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

GEOLOGICAL SURVEY CIRCULAR 891

CRITERIA FOR COAL RESOURCE CLASSIFICATION

Although not specifically noted, coal resources are classified in figures 1, 2, and 3 according to geologic assurances of existence and to the economic feasibility of recovery. The degree of geologic assurance in this system of coal classification is determined from the interrelations of (1) proximity to or closeness of spacing of points where a coal bed is measured or sampled (reliability); (2) concepts, ideas, and models of the depth, rank, quality, thickness of coal, areal extent, depositional patterns and correlations of coal beds and enclosing rocks; and (3) knowledge of associated structural features as they control the distribution, extent, thickness, depth of burial, and metamorphism of coal resources. An understanding of these elements as they relate to the three dimensional configurations of stratigraphic sequence is necessary to provide the highest degree of geologic assurance as to the existence and continuity or lack of continuity of specific coal beds.

The degree of economic feasibility is determined by interrelating the (1) thickness of coal (see specific instruction No. 3, p. 34 ); (2) thickness of overburden; (3) the rank and quality of coal as ascertained from analyses that may be from the same bed or adjacent beds and which may be projected on geologic evidence for several miles; (4) costs of mining, processing, labor, transportation, selling, interest, taxes, and demand and supply; (5) expected selling price; and (6) expected profits.

The thickness of overburden and the thickness of a coal bed are the primary factors controlling the feasibility of mining. Knowledge of the quantity of coal and rock that must be removed per unit of recovered coal, of the roof and floor conditions, and of the difficulty of separating coal from rock determine the mining method and the equipment chosen for the mining operation. The rank, purity, heat value, and selling price of the coal commonly dictate usage and marketability. Higher rank coals generally are judged more valuable than lower rank coals owing to greater heat values and chemical characteristics that are sought currently by the metallurgical and petrochemical industries. Economic variables that influence feasibility are price of coal, cost of equipment, mining, labor, processing, transportation, interest rates, and taxes. Supply and demand for coal also influence feasibility as do environmental laws, restrictions, judicial ratings, and political considerations. The relative value of coals may change markedly in the near future as the result of utilizing new techniques for converting coal to gas and or liquid fuels. Low-rank coals and coals containing pyrite that are currently of lower economic value may in the future be considered premium fuels for conversion processes.

The criteria for the principal classes of coal resources described hereafter are summarized in table 3 (above) and are to be used in preparing all U.S. Geological Survey coal resource estimates from January 1, 1983, until further revised.

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