The Rangely field, in northwestern Colorado, is the most productive field in the state. Located in the northeastern part of the Uinta Basin, it is an outstanding example of an anticlinal field, where oil is trapped in a large, gentle dome. The shape of the dome shows up well on the surface; rock layers can be seen dipping outward in all directions from the town of Rangely. Oil was found by drilling on the crest of the dome. At first, oil was produced from fractures in the Cretaceous Mancos Shale at less than 1,000 feet depth. Later, deeper drilling showed that oil had also accumulated in the Permian Weber Sandstone, at 5,000 to 7,000 feet. At present this field is producing about 28,000 barrels of oil a day, but the figure is dropping each year as the field is depleted. Oil and gas are produced in southwestern Colorado from the eastern edge of the Paradox Basin and the northern edge of the San Juan Basin. In the Paradox Basin, oil comes from Pennsylvanian limestone mounds or reefs. Production in the Colorado part of the basin has been at most a few thousand barrels per day; more is produced in adjacent Utah. In the San Juan Basin, gas and oil are trapped in thin porous layers of Cretaceous and Pennsylvanian sandstone, between impervious layers of shale. Most of the production is in New Mexico, although some oil comes from the Colorado part of the basin. The greatest known potential oil resource in the world lies in the oil shales of western Colorado. The richest of these shales cover an area of 1,600 square miles north of the Colorado River, south of the White River, and just east of the Colorado-Utah line. The oil shales are part of the Tertiary Green River Formation, which extends over much of northwest Colorado, northeast Utah, and southern Wyoming. Oily material called _kerogen_ is locked in these rocks, too solid to flow out of the fine pore spaces of the shale. To free it the shale must be mined, finely crushed, and heated until the kerogen converts to liquid oil. This is an expensive process, and as yet production of petroleum from the oil shale has not been possible at a cost which will compete with production of oil and gas from wells. The United States Bureau of Mines, as well as a number of oil companies, have sought for more than fifty years to discover a less expensive method for extracting oil from the shale. No doubt at some time in the future a competitive technique will be developed, or a growing shortage of other oil will bring world prices to a level with which present production techniques can compete.