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 trillion short tons of coal in Wyoming, not all of it is useable or accessible. The volume of coal in-place underground is called a coal resource. Coal shallower than 6,000 feet below the surface is considered the original resource. This is because coal bed natural gas resources can be found at depths up to 6,000 feet, and so this is considered the maximum depth for extraction. However, coal is usually mined safely at depths less than 3,500 feet; any deeper and the weight of the overlying rock could collapse.

Coal reserves are the portion of the total resource that is economically viable to mine. Coals are economically mineable depending on their bed thickness and burial depth, the continuity of the coal seam, as well as the current costs to mine that coal. Coal resources near faults, within steeply dipping strata, or that are too thin to mine are not included in the coal reserve total. When assigning coal resources to a reserve classification, it is assumed that the coal can be mined in an economic manner. As the price of coal and mining technologies change with time, the reserve percentage can change too.

Coal resources and reserves can be subdivided into a series of categories based on the reliability of the geologic data used to estimate the volume. The first step in determining coal resources involves locating coal occurrences using a combination of surface exploration and exploratory drilling. Each outcrop or drill hole where coal is encountered provides data that are used in the resource calculation. These drill holes and outcrops are called reliability points. At these locations the amount of coal is known; as one travels further from these reliability points, the uncertainty of the coal estimate increases.

The categories of coal resources as defined by their geologic reliability are measured coal (0–0.25 miles from the reliability point), indicated coal (0.25–0.75 miles from the reliability point), and inferred coal (0.75–3 miles from the reliability point). These three categories encompass identified coal. Any coal is considered hypothetical beyond the 3-mile radius from the reliability points. Often the term demonstrated reserves, also called the demonstrated reserve base, is used when discussing coal reserves; this is the amount of measured and indicated coal combined. Together, the demonstrated and inferred categories represent coal tonnages that have been identified with drill hole or outcrop measurements.

Resource and Reserve
Resource and Reserve:  Diagram illustrating coal resource categories. Modified from USGS Circular 891. [Credit: WSGS]


Kelsey Kehoe,