ABSTRACT: Ecopetrol discovered the Yarigui field in the Magdalena Valley in 1943 and has drilled about 150 producer and 10 injector wells. The producing intervals are Tertiary age thin reservoirs of unconsolidated shaly sandstone intercalated with shales. They are largely independent and not connected. Composed of fluvial deposits, they have connate formation water salinity of the order of 35-ppk NaCl. Wells are usually drilled with very fresh muds, of less than 5-ppk NaCl. The field has been under limited fresh water injection for several years. The current development program contemplates drilling 10 additional injectors and using recycled produced water to limit unwanted salinity changes and reduce the possibility of shale alteration. Ecopetrol has concentrated on producing the upper Yarigui reservoirs, of higher resistivity, without considering the lower resistivity sandstones, assumed to be water bearing. This was because the very fresh mud filtrate that invades the formation did not suggest oil displacement in the low-resistivity reservoirs based on a combined Archie-type analysis of the deep resistivity and the micro-resistivity. To better understand the lower resistivity reservoirs, Ecopetrol decided to test a new multi-frequency dielectric dispersion tool. The primary field product of this tool is a water-filled porosity, which, when compared with the total porosity, quantifies the oil present in the invaded zone. This quick-look is obtained at the well site and uses only the highest frequency generated. With it, Ecopetrol identified significant quantities of oil below their initial target and decided to extend the formation tester pressure acquisition to the new intervals, finding them to be at original pressure. A complete dielectric dispersion analysis was performed offsite. In addition to water-filled porosity, this analysis provides water salinity in the region investigated by the tool, which was found to be equal to or higher than the filtrate salinity.