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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

GEOLOGICAL SURVEY CIRCULAR 891


GLOSSARY OF COAL CLASSIFICATION SYSTEM AND SUPPLEMENTARY TERMS

Some of the following general definitions of coal resources and supplementary terms are amplified elsewhere in this report by criteria and guidelines for usage. The criteria and guidelines may be revised periodically to reflect changing national needs without affecting the definitions.

All definitions herein refer only to usage in this coal resources classification system and are not intended as definitions of the terms relative to any other usage.

Comparative values for units in the metric and English (U.S. Customary) systems of measurement are based on the Handbook of Chemistry and Physics by R. C. Weast (1971, p. F-242-F-263).

Note.--Glossary terms and specific criteria are crossreferenced within this report. To aid the reader, glossary items, beginning below, are printed in boldface type and specific criteria are printed in boldface italics.

accessed.-- Coal deposits that have been prepared for mining by construction of portals, shafts, slopes, drifts, and haulage ways; by removal of overburden; or by partial mining. See virgin coal.

acre.-- A measure of area in the United States: 43,560 square feet; 4,840 square yards; 4,046.856 square meters; 0.4046856 hectare; 0.0015625 square mile; 0.0040468 square kilometer.

acreage.-- The number of acres at the ground surface

acre-foot (acre-ft).-- The volume of coal that covers 1 acre at a thickness of 1 foot (43,560 cubic feet; 1,613.333 cubic yards; 1,233.482 cubic meters). The weight of coal in this volume varies according to rank.

acre-inch (acre-in.).-- The volume of coal that covers 1 acre at a thickness of 1 inch (3,630 cubic feet; 134.44 cubic yards; 102.79O3 cubic meters). The weight of coal in this volume varies according to rank.

agglomerating.-- Coal that, during volatile matter determinations, produces either an agglomerate button capable of supporting a 500-gram weight without pulverizing, or a button showing swelling or cell structure.

anthracite or anthracitic.-- A rank class of non agglomerating coals as defined by the American Society for Testing and Materials having more than 86 percent fixed carbon and less than 14 percent volatile matter on a dry, mineral-matter-free basis. (Anthracite is preferred usage). This class of coal is divisible into the semi anthracite, anthracite, and meta-anthracite groups on the basis of increasing fixed carbon and decreasing volatile matter. (See table 1).

ash.-- The inorganic residue remaining after complete incineration of coal. ash content.--The percentage of a laboratory sample of coal remaining after incineration to a constant weight under standard conditions following D-2795-69 (ASTM, 1981, p. 335-342).

ash free.-- A theoretical analysis calculated from basic analytical data expressed as if the total ash had been removed.

as-received condition or as-received basis.-- Represents an analysis of a sample as received at a laboratory.

assess.-- To analyze critically and judge definitively the geologic nature or economic potential, significance, status, quality, quantity, potential usability, and other aspects of coal resources and reserves.

assessment.-- A critical analysis based on integrating, synthesizing, evaluating, and interpreting all available data aimed at a judgment of the geologic nature or economic potential of the coal resources and reserves of an area, field, district, basin, region, province, county, state, nation, continent, or the world. An assessment differs from an estimate, which is a determination of the amount of coal in an area. An estimate or estimates may be the principal data used to assess the coal resources and reserves of an area. See economic assessment and geologic assessment.

auger mining.-- A method often associated with contour strip mining to recover additional coal after the overburden to coal ratio has become too great for further contour mining. Coal is produced by boring into the coal bed much like a carpenter's wood bit bores into wood. An auger consists of a cutting head and screw-like extensions.

bed.-- All the coal and partings lying between a roof and floor. The terms "seam" and "vein" should not be used.

bench.-- A subdivision and (or) layer of a coal bed separated from other layers by partings of non-coal rock.

bituminous coal.-- A rank class of coals as defined by the American Society for Testing and Materials (ASTM) high in carbonaceous matter, having less than 86 percent fixed carbon, and more than 14 percent volatile matter on a dry, mineral-matter-free basis and more than 10,500 Btu on a moist, mineral-matter-free basis. This class may be either agglomerating or non agglomerating and is divisible into the high-volatile C, B, A; medium; and low-volatile bituminous coal groups on the basis of increasing heat content and fixed carbon and decreasing volatile matter. (See table 1).

