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Coal Resource Classification System of the U.S. Geological Survey

By Gordon H. Wood, Jr., Thomas M. Kehn, M. Devereux Carter, and William C. Culbertson



The classification system presented herein is an expansion of the system adopted in 1976. It employs a concept by which coal is classified into resource/reserve base/reserve categories on the basis of the geologic assurance of the existence of those categories and on the economic feasibility of their recovery. Categories are also provided for resources/reserve base/reserves that are restricted because of legal, environmental, or technologic constraints. Geologic assurance is related to the distance from points where coal is measured or sampled; thicknesses of coal and overburden; knowledge of the rank, quality, depositional history, areal extent, and correlations of coal beds and enclosing strata; and knowledge of the geologic structure. Economic feasibility of recovery is affected not only by such physical and chemical factors as thicknesses of coal and overburden, quality of coal, and rank of coal, but also by economic variables--such as price of coal, cost of equipment, mining, labor, processing, transportation, taxes, and interest rates, demand for and supply of coal, and weather extremes--and by environmental laws, restrictions, and judicial rulings. For example, the Clean Air Act of 1970 issued standards that severely limited the emission of sulfur by new coal-burning power plants and, as a result, made the low-sulfur, low-rank coal deposits of the Northern Great Plains economically competitive. Similarly, environmental restrictions on the surface mining of coal and the need for adequate reclamation of mined areas has adversely affected the economic and technologic feasibility of extracting coal from some near-surface deposits.

The classification system is designed to quantify the total amounts of coal in the ground before mining began (original resources) and after any mining (remaining resources). It is also designed to quantify the amounts of coal that are known (identified resources) and the amounts of coal that remain to be discovered (undiscovered resources). The system also provides for recognizing amounts of coal that are (1) standard distances from points of thickness measurements --measured, indicated, inferred, and hypothetical; (2) similar to coal currently being mined (reserve base and inferred reserve base); (3) economically recoverable currently (reserves and inferred reserves); (4) potentially recoverable with a favorable change in economics (marginal reserves and inferred marginal reserves); and (5) subeconomic because of being too thin, too deeply buried, or lost-in-mining. Finally, the system allows tabulation of coal amounts that are restricted from mining by regulation, law, or judicial ruling.

Two factors have created difficulties in categorizing resources and reserves in all classification systems. First, most geologists and engineers who classify resources and reserves are not experts in the economics of mining, transportation, processing, and marketing. Second, economic conditions change with time, so that the economic viability of coal is relatively fluid. For example, subeconomic resources of today can become reserves of tomorrow as the price of coal rises; conversely, reserves can become subeconomic resources as the price of coal drops. Finally, changing regulations, laws, and judicial rulings can affect mining, transportation, processing, and marketing, and thus the classification of coal resources. The concept of a reserve base was developed to alleviate these difficulties (U.S. Geological Survey, 1976, p. B2).

The reserve base is identified coal defined only by physical and chemical criteria such as thicknesses of coal and overburden, quality, heat value, rank, and distance from points of measurement. The criteria for thickness of coal and for overburden have been selected so that the reserve base includes some currently subeconomic coal. The concept of the reserve base is to define a quantity of in-place coal, any part of which is or may become economic depending upon the method of mining and the economic assumptions that are or will be used. An additional purpose is to aid in long-range public and commercial planning by identifying coal suitable for economic recovery. Thus, resource specialists need not expend their time identifying the component parts of coal deposits that are currently economically recoverable (reserves) because the reserve base category contains much of the coal that will be classed as reserves in the foreseeable future. Those required to classify coal as being economically recoverable, marginally recoverable, or subeconomic can examine reserve base estimates to locate such coal.

Figures 1 and 2 are conceptual diagrams modified from Circular 831 (U.S. Geological Survey, 1980) that show the relationship of the various classes of coal resources, the reserve base, and reserves. The classes are categorized in both figures according to their degree of geologic assurance (geologic assurance or proximity to points of control increases to the left), and according to their degree of economic feasibility of recovery (economic feasibility of mining increases upward). The resource/reserve base/reserve categories (classes) that can be used are not limited to those shown in figures 1 and 2 nor to the categories described in succeeding pages. For example, a particular bed of coal may be identified as being low-sulfur (0-1 percent), low-ash (0-8 percent), high-volatile A bituminous, and premium coking coal; other beds of coal may be identified as medium-sulfur (1.1-3.0 percent), high-ash (> 15 percent), high-volatile bituminous, surface-minable cannel coal, and so forth. The ability of the classification system to precisely describe the characteristics of a body of coal allows the coal resources of the United States to be divided into many hundred resource classes or categories.

The hierarchy of coal resources shown in figure 3 illustrates the conceptual relationships between the classes of resources as distinguished by their definitions and criteria. Examination of figures 1, 2, and 3 makes clear that each succeeding class in the hierarchy from original and remaining resources to reserves is included in the overlying classes. Original resources include remaining resources and cumulative depletion. Remaining resources include identified and undiscovered resources (divisible into hypothetical and speculative resources). Identified resources include measured, indicated, inferred, and demonstrated resources. Measured and indicated resources contain coal classed as reserve base, and inferred resources contain coal classed as inferred reserve base. Some measured, indicated, and inferred resources are subeconomic because they are too thin to mine or are buried too deeply to be mined by current extraction techniques; furthermore, parts of the reserve base and inferred reserve base are potentially subeconomic because they will be lost-in-mining. Reserves and inferred reserves are economically minable as of the time of classification. The reserve base and inferred reserve base also contain some coal that is believed to be potentially economic and which is classed as marginal and inferred marginal reserves.


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