Coal Reserves vs. Coal Resources 101

The question is often posed, “How much coal does Wyoming have?” This question does not have a simple answer. While there remains a large amount of coal in the earth, scientists must answer the question by determining whether or not the coal can be mined or if it is too deep to be mined. The volume of coal in-place underground is called a coal resource. It economically represents a large number of coal underground based on drill hole data and thickness. Most of this coal is less than 6,000 feet (Fig 1). Coal is usually mined less than 3,500 feet deep: any deeper and the weight of the overlying rock could collapse. Coalbed methane resources are also found at a depth greater than 3,500 feet, but can go as deep as 6,000 feet. All coal deeper than 6,000 feet are considered uneconomic. As a result this is considered the maximum depth for extraction.

Coal Reserves
Figure 1. Chart showing coal resource categories as established by the U.S. Geological
Survey and the former U.S. Bureau of Mines. Modified from USGS Circular 891.

Most mining occurs at depths less then 3,500 feet, where the coal is shallow and considered safer to mine. In addition, 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, has a coal deposit thickness appropriate enough to fit coal mining equipment, and does not have 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 and that has to do with the inconsistent terminology due to the economics of coal. 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 coal resources that can be mined economically 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 from the two holes that there may be a correlation of the coals, which when combined can determine the count of 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. 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 that coal which 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 everywhere there is coal, then the amount of undiscovered coal in the U.S. if 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) counts the ‘Demonstrated Reserve Base,”or DRB. This was based on the number of drill holes and measured sections of coal by 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 the 1970s. The latest study on coal reserves in Wyoming was conducted by the U.S. Geological Survey’s for the Powder River Basin (PRB). As of 2013, USGS fact sheet 2013-3143, at USGS Newsroom, states that the PRB of Wyoming and Montana contain an estimated in-place coal resource of 1.07 trillion short tons of coal (Fig. 2). 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. The economically recoverable coal resources portion of this amount, at today’s coal prices, is 24.6 BT.

DRB   Reserves
The 1973 estimate indicates Wyoming holds 12%
of the U.S. Demonstrated Reserve Base.
Wyoming Estimated Recoverable Reserves are 15% of the U.S. total (1973).

For the Wyoming part of the PRB this is 855 BT for original in-place coal resource. The coal availability of Wyoming’s PRB is 768 BT. The recoverable coal for Wyoming’s part of the Powder River Basin is about 127 BT.

Wyoming has the greatest resource of coal in the nation. If this is extrapolated to the entire state, then a new conservative estimate for the Wyoming DRB would be approximately 165.1 billion tons.

  Original Resource Coal Availability Recoverable Coal Economic Recoverable Reserve Today
USGS WY-MT PRB (2013) 1,070 930 162 24.6
Wyoming PRB only 855 768 127 11.6
Estimated Wyoming Entire State (From N. Jones, 2010, Keystone Industry Coal Atlas) 1,463 --- 165.1 >20

Figure 2. Comparative table showing 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).

Additional Information:

Download DRB State Ranking-Demonstrated Reserve Base State Ranking