bone coal or bone.-- Impure coal that contains much clay or other fine-grained detrital mineral matter (ASTM, 1981, D-2796, p. 344). See impure coal. Discussion: The term bone coal has been erroneously used for cannel coal, canneloid coal, and well-cemented to metamorphosed coaly mudstone and (or) claystone. Bone coal has also been applied to carbonaceous partings. The term "impure coal" accompanied by adjective modifiers such as "silty," "shaly," or "sandy," is the preferred usage because the definition of bone coal does not specify the type or weight percentages of impurities.

British thermal unit (Btu).-- The quantity of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit (degrees F) at, or near, its point of maximum density of 39.1 degrees F (equivalent to 251.995 gram calories; 1,054.35 Joules; 1.05435 kilojoules; 0.25199 kilocalorie).

burn line.-- The contact between burned and unburned coal in the subsurface. In the absence of definitive information, the subsurface position of a burn line is assumed to be vertically below the surface contact between unaltered and altered rocks. See clinker.

calorie (cal).-- The quantity of heat required to raise 1 gram of water from 15 degrees to 16 degrees Celsius. A calorie is also termed gram calorie or small calorie (equivalent to 0.00396832 Btu; 4.184 Joules; 0.001 kilogram calorie).

clinker.-- Baked or fused rock formed from the heat of a burning underlying coal bed.

coal.-- A readily combustible rock containing more than 50 percent by weight and more than 70 percent by volume of carbonaceous material, including inherent moisture. It is formed from plant remains that have been compacted, indurated, chemically altered, and metamorphosed by heat and pressure during geologic time.

Discussion: Differences in the kinds of plant materials, in the degree of metamorphism (rank), and in the range of impurities are characteristic of coal and are used in coal classification. Impure coal/coaly material containing more than 33 weight percent ash is excluded from resources and reserve estimates unless the ash is largely in associated partings so that the coal is cleanable to less than 33 weight percent ash.

coal bed.-- See bed. coal field.--A discrete area underlain by strata containing one or more coal beds. (See fig. 5 and fig. 6.)

coal measures.--Strata containing one or more coal beds. coal province.--An area containing two or more coal regions. (See fig. 8. )

coal region.--An area containing one or more coal fields. (See fig. 7.) coal reserves.--See reserves.

coal zone.--A series of laterally extensive and (or) lenticular coal beds and associated strata that arbitrarily can be viewed as a unit. Generally, the coal beds in a coal zone are assigned to the same geologic member or formation.

coke.--A gray, hard, porous, and coherent cellular-structured solid, primarily composed of amorphous carbon. Coke is combustible and is produced by destructive distillation or thermal decomposition of certain bituminous coal that passes through a plastic state in the absence of air.

concentration.--A greater than normal accumulation of substances such as (1) coal, (2) elements, (3) compounds, and (4) minerals. In coal resource terminology, concentration is used in two senses: (1) concentrations of coaly material into beds that are minable, and (2) concentrations of elements, compounds, and minerals that may add or detract from the value of the extracted coal. A concentration of a substance always exceeds the average content of that substance in the Earth's crust.

consolidated coal.--See lignite.

content.--The amount of ash, an element, an oxide, other types of compounds, or a mineral in a unit amount of coal, expressed in parts per million or percent. Also refers to the heat value of coal as expressed in Joules per kilogram (J/kg), kilojoules per kilogram (kJ/kg), British thermal units per pound (Btu/lb), or calories per gram (cal/g).

control point.--A point of measurement, a point of observation, or a sampling point. correlate, correlation.--Demonstration of the apparent continuity of a coal bed between control, measurement, or sampling points by showing correspondence in character and stratigraphic position.

