Coal Resources & Reserves

The question is often posed, “How much coal does Wyoming have?” This question does not have a simple answer because, although there are approximately 1.4 billion short tons of coal in Wyoming, not all of it is useful or accessible. The volume of coal in-place underground is called a coal resource. Only coal shallower than 6,000 feet below the surface is considered economic. Coal is usually mined safely at depths less than 3,500 feet: any deeper and the weight of the overlying rock could collapse. Coalbed methane resources are found at a depth more than 250 feet, but can go as deep as 6,000 feet. As a result, this is considered the maximum depth for extraction.

Miners only count that part of the coal resource that is economically mineable, called the coal reserve. The coal reserve only counts that volume of drill hole information that has coal less than 3,500 feet deep, has a mineable continuity to the seam, a coal deposit thickness appropriate enough to fit coal mining equipment, and would not cause surface disturbances to civilization such as towns, major roads, power plants, or abandoned mines. There are geologic reasons not to count some coal resources as coal reserves also, such as the coal near faults or steeply dipping strata or coal that is too thin to mine. These parameters must be subtracted to come up with the coal reserve.

There is an important geological question about coal reserves: When assigning coal resources to a reserve classification it is assumed that the coal can be mined in an economic way. Coal reserves represent a small percentage of the economic coal resources that can be mined today. This is a changeable percentage, as the price of coal and the technology used to mine that coal changes over time.

Coal resources and reserves can be subdivided into a series of categories based on geologic certainty. With exploration, when a hole is drilled and coal is encountered, this coal intercept is logged in terms of its depth and thickness. When another hole is drilled in the same vicinity a measurement can be taken of the coal intercepts, which can determine the likelihood of whether that coal is the same mineable coal or not. Then it can be inferred from the data that there may be a correlation of the coals, which when combined, can determine the volume of coal in that area and whether it can be mined based on thickness and depth.

Measured coal is the amount of coal based on a close proximity to the drill hole, which is one-quarter mile, as defined by the U.S. Geological Survey, and former U.S. Bureau of Mines. This direct measurement usually includes information for coal rank and quality as well. Indicated coal is the amount of coal based on direct measurement and reasonable geologic assumptions, such as individual coal bed correlations. Indicated coal is that coal beyond measured coal at one-quarter to a distance of three-quarters of a mile from the drill hole. Often coal reserves are defined by the term “demonstrated reserves,” or the amount of measured and indicated (combined) coal near a drill hole or outcrop. Inferred coal refers to coal that is beyond the three quarter mile designation of known coal that is estimated to still contain a coal intercept. This is not widely used, especially in the western United States, where Cretaceous and Tertiary coals are not usually that contiguous for miles at a time. Inferred coal represents that coal from three quarters of a mile to three miles in diameter away from the drill hole. Together, the Demonstrated and Inferred categories represent coal tonnages that have been identified with drill hole or outcrop measurements (USGS, 1983). Since drill hole information does not exist for coal everywhere, then the amount of undiscovered coal in the United States is much greater than the identified coal.

Demonstrated Reserve Base

To count how much coal is in-place for each state, the Energy Information Administration (EIA) determines the "Demonstrated Reserve Base,” or DRB. This was based on the number of drill holes and measured sections of coal available in each state back in the 1970s. Currently, Wyoming ranks third on the list (after Montana and Illinois) at 61 billion tons. This database has not been updated for new coal exploration since 1993. The latest study on coal reserves in Wyoming was conducted by the U.S. Geological Survey for the Powder River Basin (PRB). USGS Professional Paper 1809, published 2015, states that the PRB in Wyoming and Montana contains an estimated in-place coal resource of 1.16 trillion short tons of coal. Of that total, the recoverable coal is 162 billion tons (BT). This is similar to measured and indicated coal, or the Demonstrated Reserve Base of coal.

For the Wyoming part of the Powder River Basin this is 855 billion tons for original in-place coal resource. The coal availability of Wyoming’s PRB is 768 billion tons. The recoverable coal for Wyoming’s part of the PRB is about 127 billion tons. If this is extrapolated to the entire state, then a new conservative estimate for the Wyoming DRB would be approximately 165.1 billion tons.

The table above shows the most recent studies on coal resources and reserves in Wyoming and Montana. Note that the recoverable and economically recoverable coal resources for the entire state of Wyoming are only estimated and not calculated. Original coal resources for all of Wyoming may count some coal deeper than 6,000 feet. All values in billion short tons. Data sources from USGS (2008-2013) and WSGS (2010). Approximately 2.5 billion tons of coal have been mined in Wyoming since 2010 and should be subtracted from these calculations.