About twenty miles to the southeast, near Florence, the Cretaceous Pierre shales were drilled in 1876. Oil was found in a system of intersecting fractures and [joints]. Some of the early wells in the Florence field are still producing, making this Colorado’s oldest and longest producing field. It has yielded more than 10,000,000 barrels of oil.
Small quantities of oil have been produced near Boulder since about 1900, also from Pierre sandstones and shales. In this area, wells were located by “[dowsing]” or “witching,” as was fashionable at the time. Several old rigs can be seen near Boulder Reservoir. As at Florence, oil has been trapped in fractures of otherwise dense and impervious shale. Some gas is produced and is used by local farms.
More recently, oil was found far beneath the surface in the northern part of the Prairie Province. Here, in the Denver Basin, oil is produced from several levels in the Dakota Sandstone. The oil has accumulated in lenses of beach sand deposited along the shoreline of the Cretaceous sea. The general trend of the shoreline, and of the oil fields, is northeast-southwest. The shore appears to have been similar to Georgia’s present coastline: a swampy tidal zone separated from open sea by lagoons, sandy bars, and clean sand beaches.
Individual oil pools in the Denver Basin are small, but there are many of them; they lie nearly a mile below the surface, under much of Morgan and Logan Counties and adjacent parts of Nebraska. Exploratory and development drilling keeps total oil production at about 50,000 barrels a day. Oil and gas produced here is piped to Denver and other Colorado cities.
In southeastern Colorado, oil and gas occur in late Paleozoic limestones and sandstones similar to those which outcrop at the edge of the Wet Mountains. Prospecting by geophysical methods and by drilling has revealed several small, rich accumulations, one of which is thought to contain about 30,000,000 barrels of oil.
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.
Oil and gas production in Colorado is decreasing at present, even though great efforts are being made to find new oil pools. Petroleum prospecting and wildcat drilling are carried out in as yet unproductive basins in the [Plateau] Province, in intermontane basins such as the San Luis Valley, and on the Plains. Known reserves will continue to provide the state with significant income for many years to come, and if oil shale recovery becomes profitable. Colorado’s hydrocarbons will become the most prominent of her commodities.