Discussion: Correlations of coal beds are based on a knowledge of the stratigraphy of the coal beds and of the enclosing rocks and of the unique characteristics of individual coal beds. Confidence in correlations increases as the knowledge and abundance of data increases. Where a coal bed is continuously exposed along an outcrop or strip-mine face, continuity of the coal bed becomes an established fact and not a correlation. Where data indicate that correlation of a coal bed is possible or probable among data points within an area, an estimate of the resources of that coal bed can be made for the entire area. However, where a coal bed at single data point cannot be correlated with beds at other data points, or where there is only one data point, resources can be calculated for that coal bed using the single data point as the center of circles defining measured, indicated, and inferred.

cumulative depletion.--The sum in tons of coal extracted and lost-in-mining to a stated date for a specified area or a specified coal bed. (See cumulative depletion; and fig. 3.)

cumulative production.--The sum in tons of coal extracted to a stated date for a specified area or a specified coal bed. (See cumulative production; and fig. 1, 2, and 3.)

demonstrated.--A term commonly used for the sum of coal classified as measured and indicated resources. Used when not feasible or desirable to subdivide into measured and indicated. (See fig.1, 2, and 3.)

demonstrated reserves.--Same as reserves. (See also, demonstrated reserves and fig. 1, 2, and 3.)

demonstrated reserve base.--Same as reserve base. (See also demonstrated reserve base and fig. 1, 2, and 3.)

demonstrated resources.--See resources. (See also, demonstrated resources; and fig 1, 2, and 3.)

density.--Mass of coal per unit volume. Generally expressed in short tons/acre-foot or metric tons/hectare/square hectometer-meter of coal. See specific gravity.

depleted resources.--Resources that have been mined; includes coal recovered, coal lost-in-mining, and coal reclassified as subeconomic because of mining. See cumulative depletion. (See also cumulative depletion and fig. 3.)

depth (overburden) categories.--Coal tonnage data are divided into classes by the thickness of overburden: 0-500 feet (0-150 m); 500-1,000 feet (150-300 m); 1,000-2,000 feet (300-600 m); 2,000-3,000 feet (600-900 m); and 3,000-6,000 feet (900-1,800 m). See overburden.

Discussion: The depth categories or overburden categories (see table 3 and specific instruction No. 2) were decided after consultation among personnel from the U.S. Geological Survey, the Bureau of Mines, and various State Geological Surveys, mining companies, and agencies of foreign nations.

dry, mineral-matter-free basis.--A type of calculated analytical value of a coal sample expressed as if the total moisture and mineral matter had been removed. Mineral-matter-free is not the same as ash-free.

economic.--This term implies that profitable extraction or production under defined investment assumptions has been established, analytically demonstrated, or assumed with reasonable certainty.

economic assessment.--A critical analysis resulting in a judgment of the economic nature, significance, status, quantity, quality, market, demand, supply, costs, transportation, cash flow, capital, and processing of the coal resources of a mine, area, district, field, basin, region, province, county, state, or nation. See assessment.

estimate.--A determination as to the amount or tonnage of coal in an area. The term estimate indicates that the quantities of resources are known imprecisely. An estimate differs from an assessment, which is an analysis of all data concerning an area's coal resources and reserves with the objective of reaching a judgment about the geologic nature and economic potential of the coal resources and reserves of the area.

existing market conditions.--The relations between production, selling and transportation costs, supply, demand, and profit at any time.

extraction.--The process of removing coal from a deposit.

feasibility.--The possibility of extracting coal. fixed carbon.--The solid residue, other than ash, obtained by destructive distillation of a coal, determined by definite prescribed methods (ASTM, 1981, p. 183).

floor.--Stratigraphically, the rock immediately underlying a coal bed. Where the bed is overturned, the stratigraphic floor is the mining roof.

gasification, underground (in situ).--A method of utilizing coal by burning in place and extracting the released gases, tars, and heat. See in situ mining.

geologic assessment.--A critical analysis resulting in a judgment of the geologic nature, significance, status, quality, and quantity of the coal resources of an area, district, basin, region, township, quadrangle, province, county, state or political province, nation, continent, or the world. See assessment and economic assessment.

geologic assurance.--State of sureness, confidence, or certainty of the existence of a quantity of resources based on the distance from points where coal is measured or sampled and on the abundance and quality of geologic data as related to thickness of overburden, rank, quality, thickness of coal, areal extent, geologic history, structure, and correlations of coal beds and enclosing rocks. The degree of assurance increases as the nearness to points of control, abundance, and quality of geologic data increases.

