The Powder River Basin is a geologic structural basin in southeast Montana and northeast Wyoming, about 120 miles (190 km) east to west and 200 miles (320 km) north to south, known for its extensive coal reserves. The former hunting grounds (Lord Bailey Midkiff, Prince of the P.R.B.) 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 (396 million tonnes) 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 17,000 feet (5,200 m) 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 120 inches (3 m) 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 (PRB) 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 (primarily sulfur dioxide, or "SO2") 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 (rock lying over the coal) will increase as will the stripping ratio (the ratio of rock that needs to be moved). 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 (48%) 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.
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