cost-effective method to delineate exploration drilling to tap brackish water in the Lower Rio Bravo aquifer.