The Great Plains region is a broad expanse of flat land underlain by thick sequences of sedimentary rock and primarily covered in grassland and prairie. Ancient sedimentation patterns and tectonic activity have favored the placement of widespread fossil fuel resources in this region. Organic-rich sediments were deposited in inland seas that spread across much of the region, and Cenozoic swamps contributed plant matter to form thick beds of coal. The Great Plains' sedimentary basins contain vast oil, gas, and coal reserves that dominate energy production here (see Figures 6.3 and 6.4), but the area's topography and climate also make it favorable for large wind farms.

Oil and Gas

The Southwest is rich in fossil fuel resources, in part because of its history as an area of deposition. A variety of fine-grained organic-rich shales and coals, porous sandstones, and carbonate rocks—all excellent reservoirs and source rocks for fossil fuels—are found in the Great Plains' sedimentary basins. In this area, many vertically stacked reservoir rocks can be accessed through one well due to the large thicknesses of sedimentary rocks in these basins.

The Denver Basin is, in area, the largest of the basins in the Southwestern states, covering a large part of northeastern Colorado and the corners of Wyoming, Nebraska, and Kansas where they intersect Colorado. The Denver Basin contains thick sequences of Pennsylvanian through Paleocene rocks, as well as Cretaceous sediments from the Western Interior Seaway. Oil and gas have been produced from the basin since 1901, when oil was discovered in the Cretaceous Pierre Shale near Boulder (Figure 6.20). Just north of Denver, the Wattenberg Gas Field has been a major producer of natural gas since the 1970s, and has produced more than 113 billion cubic meters (4 trillion cubic feet) of natural gas. It is the ninth largest source of natural gas in the United States. Other well-known reservoir formations in the basin include the early Cretaceous Muddy Sandstone and the late Cretaceous Codell Sandstone.

Directly overlying the Codell Sandstone is the fine-grained Niobrara Formation. The Niobrara is formed of considerable chalk deposits, made up of the fine-grained carbonate skeletons of phytoplankton, and of shale weathered from mountains to the west. The Niobrara Formation was deposited in the deepest parts of the Western Interior Seaway, along its eastern margin; it is so widespread that it occurs in three of the four Southwestern regions in the Southwest (the Great Plains, Rockies, and Colorado Plateau) and in all the major Southwestern basins except the Permian Basin. The formation has been a major reservoir for oil and natural gas since the 1920s, and it has been drilled most intensively in the Denver Basin, particularly in Weld County. In the past decade, oil production rates in the Niobrara Formation have expanded enormously through the application of "unconventional" drilling, using horizontal drilling combined with high volume hydraulic fracturing. This method fractures rocks beneath the surface, releasing gas and oil trapped in source rocks that have very low permeability (also known as "tight" layers).

Figure 6.20: An oil pumpjack in the Pawnee National Grasslands, Colorado, part of the Denver Basin.

Hydraulic fracturing uses high volumes of water introduced at high pressure through horizontal wells along the source rock layer, to create thousands of tiny fractures (Figure 6.21). Most horizontal wells are drilled where the source rock is approximately 100–150 meters (330–490 feet) thick. The fractures are held open by small grains of sand carried by gel in the water, increasing its viscosity. A number of chemicals are added to the water to increase the recovery of fossil fuels, including a chemical to reduce friction as the mixture is introduced (thus the term "slickwater"). "Slickwater, high-volume hydraulic fracturing"—often shortened to "hydraulic fracturing" or simply "fracking"—has greatly increased the accessibility of available fossil fuel resources and the production rate of oil and gas. It has also been controversial, in part because of the use of large volumes of water, concerns about protection of shallow aquifers, the proper disposal of flow-back fluids, and associated environmental impacts. Unconventional drilling through low-permeability source rock layers has generated shale gas and shale oil booms around the US.

Another unconventional fossil fuel source in Colorado and Utah is Eoceneaged "oil shale" (as opposed to shale oil, see box on p. 247), in which the immature source rock is mined and heated to generate liquid hydrocarbons. This resource has been known and produced for many years elsewhere in the world, but has only recently been extracted in the Southwest because it is generally only economically viable to do so during times of high oil prices. Oil shale is more expensive to produce per unit of energy than oil and gas obtained through regular extraction, and has similar environmental consequences.

