The Colorado Plateau's sedimentary basins have yielded oil, gas, and coal for the past century, and they contain large reserves of additional oil that could potentially be tapped through hydraulic fracturing and horizontal drilling. The high average wind speed and solar intensity on the Colorado Plateau make wind and solar energy potential future sources of electrical energy, but they currently remain minor compared to existing coal and natural gas-powered plants. A number of power plants are associated with hydroelectric dams along the Colorado River system, and the region, particularly southeastern Utah, has long been a source of uranium for use in nuclear power.

Oil and Gas

The Colorado Plateau contains numerous sedimentary basins, each of which contains many organic-rich layers (Figure 6.6). It is possible to make sense of why we find petroleum and natural gas in these areas by understanding the region's geologic history. Mud with relatively high organic matter content tends to accumulate in shallow continental seas and in coastal marine environments. The history of the Southwest's sedimentary basins extends back to the Cambrian period, when a broad shallow sea covered much of the area. Thick sequences of carbonate rocks accumulated in these basins. During the Carboniferous and Permian, as sea levels dropped and tectonic changes affected the landscape, parts of these basins became more restricted. As land emerged and weathered into silt and sand, layers of sandstones and organic-rich shales were laid down and organic material was preserved on the seabed. When seawater in the basins evaporated, evaporites were also deposited. With time, pressure, and heat, organic material in the shale was changed into petroleum and gas, and the organic-rich shales became source beds for hydrocarbons. Later deposition of non-marine sandstones in the Mesozoic created additional reservoirs for the oil. The Cretaceous saw the development of the Western Interior Seaway, which accumulated additional organic-rich shales, along with coastal coals, sandstones, and deeper marine limestones. Finally, terrestrial sedimentation in large lake basins during the early Cenozoic trapped yet more organic-rich sediments in lacustrine shales. The thick set of sediments that built up over millions of years created heat and pressure in deeper layers, compacting and "cooking" much of the organic matter into forms that are now used as fuels.

Conventionally, finding oil and gas has not been as simple as finding organicrich rock layers. Oil and gas can flow both within and between rock layers, wherever the number and size of paths between pores, fractures, and other spaces (permeability) is large enough. Because oil and gas are under pressure and are more buoyant than pore-filling waters, they will move gradually upward to areas of lower pressure and will rise all the way to seeps at the surface unless they are blocked by a caprock or seal—that is, one or more layers with permeability so low that they effectively block the flow of liquids and gases. If the fossil fuel happens to rise beneath a caprock in the shape of a concave surface (such as an anticline or certain faults), the fossil fuels may accumulate in what geologists call a "petroleum trap." Petroleum traps accumulate oil and gas in porous sedimentary layers with thin natural fractures, called reservoirs. Most oil and gas has been extracted using the conventional technique of searching for such reservoirs and then drilling vertically into them, which allows the gas or oil to come to the surface through the well pipe. Reservoirs in the Southwest range from Devonian to Eocene in age.

Oil and Gas

Oil and gas form from organic matter in the pores of sediments subjected to heat and pressure. The organic matter is primarily composed of photosynthetic plankton that die and sink to the bottom of large water bodies in vast numbers. Shale in particular is often organic rich, because organic matter settles and accumulates in the same places that mud (clay and silt particles) settles out of the water. In most environments, organic matter is recycled by bacteria before it can be buried, but the quiet waters where mud accumulates are often relatively stagnant and low in oxygen. In these places, the bacterial decay rate is low relative to the rate of organic matter sinking and to the rate that the organic matter becomes buried in muddy sediments. Under such conditions, organic matter may accumulate enough to make up several percent or more of the deposited sediment.

Because oil and gas are under pressure, they will move gradually upward to areas of lower pressure through tiny connections between pore spaces and natural fractures in the rocks. Reservoir rocks typically have a considerable amount of pore space, and to be viable there must be a way of trapping the oil and gas, such as through a geologic structure or a change in rock type that will prevent the resource from escaping. Often, natural gas and oil are trapped below the surface under impermeable layers that do not have sufficient spaces for liquids and gases to travel through. Folds or “arches” in impermeable layers, or faults in rock layers, are common ways of trapping oil and gas below the surface.

Figure 6.6: Sedimentary basins across the Southwestern US contain significant fossil fuel accumulations.

The Paradox Basin of southeast Utah and southwest Colorado produces oil and gas, as well as a variety of other mineral resources. The basin contains a thick sequence of Paleozoic and Mesozoic sediments. Shallow marine carbonate rocks of Devonian, Mississippian, and Pennsylvanian age act as primary reservoirs for the basin's oil. The source rock for this oil is largely organic-rich shale that is interbedded with the Pennsylvanian carbonates; the shale accumulated from the eroded sediments of the Ancestral Rockies that were uplifted immediately to the east. As oil migrated stratigraphically into the overlying sandstone, it ultimately pooled in reservoirs trapped under a variety of impermeable sedimentary deposits such as gypsum, anhydrite, limestone, and dolomite.

