Abstract The “Sand Belt” area of Ward and Winkler counties, Texas, and Lea County, New Mexico, forms an almost continuous north-south line of production, about 90 miles in length. Porosity is present in sandstone and sandy dolomite immediately behind the Capitan reef. The trap is formed in part by the regional stratigraphy and in part by regional and local structure. The reservoir sands are fine-grained and contain much silt, shale, and dolomite cement.