Determining the Hydrocarbon Saturation in Reservoir Rocks with Fresh Formation Water, Pilar Field, Alagoas - Sergipe Basin

We performed laboratory resistivity measurements on sandstones from the Pilar field (State of Alagoas) in order to investigate their behaviour with partial water saturations. The data were fitted to a number of resistivity models, notably those of Archie, Patnode & Wyllie, Waxman & Smits and Sen, Goode & Sibbit. Those models which feature a solid or surface conductivity term acting in parallel to the electrolytic conductance of the brine showed a much better fit than those which did not. The magnitude of this constant conductance is found to correlate well with the amount of mica. We propose a modified Patnode & Wyllie equation as a workable model in the upper reservoir zones where the water is fresh and the solid conductance contribution is therefore more important. For it to be pratically applicable, a simultaneous determination of the solid conduction and the cementation exponent m is required, possibly through a combination of nuclear and resistivity logs. If one were to use Archie's equation, the hydrocarbon saturation would be underestimated at low water saturations and overestimated at high water saturations.