The Paradox Basin contains substantial quantities of evaporites, particularly halite, which greatly influenced the structure of petroleum traps in the basin. These evaporites formed during an interval of repeated basin restrictions during Pennsylvanian and Permian sea level fluctuations. These underlying salt structures explain the geographic distribution of many oil and gas reservoirs in the Paradox Basin. Impermeable rocks pushed up by salt domes became caprock where oil could be trapped—this is the case in the Hermosa Formation of southeastern Utah. Collapsed domes are also responsible for many of the region's landscape features. For example, the name "Paradox Basin" came from the paradoxical observation that the Doloros River cuts across instead of flowing down the landscape. The river is superimposed upon a buried salt anticline that collapsed, leaving a long dry valley now occupied by the river (Figure 6.7).

Figure 6.7: The formation and partial collapse of a salt dome can trap fossil fuels and affect the landscape.

Salt domes can also be used to store large quantities of oil and gas. Storage caverns are created by injecting the salt with water to dissolve a cavity within the salt structure—a process called solution mining (Figure 6.8). Many salt domes along the Gulf Coast are used for this purpose; in the Southwest, however, the first such storage complex is being developed by the Magnum Gas Storage Project. The caverns, hollowed out of a 2100-meter-thick (7000-footthick) salt dome in Millard County, Utah, will be capable of storing 1.5 billion cubic meters (54 billion cubic feet) of natural gas and will be connected to an interstate natural gas pipeline system.

Figure 6.8: Solution mining is used to create a storage cavern inside a salt dome.

The San Juan Basin of northwest New Mexico and southwest Colorado contains important reservoir rocks for oil and gas, primarily late Cretaceous marine sandstones associated with the Western Interior Seaway. The basin's reservoir rocks also include late Cretaceous fluvial sandstones and coals associated with the filling of the Seaway, Jurassic aeolian sandstones, and limestones and sandstones in the shallow marine Pennsylvanian-age Paradox Formation. Source rocks in the San Juan Basin are equally varied, including organic-rich marine shales from the Paradox Formation and the Mancos and Lewis shales of the Western Interior Seaway.

The Piceance Basin, located in northwest Colorado, and the Uinta Basin, located in northeast Utah, also contain a wide variety of fossil fuel-bearing deposits dating from the late Paleozoic to the early Cenozoic. These two basins share much of their geological history—they are effectively part of the same east-west trending basin that developed across the Utah-Colorado state boundary during the late Cretaceous. The Rangely oil field, near the boundary between the Uinta and Piceance basins, has been producing oil since the 1940s. Oil was discovered at the site in 1901, but the field was not developed until after World War II due to its remoteness. Since development began there, Rangely has produced nearly 800 million barrels of oil, and it was the most prolific oil field in Colorado for many years. The oil at Rangely comes primarily from the late Pennsylvanian and early Permian Weber Sandstone, which includes permeable and porous cross-bedded aeolian dune deposits. The source for this oil is probably Carboniferous strata deep in the Uinta Basin, or the shales of the early Permian Phosphoria Formation. Some oil has also been produced from the late Cretaceous Mancos Shale, deposited as part of the Western Interior Seaway. Today, carbon dioxide (CO₂) is injected into the field to increase pressure in the depleted reservoir and enable more oil extraction.

The Uinta and Piceance basins contain the largest oil shale deposits in the US. These are part of the Green River Formation, where Eocene lacustrine deposits overlie Cretaceous marine sediments from the Western Interior Seaway. These Colorado and Utah oil shales formed during the greatest extent of Lake Uinta in the Eocene, at a time of relatively high salinity in the lake and high algal productivity. Within the Uinta Basin, some organic-rich Green River shales have been buried deeply enough to generate large quantities of kerogen (a form sometimes described as "waxy oil"). Some of this kerogen migrated and was trapped in porous sandy beds of the Wasatch and Green River formations, while some is still found in its organic-rich Green River source rock. Because the cost of processing oil shale into usable hydrocarbons is so high relative to other forms of producing oil, oil shales have remained mostly untapped.

Gas is produced in the Colorado Plateau as a byproduct of oil extraction, and from reservoirs in porous rock. In the Paradox Basin, gas is extracted from Pennsylvanian and Permian sandstones; helium is also a byproduct of gas extraction from the basin's Lisbon Field. The sands of the Cretaceous-aged Mesaverde Group in the Piceance Basin have also yielded substantial volumes of gas. Coals in the Williams Fork Formation of the Mesaverde Group have been a major source of that gas, but the Mancos and Niobrara strata that underlie the northern part of the Piceance Basin may also contribute to gas production there.