geologic evidence.--Information derived from geologic observations that can be used to substantiate the existence, size, depth, attitude, structure, tonnage, and physical and chemical characteristics of a body of coal.

geologic identification.--State of being identified as to location, areal extent or size, depth, volume, quantity, magnitude, and quality of coal resources.

grade.--A term indicating the nature of coal as mainly determined by the sulfur content and the amount and type of ash. This term is not recommended for usage in coal resource estimations; definitive statements as to the contents and types of sulfur and ash are preferable. Statements indicating high, medium, or low grade are inappropriate without documentation. See quality.

heat value or heat of combustion.--The amount of heat obtainable from coal expressed in British thermal units per pound, joules per kilogram, kilojoules or kilocalories per kilogram, or calories per gram. To convert Btu/lb to kcal/kg, divide by 1.8. To convert kcal/kg to Btu/lb, multiply by 1.8. hectare (ha) or square

hectometer (hm**2).--A metric unit of area equal to 10,000 square meters; 0.010 square kilometer; 2.4710538 acres; 107,639.10 square feet; 11,959.9 square yards; 0.003861 square mile.

high-ash coal.--Coal containing more than 15 percent total ash on an as-received basis. See ash-content, medium-ash coal, and low-ash coal.

high-sulfur coal.--Coal containing 3 percent or more total sulfur on an as-received basis. See low-sulfur coal and medium-sulfur coal.

high-volatile bituminous coal.--Three related rank groups of bituminous coal as defined by the American Society for Testing and Materials which collectively contain less than 69 percent fixed carbon on a dry, mineral-matter-free basis; more than 31 percent volatile matter on a dry, mineral-matter-free basis; and a heat value of more than 10,500 Btu per pound on a moist, mineral-matter-free basis. (See table 1).

hypothetical.--A low degree of geologic assurance. Estimates of rank, thickness, and extent are based on assuming continuity beyond inferred. Estimates are made, not exceeding a specified depth beyond coal classed as inferred, by projection of thickness, sample, and geologic data from distant outcrops, trenches, workings, and drill holes. There are no measurement sites in areas of hypothetical coal. Used as a modifier to resource terms. See resources and undiscovered. (See also figs. 1, 2, and 3.)

hypothetical resources.--See Undiscovered Resources; (See also hypothetical resources and figs. 1, 2, and 3.)

identified resources.--See Identified Resources; (See also identified resources and figs. 1, 2, and 3.)

impure coal.--Coal having 25 weight percent or more, but less than 50 weight percent ash on the dry basis (ASTM, 1981, D-2796, p. 344). Impure coal having more than 33 weight percent ash is excluded from resource and reserve estimates unless the coal is cleanable to less than 33 weight percent ash. See bone coal.

indicated.--A moderate-degree of geologic assurance. Estimates of quantity, rank, thickness, and extent are computed by projection of thickness, sample, and geologic data from nearby outcrops, trenches, workings, and drill holes for a specified distance and depth beyond coal classed as measured. The assurance, although lower than for measured, is high enough to assume continuity between points of measurement. There are no sample and measurement sites in areas of indicated coal. However, a single measurement can be used to classify coal lying beyond measured as indicated and to assign such coal to resource and reserve base categories (fig4). Used as a modifier to resource terms.

indicated reserves and indicated marginal reserves.--See reserves and indicated. (See also indicated reserves and indicated marginal reserves, and figs. 1, 2, and 3.) indicated reserves base and indicated marginal reserve base.-- See reserve base. (See also indicated reserve base and figs. 1, 2, and 3.)

indicated resources.--See Indicated Resources. (See also indicated resources, and figs. 1, 2, and 3.)

inferred.--A low-degree of geologic assurance. Estimates of quantity, rank, thickness, and extent are based on inferred continuity beyond measured and indicated for which there is geologic evidence. Estimates are computed by projection of thickness, sample, and geologic data from distant outcrops, trenches, workings, and drill holes for a specified distance and depth beyond coal classed as indicated. There are no sample and measurement sites in areas of inferred coal. However, a single measurement can be used to classify coal lying beyond indicated as inferred and to assign such coal to inferred resource and inferred reserve base categories (fig. 4). Used as a modifier to resource terms.

