Region 1: The Central Lowland

Ancient sedimentation patterns and tectonic activity have favored the placement of widespread fossil fuel resources in this region. Organic-rich sediments were deposited in Paleozoic inland seas that spread across much of the region. Parts of the Central Lowland also favor the production of wind energy.

Conventional Oil and Gas

The Central Lowland is home to several basins with thick sections of late Paleozoic fossil fuel-bearing deposits, including the Forest City and Cherokee Basins in Kansas, the Arkoma and Anadarko Basins in Oklahoma, and the Fort Worth Basin in northern Texas (Figure 7.5). 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, especially beyond the mouths of large rivers. The South Central has long been home to both of these environments—when organic matter originally accumulated on sedimentary deposits in these basins, they were situated along the southern part of the continent, covered by a shallow sea.

Figure 7.5: Sedimentary basins containing significant fossil fuel accumulations.

Figure 7.5: Sedimentary basins containing significant fossil fuel accumulations, and the area of these basins covered by shale formations in which horizontal drilling and hydraulic fracturing are used.

Fossil Fuels

Fossil fuels—oil, natural gas, and coal—are made of the preserved organic remains of ancient organisms. Coal and lignite result from the burial, compaction, and heating of preserved plant matter, whereas petroleum and natural gas originate deep underground through a slow process involving the low-grade heating of sedimentary source rocks that contain an abundance of organic matter. In either case, organic matter only preserves when the rate of accumulation is higher than the rate of decay. This happens most often when the oxygen supply is sufficiently low that oxygen-loving bacteria cannot thrive, greatly slowing breakdown of organic matter. In this way, the organic matter can be incorporated into the buried sediment. The organics are compacted and heated with the rest of the rock, eventually transforming into fossil fuels.

The history of surface environments, evolution of life, and geologic processes beneath the surface have all influenced where fossil fuel deposits formed and accumulated. The largest oil and gas reserves were at one time nutrient-rich seas with abundant surface phytoplankton and organic-rich bottom sediments; the largest coal beds were swampy environments where fallen forest trees and leaves were buried in stagnant muds.

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, certain faults, or a salt dome), 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 seeking such reservoirs and drilling into them, allowing the gas or oil to come to the surface through a vertical well.

Reservoir rocks in the Central Lowland include both sandstones and carbonate rocks. In this area, many reservoir rocks can be accessed through one well due to the large thicknesses of sedimentary rocks in the many depositional basins. A number of the reservoirs are Pennsylvanian in age, often covering thousands of meters of sandstone and shale.

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.

Shale Gas

Some impermeable, or “tight,” layers contain oil and gas that has never escaped. Since the early 2000s, horizontal drilling has been combined with a method to fracture rocks beneath the surface, releasing gas and oil trapped in source rocks that have very low permeability. This method uses high volumes of water introduced at high pressure through horizontal wells along the source rock layer, to create thousands of tiny fractures (Figure 7.6). Most horizontal wells are drilled where the shale 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.

See Chapter 10: Earth Hazards to learn about the recent increase in Oklahoma’s earthquakes, linked to destabilization related to hydraulic fracturing.

The Fort Worth Basin is best known today for the Barnett Shale, an organic-rich shale that made international news when horizontal drilling and hydraulic fracturing were first used together to extract natural gas directly from layers in which the gas formed and was still trapped. Like other such shale gas layers, the Barnett Shale was originally mud that accumulated on a poorly oxygenated ocean bottom, in this case during the Mississippian period. The Barnett Shale extends to the southwest into the Permian Basin, part of the Great Plains region.

Another “tight shale” in the news is the Woodford Shale, which is mostly late Devonian and partly early Mississippian in age. The most productive geological units are found in the Arkoma Basin of southeast Oklahoma, but they do extend west into the Anadarko Basin.


See Chapter 3: Fossils for more on the swamps and forests of the Carboniferous period.

Unless rock layers are overturned, older rocks are found at the bottom and younger rocks are found at the top of a sedimentary sequence. This is known as the Law of Superposition.

The Central Lowland is also known for its coal deposits (see Figure 7.4). Coastal deltaic swamps and Paleozoic forests provided plant matter that, upon submergence and burial, eventually produced deposits of coal. These deposits are found in Pennsylvanian-age rocks which are stratigraphically above (and thus younger than) the Barnett Shale.

Figure 7.6: Oil wells.

Figure 7.6: 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.

Bituminous coal in the Central Lowland was mined heavily, especially in the early 1900s, and mostly from relatively thin Pennsylvanian-age beds (“seams”) near the surface. Bituminous coal deposits of a similar age are mined to the north in the Midcontinent Basin, sometimes known as the “Western Interior Coal Region,” which covers parts of eastern Oklahoma and Kansas and western Missouri (as well as the tip of eastern Nebraska and southern Iowa). These thin coal seams are deposited in cycles of marine and terrestrial sediments known as cyclothems (Figure 7.7). Such thin, shallow coal seams were excavated using electrically-powered mining shovels to scoop enormous quantities of fragmented rock and overburden. West Mineral, a town in southeasternmost Kansas, is home to Big Brutus (Figure 7.8), a 49-meter-tall (160-foot-tall) giant stripping shovel designed to dig coal from depths of 6 to 21 meters (20 to 69 feet). Although it is not the largest such shovel ever built, it was the second largest of its kind in operation during the 1960s and 70s.

Figure 7.7: An example of a cyclothem.

Figure 7.7: An example of a cyclothem: alternating sequences of marine and non-marine sedimentary rocks, characterized by their light and dark colors.

Figure 7.8: Big Brutus, a 49-meter-tall (160-foot-tall) giant stripping shovel.

Figure 7.8: Big Brutus, a 49-meter-tall (160-foot-tall) giant stripping shovel used in the 1960s and 70s to mine coal in southeastern Kansas. Today, it has been preserved as a national landmark, and is the largest electric shovel still in existence. Inset shows people standing next to rear treads, for scale.


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. A remarkable amount of today’s coal formed from the plants of the Carboniferous, which included thick forests of trees with woody vascular tissues.

The southernmost coal-bearing units in the Central Lowland are found in the Arkoma Basin, covering parts of Arkansas and Oklahoma. These are known especially for “coalbed methane”—natural gas extracted from subsurface coal deposits several hundred meters below the surface. An additional byproduct of natural gas extraction is the recovery of large quantities of helium gas at processing plants in Texas and Kansas.

Energy Production

In addition to the infrastructure associated with the extraction of fossil fuel resources, oil refineries, coal, and natural gas plants dot the Central Lowland from Missouri through Texas. Wind in the southwest part of the region, along the boundary with the Great Plains, also plays a role in local energy production (see Figure 7.19).

The only major nuclear power plant in the region is the Callaway Nuclear Generating Station in Auxvasse Township in Missouri (Figure 7.9). This plant, which began operating in 1984, produces about 1300 MW—9% of the state’s energy consumption. In 2012, the plant entered a proposal to build a second tower, which would more than double its capacity.

Figure 7.9: The containment building and cooling tower of Callaway Nuclear Generating Station near Fulton, Missouri.

Figure 7.9: The containment building and cooling tower of Callaway Nuclear Generating Station near Fulton, Missouri.