Energy in the Great Plains

Overview

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, but the area’s topography and climate also make it appropriate for large wind farms.

Oil and gas

Oil deposits from the Great Plains region are also among the largest in the U.S. It is possible to make sense of why we find petroleum and natural gas in these areas by understanding the history of marine environments. Mud with relatively high organic matter content tends to accumulate in shallow continental seas and in coastal marine environments. The northwest-central U.S. has been home to both types of environments throughout its geologic past.

Reservoir rocks in the Great Plains include dolomites, chalks, and organic-rich shales. There have been estimates of some 400 billion barrels of untapped oil in the Bakken Formation and large reserves of both oil and natural gas in the Niobrara Formation, although estimates of the size of oil and gas reserves that can or will be economically extracted are in dispute. Thanks to these geological units, this region is a net exporter of energy, providing much of the central U.S. with its oil and gas.

Williston Basin (Bakken Formation)

The Bakken Formation formed in the late Devonian and early Mississippian, in a continental sea that filled what we now call the Williston Basin. The Bakken is known only from coring, as it does not outcrop at the surface. The source rock for the formation’s oil is present in its upper and lower dark shale layers, and a reservoir layer of dolomite lies between the shales. Since 2000, oil production rates in the Bakken Formation have expanded enormously through the application of horizontal drilling combined with high volume hydraulic fracturing. 

Unlike some famous “fracked” formations in other areas, such as the Barnett Shale in Texas and Marcellus Shale in Pennsylvania, the part of the Bakken Formation most intensively hydraulically fractured has been its dolostone reservoir unit rather than the dark shale source rock. This unconventional drilling activity is centered in North Dakota, which has become the nation’s second largest oil-producing state after Texas.

Niobrara Formation

The Niobrara Formation, also known as the Niobrara Chalk or Niobrara Shale, extends from the Gulf of Mexico to the Arctic, and originates from sediments deposited in the Late Cretaceous Western Interior Seaway. Where the formation outcrops at the surface, it is famous for its fossil faunas. The Niobrara is tapped for fossil fuels in the Denver Basin (also known as the Julesburg or D-J Basin), which underlies northeastern Colorado, a small corner of southeast Wyoming, and southwest Nebraska. The formation contains alternating chalks and organic-rich marls and shales; the marls and shales are a source of petroleum, and the adjacent chalks have become reservoir rocks. Natural gas and oil from conventional drilling have been extracted from the Niobrara since the early 1900s. Unconventional drilling below about 1830 meters (6000 feet) has greatly increased oil production in southeastern Wyoming in recent years.

Coal and coalbed methane

Williston Basin (North Dakota)

The world’s largest known lignite coal deposit, weighing in at an estimated 351 billion tons, is found in western North Dakota’s Williston Basin. This area is known as the Fort Union coal region, named after the Fort Union Formation, a thick sequence of Paleocene-aged coal deposits lying above Cretaceous-aged marine sediments from the Western Interior Seaway. North Dakota’s supply of lignite is estimated to last more than 800 years, and the deposits are used for synthetic fuels (made of carbon monoxide and hydrogen) as well as fuel for nearby power plants. Coal mining in this area began in the 1870s, when small seasonal mines sprung up along the main routes of transportation in the area. Over 250 mines were in operation by the 1920s. Today, there are only six large coal mines in western North Dakota, from which 32 million tons of coal are extracted annually. One of these, the Freedom Mine, is the 12th largest coal mine in the US.

Powder river basin (Wyoming)

In Wyoming, great quantities of coal are produced annually from the Powder River Basin. Like the Williston Basin, the Powder River Basin contains a thick sequence of Cretaceous marine shales and sandstones formed in the Western Interior Seaway, overlain by Paleocene-aged coals of the Fort Union Formation. These coals have experienced greater heat and pressure from burial than those in the Williston Basin, and thus are higher-grade sub-bituminous coals. In fact, the Powder River Basin contains the largest resources of low-sulfur, low-ash, sub-bituminous coal in the U.S. These deposits provide more than 40% of the U.S. coal supply, making Wyoming the largest coal-producing state. The Black Thunder Coal Mine is currently the most productive coal mine in the U.S., providing 8% of the country’s coal and 20% of Wyoming’s total coal production. This mine utilizes the world’s largest dragline excavator, employed to strip the overlying rock and sediment and expose the underlying coal seams.

The Powder River Basin hosts significant quantities of coalbed methane. Coal mines have long been vented to the atmosphere, in part because of the buildup of methane (CH₄, the primary gas in natural gas) released from fissures around the coal. This methane is a byproduct of the process of coalification, by which ancient plant material was transformed into coal, and it accounts for over 5% of US methane production. Wyoming is one of the three leading US states for coalbed methane production (approximately equal to that of Colorado and New Mexico), each of which account for about 25% or more of the national total.

Montana

The Great Plains of Montana also produce sub-bituminous coal from the northern extension of the Powder River Basin and lignite from the western extension of Williston Basin deposits. Montana ranks 6th in the nation among coal-producing states. Considerably more coal resources lie below currently mineable depths, that is, below about 150 meters (500 feet). Not surprisingly, these zones are being considered for potential underground coal gasification projects that would convert coal to gas below the surface and then bring the gas to the surface through wells.

Wind 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. Wind energy provides about a third of the renewable energy produced in the U.S., with hydroelectric representing about half; solar, geothermal, and biomass account for the remaining sixth.

In contrast to hydroelectric, wind energy is growing rapidly. Wind grew tenfold on a national scale from 2004 to 2014, and wind farms on the Great Plains have played a significant role in that growth. In the northwest-central region, the five Great Plains states are among the top 16 states for wind energy as a percentage of state electricity generation (South Dakota 25%, North Dakota 18%, Wyoming 9%, Nebraska 7%, and Montana 7%). This is all the more remarkable considering the rate of local petroleum and coal extraction.

Nuclear power

Uranium used in nuclear power plants is mined from certain sedimentary rocks in the Great Plains. Economic deposits of uranium are found in Paleocene and Eocene sandstones in the southern Powder River Basin of Wyoming and in Oligocene rocks in northwest Nebraska (Crow Butte). The Paleocene lignitic coals of North Dakota also contain significant uranium content. Despite the prevalence of uranium resources throughout the Great Plains, however, nuclear power is not generated here.