Volumes of sandstone, shale, carbonate rocks, and evaporite, and pore volumes within each of these, have been computed for 28 stratigraphic units in the plains region of western Canada. This region covers 486,785 sq mi and contains a total sedimentary fill of 537,895 cu mi, with an average depth of 5,834 feet. Tertiary strata comprise 1.3 percent, Mesozoic strata 45.5 percent, and Paleozoic strata 53.2 percent of the basin fill. The average lithologic percentages for the basin are: sandstone, 11.0; shale, 56.9; carbonate, 26.4; and evaporite, 5.7. The average basin porosity is 11.8 percent, with more than half the pore volume in Mesozoic shale which comprises only 38 percent of the total fill. The average basin porosity for sandstone, shale, and carbonate is 15.7, 13.9, and 8.2 percent, respectively.