Region 2: The Great Plains
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 Figure 7.5), but the area’s topography and climate also make it appropriate for large wind farms.
Coal
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.
In Wyoming, great quantities of coal are produced annually from the Powder River Basin (see Figure 7.4). 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 US. These deposits provide more than 40% of the US coal supply, making Wyoming the largest coal-producing state (Figure 7.7). The Black Thunder Coal Mine is currently the most productive coal mine in the US, 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.
Figure 7.7: Coal mining in Wyoming. Mining of thick sub-bituminous coal beds in the Paleocene Fort Union and other formations make Wyoming the largest coal-producing state in America.
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 (see Figure 7.4). 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.
Oil and Gas
Oil deposits from the Great Plains region are also among the largest in the US. 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 has been home to both types of environments throughout its geologic past.
See Chapter 1: Geologic History to learn more about the changing face of the Northwest Central through geologic time.
Conventionally, finding oil and gas has not been as simple as finding organic-rich 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, 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—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 “reservoir.” Reservoirs are typically found in porous sedimentary layers and thin natural fractures. Most oil and gas has been extracted using the conventional technique of searching for such reservoirs and then drilling into them, which allows the gas or oil to come to the surface through a vertical well. Reservoir rocks in the Great Plains include dolomites, chalks, and organic-rich shales.
Coal
As leaves and wood are buried more and more deeply, 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. However, in the Northwest Central US most coal is from plants of the Paleocene and Eocene epochs.
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 in vast numbers to the bottom of large water bodies. 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, enough organic matter may accumulate 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.
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, though estimates of the size of oil and gas reserves that can or will be economically extracted are in dispute. The Fort Union Formation in the Powder River Basin is also a significant source of coalbed methane. Thanks to these geological units, this region is a net exporter of energy, providing much of the central US with its oil and gas.
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 expanded enormously through the application of 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). 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 7.8). Most horizontal wells are drilled where the source rock is about 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 associated environmental impacts. 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 (Figure 7.9).
Figure 7.8: 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 a mile or more along layers with oil or gas trapped in pore spaces.
Figure 7.9: Oil pumpjacks in McKenzie County, North Dakota. The flame on the right-hand side is a flare that burns off natural gas separated from the oil.
See Chapter 3: Fossils to learn about fossils of the 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 sedimentary deposition 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, and in the past decade unconventional drilling below about 1830 meters (6000 feet) has greatly increased oil production in southeastern Wyoming.
How does oil drilling work?
Once an oil trap or reservoir rock has been detected on land, oil crews excavate a broad flat pit for equipment and supplies around the area where the well will be drilled. Once the initial hole is prepared, an apparatus called a drilling rig is set up. The rig is a complex piece of machinery designed to drill through rock to a predetermined depth. A typical drilling rig usually contains generators to power the system, motors and hoists to lift the rotary drill, and circulation systems to remove rock from the borehole and lubricate the drill bit with mud. It also contains high-pressure blowout prevention equipment to prevent pressurized oil or gas from rising uncontrollably to the surface after being tapped. The support structure used to hold the drilling apparatus is called a derrick. In the early days of oil exploration, drilling rigs were semi-permanent structures and derricks were left onsite after the wells were completed. Today, however, most rigs are mobile and can be moved from well to well. Once the well has been drilled to a depth just above the oil reservoir, a cement casing is poured into the well to structurally reinforce it. Once the casing is set and sealed, oil is then allowed to flow into the well, the rig is removed, and production equipment can be put in place to extract the oil.
The Powder River Basin hosts significant quantities of coalbed methane. Coal mines have long been vented to the atmosphere, in part because of the build-up 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. While originally considered a hazard to be mitigated in subsurface mines, methods have been developed to trap this methane as an additional energy source. In some subsurface coal seams, water saturates fractures in the seam, transforming it into 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 in the coal fractures may be trapped in the coal. To extract this methane, water can be removed via wells, thereby reducing pressure and allowing the gas to escape toward lower pressures along the well bore (Figure 7.10). 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 that infiltrates below the surface to the aquifer. Production rates climbed steeply beginning in the early 1990s, though in recent years it has decreased both in absolute and relative quantity as shale gas methane production has increased. 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.
Figure 7.10: 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.
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 (Figure 7.11). Wind energy provides about a third of the renewable energy produced in the US, with hydroelectric representing about 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 have played a significant role in that growth. In the Northwest Central, 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.
Figure 7.11: Wind energy potential in the Northwest Central US.
Wind Energy and Landscape
Economically useful wind energy depends on steady high winds. Variation in wind speed is in large part influenced by the shape and elevation of the land surface. For example, higher elevations tend to have higher wind speeds, and flat areas can allow winds to pick up speed without interruption; thus high plateaus are especially appropriate for large wind farms. Since plateaus with low grass or no vegetation (or water bodies) have less wind friction than do areas of land with higher crops or forests, they facilitate higher winds. For all these reasons, the Great Plains region has high average wind speeds throughout its extent.
The Rockies and the Basin and Range, however, may have locally high wind speeds that can support strategically placed wind farms. For example, constricted valleys parallel to wind flow may funnel air into high velocities. Elevated ridges perpendicular to wind flow can also force fast winds across them. Thus, the wind velocities of these areas can vary geographically in quite complicated ways.
Uranium
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.
