A computer mapping technique for improving the accuracy of original oil-in-place estimates for the Tensleep and Phosphoria formations in South Dome of Oregon basin field, Park County, Wyoming, is presented. Grids representing top and base structure, average porosity, net-to-gross ratio, and integrated average oil saturation from capillary pressure curves, are combined to create a grid of hydrocarbon pore thickness for each reservoir.

The average-oil-saturation model is created by correlating capillary pressure curves to mapped porosity. This model incorporates more information about the three-dimensional distribution of reservoir fluids than was used in methods applied previously. Because of this, it provides a more accurate representation of the fluids' areal distributions. Volumes resulting from use of this technique are compared with methods that use alternative average oil saturation modeling approaches: (1) a constant oil saturation, (2) a grid derived from average-oil-saturation at each well, (3) an average oil saturation grid from a single capillary pressure curve, and (4) a dry-oil production contact used in place of the free-energy surface. Maps show the steps in the process and allow comparison of the ef ects of the five oil saturation modeling methods.

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