Petroleum and natural gas have been found in large quantities in the
Prairie and Plateau Provinces in Colorado, as well as in smaller
quantities in North Park in the Mountain Province. They generally occur
in porous sandstone and limestone layers, where they have been trapped
by overlying finer-grained, less permeable layers in or near folds and
faults.
Several oil and gas seeps were found along the mountain front shortly
after the arrival of the earliest settlers. Near Canon City, on Oil
Creek, a plaque commemorates the first production:
Oil Creek—site of the first oil well in the west—second place in the
United States to produce petroleum from wells. In 1862 ... A. M.
Cassedy drilled an oil well 50 feet deep. By February, 1863,
production was one barrel a day. Later, several thousand gallons of
petroleum were produced by primitive methods, and kerosene and
lubricating oil were shipped by ox team as far as Denver and Santa Fe.
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.