Mexico’s oil production originates mainly from low-porosity carbonate reservoirs that benefit from secondary porosity associated with natural fractures and vugs. Traditionally, empirical models were built to assess oil/water contacts in these complex carbonates, with a goal of making the models both reliable and quick to implement. However, the complexity of these carbonates challenged this approach.One methodology empirically defined the variable m but fails to clearly identify oil/water contacts in these low-porosity formations. A more recent approach attempted to use nuclear magnetic resonance (NMR) diffusion maps. However, in an environment drilled with oil-base mud and where mud losses are common, often only a mud filtrate signal can be seen, making the interpretation subjective.The advent of a new logging technology from a multifrequency dielectric tool brings previously unavailable information that has changed the way that these carbonate reservoirs are evaluated. Dielectric permittivity is used to provide formation water saturation and salinity in the near-wellbore area and to determine flushed-zone resistivity in oil-base mud. Given that oil-base mud filtrate is the invading phase, this method independently provides irreducible water saturation. In addition, dielectric dispersion provides a textural parameter sensitive to the formation tortuosity, along with its wettability. This parameter, referred to as MN, can be used in an Archie-type saturation equation. A deep saturation from resistivity can easily be compared with a shallow dielectric saturation to infer formation water displacement by mud filtrate and define the water leg.Integration of conventional induction deep-reading measurements with multifrequency dielectric measurements in deep offshore wells of Mexico drilled with oil-base mud (OBM) is used to easily identify the oil/water contact. Wireline formation tester and extended production tests confirm the petrophysical evaluation.This approach is now applied routinely in Mexico’s offshore operations and has improved both the static and dynamic characterization of the reservoir. A better understanding of fluid levels in newly drilled wells has a major impact on completion time and success.
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