Vertical modified in-situ processes for oil-shale development require an accessible, thick and continuous oil-shale section. In Colorado's Piceance Creek Basin the Mahogany zone and its overlying oil-shale is the Green River Formation section that best meets these requirements. This oil-shale section is described and evaluated in detail with contour maps of richness, thickness, and oil resource in place. Seven cross sections were constructed to provide the frame work necessary for basin-wide correlation of assay results from 189 cores. These correlations form the basis for detailed contouring and source evaluation of the Mahogany zone, A-groove, and 5 overlying oil-shale sections. Total oil in place basin-wide in this primary resource is 419 billion barrels with 85 percent occurring in areas with resource exceeding 400,000 bbls per acre. Ninety-five percent of the resource occurs where average richness of the total section exceeds 14 gallons per ton. Thickness of the continuous section reaches 600 feet and virtually all the resource occurs in sections thicker than 300 feet (91 meters).