Figure 6.21: Oil wells (not to scale). A) A conventional vertical well. B) An unconventional horizontal well. Hydraulic fracturing may be carried out along horizontal wells running for 1.6 kilometers (1 mile) or more along layers with oil or gas trapped in pore spaces.

The Permian Basin, present partly in southeast New Mexico with a larger portion in western Texas, contains one of the world's thickest Permian rock sequences and accounts for nearly a fifth of US crude oil production. The "Greater" Permian Basin is composed of several sub-basins and other structural features: one of the two primary basins, the Delaware Basin, is present in southeast New Mexico (parts of Eddy and Lea Counties) and the western tip of Texas; the Midland Basin, located entirely in Texas, is the other. It contains a thick sequence of limestones and dolostones from reef and adjacent carbonate environments, together with evaporites, sandstones, and other reservoir structures that allowed hydrocarbons to accumulate. The Permian Basin's source rocks are largely organic-rich, shaly carbonates interbedded within the sequence. The first oil wells in the Permian Basin, in both New Mexico and Texas, were completed in the 1920s, and exploration in the 1940s led to significant production by the 1950s. In the past two decades, unconventional drilling in a number of carbonate-rich formations has increased oil production further. In New Mexico, new drilling has focused on Delaware Basin carbonates and a structure known as the Northwest Shelf, an area where shallow marine sediments accumulated just northwest of the Delaware Basin. Other formations of special interest include the early Permian Wolfcamp Formation, which occurs throughout the Permian Basin, as well as the more locally occurring early Permian Glorieta-Yeso, Abo-Yeso, and Bone Spring formations, and the late Permian Delaware Group.

Coal

Significant amounts of coal are found in the Raton Basin, which lies along the boundary of Colorado and New Mexico. The basin contains a sequence of sediments that accumulated over a similar time interval to those of the Denver Basin. Cretaceous-aged coals in the Vermejo Formation formed along the Western Interior Seaway's deltas, and Cretaceous-Paleocene coals of the Raton Formation formed in swampy alluvial environments after the Western Interior Seaway retreated. Raton Basin coals have been mined since the 1870s. Coal mining declined substantially by the 1950s (Figure 6.22), but extraction of coalbed methane from the same formations began in the 1980s. For a time, coalbed methane from the Raton Basin became one of the largest sources of natural gas in the US, though it has since been eclipsed by the shale gas boom. Bituminous and lignite coals have historically been extracted from the Denver Basin, but coal mining ended there in 1979.

Figure 6.22: Ovens in Cokedale, Colorado, used to produce a fuel called coke by baking bituminous coal. The coal mines and coke ovens of Cokedale ceased operation in 1946.

Alternative Energy

The Great Plains (in this case referring to the full area that runs from Texas to Montana and into Canada) has been called the "Saudi Arabia of Wind Energy," at least in terms of potential (Figure 6.23). Wind energy provides approximately a third of the renewable energy produced in the US, with hydroelectric representing approximately half; solar, geothermal, and biomass account for the remaining sixth. In contrast to hydroelectric, wind energy is growing rapidly: it grew tenfold on a national scale from 2004 to 2014, and wind farms on the Great Plains played a significant role in that growth. In the Southwest, the two Great Plains states are among the top 19 states for wind energy as a percentage of state electricity generation (Colorado 14%, New Mexico 6%). This is all the more remarkable considering the rate of local petroleum and coal extraction.

Figure 6.23: Wind energy potential in the Southwestern US, with locations of active wind farms.

Wind energy is strong and persistent on the high plains and the uplands of eastern Colorado and New Mexico. There are especially consistent high wind speeds in the upland area of northeastern Colorado, northeast of Denver, where over half-a-dozen wind farms of over 200 MW each have been built. The largest wind farm in the state, the Peetz Table Wind Energy Center, has a capacity of 430 MW. All together, the wind farms of Colorado's Great Plains make the state 10th in the nation for total wind energy. New Mexico's largest wind farm, the 250 MW capacity Roosevelt Wind Farm, is located on the Great Plains south of Clovis; the plant began operation in December 2015.

While the Great Plains also provide good opportunities for solar power capacity, this industry is still in the early stage of development in the region and only produces approximately 80 MW annually.