Coal

Billion-ton coal deposits formed in the Colorado Plateau during the late Cretaceous, as the Western Interior Seaway grew and shrank with sea level and local tectonic changes. North American Cretaceous coals are nearly as extensive as those of Carboniferous age, largely due to the widespread extent and long history of the Western Interior Seaway. The thickest coals accumulated as peat in coastal swamps while the basins in the area subsided, and others accumulated farther inland on alluvial plains. Some later Paleocene coals were also deposited in intermontane basins. These Cretaceous and Paleocene deposits are typically high-volatile (low-grade) bituminous coals. Most of the region's coals are deeply buried toward the centers of the major basins, and crop out around the exposed edges of the Piceance Basin in Colorado, the Kaiparowits and Uinta basins in Utah, the Black Mesa Basin in Arizona, and the Raton and San Juan basins in Colorado and New Mexico (see Figure 6.3). Coal mining has continued for over 115 years in some of these basins.

Since approximately 1980, large reserves of natural gas have been exploited in tandem with coal seams. This gas, called coalbed methane, is a byproduct of the process of coalification, and it accounts for over 5% of US methane production. Coal seams have long been vented to protect underground coal miners from potentially explosive build-ups of methane (CH₄, the primary gas in "natural gas") released from fissures around the coal. While long considered primarily a hazard to be mitigated in subsurface mines, methods have been developed to trap the methane as an additional energy source. In deep subsurface coal seams, water saturates fractures (or cleats) in the seam, making the seam an aquifer (which in some places may be clean enough to be part of the local water supply). If there is sufficient water pressure, methane present within the coal fractures may be trapped in the coal. To extract this methane, water can be removed via wells, thereby reducing pressure and allowing methane to escape toward lower pressures along the well bore (Figure 6.9). Methane is then separated from the water. After the water is removed it may take some years for the aquifer to be recharged, that is, refilled with water from rain at the surface that filters down to the aquifer. Coal bed methane accounts for over 5% of US methane production. Production rates for coalbed methane climbed steeply beginning in the early 1990s, though in recent years they have decreased both in absolute and relative quantity as shale gas production has increased. Some subsurface coals on the Colorado Plateau are significant sources of coalbed methane. For example, gas from the Fruitland Formation in the San Juan Basin is among the largest non-shale gas sources of natural gas in the US, and is by far the largest coalbed methane source in the country. Another notable coalbed gas-producing area is the Ferron Sandstone Member of the Mancos Shale, in the southwestern part of the Uinta Basin.

Coal

As leaves and wood become more deeply buried, pressure on them builds from overlying sediments, squeezing and compressing them into coal. The coal becomes gradually more enriched in carbon as water and other components are squeezed out: peat becomes lignite, bituminous, and eventually anthracite coal, which contains up to 95% carbon. Anthracite has the fewest pollutants of the four types of coal, because it has the highest amount of pure carbon. By the time a peat bed has been turned into a layer of anthracite, the layer is one-tenth its original thickness.

The Carboniferous period takes its name from the carbon in coal. Globally, a remarkable amount of today's coal formed from the plants of the Carboniferous, which included thick forests of trees with woody vascular tissues.

Figure 6.9: Coalbed methane production involves using water or other fluids to reduce pressure on the coal seam by creating a crack through which the methane can escape into a well.

Alternative Energy

The Paradox Basin contains uranium in the Triassic and Jurassic fluvial sandstones of the Chinle and Morrison formations. Other deposits include breccia pipes in northwest Arizona, which are sediment-filled cavities in ancient karst terrain. Uranium was actively mined in the second half of the 20th century as a source fuel for nuclear energy, though mining has significantly declined since (in part due to more stringent environmental regulations). Uranium mining and processing in the Four Corners area has continued irregularly in the late 20th and early 21st centuries, influenced primarily by uranium prices. Almost all uranium mined in the Southwest is exported; there are no nuclear power plants in the Colorado Plateau.

The Colorado River and its tributaries provide the Southwest with the potential for hydroelectric power (Figure 6.10), which uses the gravitational force of falling or rushing water to rotate turbines that convert the water's force into energy. There are also several pumped storage facilities within the region, where water is pumped uphill into reservoirs in times of excess production, essentially acting as batteries. Many of the hydroelectric plants scattered through the Colorado Plateau are associated with the topographic drop along the region's rim. The most notable is the Glen Canyon Dam (Figure 6.11) near the Arizona-Utah state line, which provides an average of 3.4 billion kWh per year. The massive concrete dam was completed in 1966 and retains Lake Powell, the second largest reservoir in the United States, with a capacity of 30 billion cubic meters (1 trillion cubic feet).

Figure 6.10: Hydroelectric power plants in the Southwestern US.

Figure 6.11: The Glen Canyon Dam in Coconino County, Arizona, with Lake Powell in the distance. The concrete arch of the dam is 220 meters (710 feet) high and 48 meters (1560 feet) wide.

The Colorado Plateau has only a fragmented capacity for wind energy (see Figure 6.22), but it does host a few wind farms. The Dry Lake Wind Power Project in Navajo County, Arizona was the state's first utility scale wind farm, and generates approximately 127 MW of power. Perrin Ranch Wind Farm in Coconino County, Arizona, also generates approximately 100 MW. A few smaller plants are found in the Colorado Plateau of Utah and New Mexico.