ABSTRACT

Values of irreducible water saturation calculated from conventional electric and porosity logs at Bayou Middle Fork Field are often greater than 60 percent. Mercury injection curves reveal that the true water saturation averages about 40 percent. Scanning electron micrographs show oolitic grains that have been recrystallized and have rough irregular surfaces as well as intercrystalline porosity within the particles. Water adheres to these grain surfaces and occupies some of the intercrystalline porosity in productive zones, causing the calculated water saturations to read too high. Application of the Archie equation, the Guillotte et al. technique, and the Rocky Mountain technique each provide a different suite of water saturations.

Archie water saturations average about 75 percent when density-neutron porosity values are used. Density-neutron porosity values tend to read about 2 porosity units low when compared with core porosity values throughout the field. When core-correlated porosity is used, the Archie saturation averages about 60 percent.

The Guillotte et al. technique employs a variable textural parameter "w" which is dependent on porosity and permeability. Mean water saturation with this method averages about 49 percent, which is in better agreement with the mercury injection data.

The Rocky Mountain ratio yields water saturations that are too high until the porosity balance technique is applied. Since R_t is assumed to read too low due to the excess immobile water, the Rocky Mountain derived porosities are too low. When R_t is increased to balance the porosity the water saturation averages about 43 percent, using z = .025. When the Rocky Mountain R_ta is used in the Archie equation, water saturations more consistent with the mercury injection data result. In addition the difference between the observed R_t and the Rocky Mountain derived R_ta can be plotted as a function of porosity.