Powder River Basin


The Powder River Basin is a geologic structural basin in southeast Montana and northeast Wyoming, about east to west and north to south, known for its extensive coal reserves. The former hunting grounds of the Oglala Lakota, the area is very sparsely populated and is known for its rolling grasslands and semiarid climate.
The basin is both a topographic drainage and geologic structural basin, drained by the Powder River, after which it is named, Cheyenne River, Tongue River, Bighorn River, Little Missouri River, Platte River, and their tributaries.
The major cities in the area include Gillette and Sheridan, Wyoming and Hardin, Montana.
In 2007, the region produced 436 million short tons of coal, more than twice the production of second-place West Virginia, and more than the entire Appalachian region. The Powder River Basin is the largest coal-producing region in the United States. The region includes the Black Thunder Coal Mine, the most productive in the United States, and North Antelope Rochelle Mine, the second most productive. In recent years, the region has become a major producer of natural gas, both conventional natural gas and coal-bed methane.

Geologic history

The Powder River Basin contains a section of Phanerozoic rocks up to thick, from Cambrian to Holocene.

Cretaceous

The thickest section of the Powder River Basin is composed of Cretaceous rocks, an overall regressive sequence of mostly marine shales and sandstones deposited in the Western Interior Seaway.

Tertiary

The coal beds of the region began to form about 60 million years ago when the land began rising from a shallow sea. The rise of the Black Hills uplift on the east and the Hartville uplift on the southeast side of the basin created the present outline of the Powder River Basin.
When the coal beds were forming, the climate in the area was subtropical, averaging about of rainfall a year. For some 25 million years, the basin floor was covered with lakes and swamps. Because of the large area of the swamps, the organic material accumulated into peat bogs instead of being washed into the sea. Periodically the layers of peat were covered with sediments washed in from nearby mountains. Eventually the climate became drier and cooler. The basin filled with sediment and buried the peat under thousands of feet, compressing the layers of peat and forming coal. Over the last several million years, much of the overlying sediment has eroded away, leaving the coal seams near the surface.

Coal

Powder River Basin coal is classified as "sub-bituminous" and contains an average of approximately 8,500 btu/lb, with low sulfur. Contrast this with eastern, Appalachian bituminous coal containing an average of 12,500 btu/lb and high sulfur. PRB coal was essentially worthless until air pollution emissions from power plants became a concern. A coal-fired plant designed to burn Appalachian coal must be modified to remove SO2 at a cost estimated in 1999 to be around $322 per ton of SO2. If it switched to burning PRB coal, the cost dropped to $113 per ton of SO2 removed. Removal is accomplished by installing scrubbers.
The Powder River Basin is the largest coal mining region in the US, but most of the coal is buried too deeply to be economically accessible. The Powder River Basin coal beds are shaped like elongated bowls and as mines expand from east to west in the Powder River Basin, they will be going "down the sides of the bowl". This means that the overburden will increase as will the stripping ratio.
The United States Geological Survey has conducted a series of studies on the economic accessibility of coal in the major coal-producing regions of the country. The studies typically found that only a small fraction of the coal would be economically accessible at the price then of $10.47/ton. In August 2008, the USGS issued an updated assessment of coal in the Powder River Basin. After considering stripping ratios and production costs, the USGS concluded that at that time, only 6% of the original resource, or 10.1 billion short tons of coal, was economically recoverable. At a price of $60/ton, however, roughly half of the coal would become economic to produce. Increasing the price paid for coal can increase the amount of economically recoverable coal, but increasing the price of coal also increases its production cost. Because coal is a solid, it cannot be produced from many scattered wells like oil and gas can be. Rather, coal has to be produced from mines that expand slowly by moving massive quantities of overburden.

Coal mining

Fifteen mines operate in the Powder River Basin, with most of the active mining taking place in drainages of the Cheyenne River. The US uses about 600 million tons of coal a year, with about 40% of the coal coming from the Powder River Basin. The amount of coal coming from the Powder River Basin has been increasing over the last 20 years. In 2019, 43% of the nation's coal was mined in the Power River basin.
The mines in the Powder River Basin typically have less than 20 years of life remaining. Almost all of the coal in the Powder River Basin is federally-owned, and further mine expansions will require a series of federal and state approvals, as well as large investments in additional mine equipment to begin the excavations.
The majority of the coal mined in the Powder River Basin is part of the Fort Union Formation, with the low sulfur and ash content of the coal in the region making it very desirable. Coal supplies about one-fifth of the United States' electricity supplies. The Powder River Basin mines supply approximately 40% of the coal that fuels those stations for generating electricity.
The mines work in areas where the stripping ratio is between 1:1 and 3:1. As the mines expand the stripping ratio will increase. As more rock must be moved the production cost will also increase.
The mines are largely non-union operations with a history of squelching labor activity. According to historian Ryan Driskell Tate, surface mining in remote areas happened to reduce some of the "occupational togetherness" typically associated with coal miners working shoulder-to-shoulder underground in Appalachia. The environmental impact of mining on grass and aquifers has been a concern for surrounding ranchers who organized to resist new mines in the 1970s.

Coal mining companies operating in the Powder River Basin

Southern Powder River Basin
Northern Powder River Basin
  • Lighthouse Resources
  • Navajo Transitional Energy Company
  • Westmoreland Coal Company
In June 2019, Peabody Energy and Arch Coal announced a joint venture for their combined Powder River Basin assets. In September 2020, Peabody Energy and Arch Coal abandoned their plans for a joint venture following regulatory intervention by the Federal Trade Commission.

Power plants fueled from Powder River Basin coal (incomplete list)

Oil and gas

The Powder River Basin also contains major deposits of petroleum, including the giant Salt Creek Oil Field. The oil and gas are produced from rocks ranging from Pennsylvanian to Tertiary, but most comes from sandstones in the thick section of Cretaceous rocks.
There is a recent resurgence in oil and gas production as a result of horizontal drilling and hydraulic fracturing. This resurgence is occurring mainly in the Wyoming portion of the basin, which is historically known as the source of the basin's oil. In 2009, a low of 38,000 barrels of oil per day were produced in the basin. That number has risen dramatically to 78,000 barrels per day in the first quarter of 2014.
The Bell Creek Field is a Lower Cretaceous stratigraphic trap in the Muddy Sandstone. Discovered in 1967 by the Exeter Drilling Co. No. 33-1 Federal-McCarrell well, which found 27 feet of pay at a depth of 4500 ft.

Coalbed methane

Recent controversy surrounds the extensive coalbed methane extraction in the region. In the last decade, nearly 7000 such wells have been drilled. An extensive network of gas pipelines connecting these wells has been built, along with a series of pressurization plants, as well as power lines to provide electricity to operate the system. In addition, thousands of miles of new access roads have been constructed.
Extracting the gas requires that water be pumped to the surface to release gas trapped in the coal seam. While some of the water is successfully utilized in agriculture production such as livestock water and crop irrigation, some waters are naturally high in salinity and sodium adsorption ratio. There has been controversy on how to best manage these saline waters.
In 2007, Powder River Basin coalbed field produced 442 billion cubic feet of gas, making the field the 3rd largest source of natural gas in the United States.