Generally, hydrocarbon reserves are determined by the irreducible water saturation using the volumetric equation of the original oil in place (OOIP), regardless of the initial water saturation in the transition zone and as a result overestimate hydrocarbon reserves. The objective of this work is to determine a water saturation model that allows modeling the initial distribution of fluids in the reservoir knowing the original oil-water contact and rock quality.

The Leverett J function was used as a variable independent from initial water saturation model; it controls the flow of fluid and depends on the height above the free water level, the interfacial tension, Oil-water fluid density, absolute permeability and effective porosity. In order to define this function, capillary pressure data from oil-water system of drain cycle from special core analysis were used. The core data were modeled and averaged for each rock type with the capillary pressure function from "JNM Jing XD Van Wunnik SCA-9807 (1998)". Then, these curves were converted to values of "J" and these were related with water saturation from capillary pressure curves to obtain functions type SWINIT: f (J).

This model gave excellent results in the fluid initial behavior, it means, knowing the original oil-water contact and rock quality, the initial water saturation was quantified in in wells completed in the reservoir. The aforementioned was validated with production / pressure tests. Finally, two sensitivities were performed to calculate OOIP, the first was determined with the irreducible water saturation and the second with the initial water saturation, resulting in a error difference about 5%, in this case, obtaining higher reserves were the calculated with irreducible water saturation.

The initial water saturation profile of the wells generated from the model, it is useful to initialize the numerical simulation from reservoir "A", because this model takes as an input data the original oil water contact, which represents the limit reservoir fluid. This model does not depend on the date of drilling wells to determine the initial water saturation of the reservoir.

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