Conventional domestic petroleum deposits occur in sedimentary basins throughout the United States. In the northern part of the state, permanently frozen ground is common and constitutes an operational problem in petroleum development. The Arctic Slope, also called the North Slope, includes an area in excess of 100,000 square miles, a significant portion of which is considered to have petroleum potential. The search for petroleum doubled in scope in Oregon in 1980, principally because of the discovery of commercial quantities of gas in the Columbia County Mist field in 1979. Petroleum prospects remain favorable for the Eastern Interior region, but production is expected to continue its decline. Petroleum accumulations occur in anticlinal folds and in local stratigraphic traps caused by variations in reservoir-rock porosity. Explored pull-apart basins have exhibited low petroleum content, and exploration so far in the offshore Atlantic coastal basins has not produced a commercial field.