inferred reserves and inferred marginal reserves.-- See subdivisions of reserves. (See also inferred reserves, and inferred marginal reserves; and figs. 1, 2, and 3.)

inferred reserve base.--See reserve base. (See also inferred reserve base, and figs. 1, 2, and 3.)

inferred resources.--See Inferred Resources, (See also inferred resources; and figs. 1, 2, and 3..)

inferred subeconomic resources.--See Inferred Subeconomic Resources. (See also inferred subeconomic resources; and figs. 1, 2, and 3.)

in situ.--Refers to coal "in place" in the ground. in situ mining.--Utilization of coal by burning in place and extracting the gases, tars, and heat.

joule (J).--The basic metric unit of work or energy equal to 1 x 10**7 ergs, 0.238662 gram calorie, 0.0002386 kilogram-calorie, or 0.0009471 Btu.

kilogram-calorie (kcal).--A metric unit of heat equal to 1,000 gram-calories; 3.9683207 Btu; 4,184 Joules; 4.184 x 10**10 ergs; or 4,184 Watt seconds. Also known as "large calorie."

kilogram (kg).--The basic metric unit of weight measurement equal to 1,000 grams; 0.001 metric ton; 2.2046 pounds; 0.0011023 short ton; 0.0009842 long ton.

kilojoule (kJ).--A metric unit of work or energy equal to 1,000 joules; 0.948708 Btu; or 238.662 gram-calories.

known coal.--Coal whose existence has been perceived from measurements and observations at the outcrop, in mines, from drill holes, and from exploratory trenches. Data confirming existence may be projected for several miles (kilometers) if based upon reasonable geologic assumptions. See identified resources. Coal fields, basins, regions, provinces, and occurrences of coal in the United States are illustrated in figures 5, 6, 7, and 8).

lignite or lignitic.--A class of brownish-black, low-rank coal defined by the American Society for Testing and Materials as having less than 8,300 Btu on a moist, mineral-matter-free basis. (See table 1) In the United States, lignite is separated into two groups: Lignite A (6,300 to 8,300 Btu) and lignite B ( < 6,300 Btu). Lignite is the preferred usage.

long ton.--A unit of weight in the U.S. Customary System and in the United Kingdom equal to 2,240 pounds (1.0160469 metric tons; 1.1200 short tons; 1,016.0469 kilograms). This term is not recommended for use in estimates of coal resources.

lost-in-mining.--Coal remaining in the ground after all extraction is completed. Lost-in-mining includes coal that is (1) left to support mine roofs, (2) too thin to mine, (3) unmined around oil, gas, water, and disposal wells, (4) unmined around shafts and electrical and water conduits, (5) unmined as barrier pillars adjacent to mine or property boundaries, (6) unmined adjacent to haulageways, tunnels, airways, and waterways, (7) unmined because of many other unspecified reasons, (8) the unrecovered or unrecoverable part of any coal bed in a mining property that has been or may be extracted, (9) all unrecoverable in beds that closely overlie a mined bed, (10) all unrecoverable in beds that closely underlie a mined bed, (11) unmined between mining properties.

Discussion: According to this system of classification, lost-in-mining equals reserve base minus reserves and marginal reserves. Thus, lost-in-mining includes all reserve base coal not economically recoverable at the time of classification or not bordering on being economically recoverable. Lost-in-mining coal is subtracted from the reserve base and is divisible into subeconomic coal or noneconomic coal according to its potential for being reclassified as economic. (See fig. 3.)

low-ash coal.--Coal containing less than 8 percent total ash on an as-received basis. See ash content, high-ash coal, and medium-ash coal.

low-sulfur coal.--Coal containing 1 percent or less total sulfur on an as-received basis. See high-sulfur coal and medium-sulfur coal.

low-volatile bituminous coal.--A rank group of bituminous coal as defined by the American Society for Testing and Materials containing more than 78 percent and less than 86 percent fixed carbon, and more than 14 percent and less than 22 percent volatile matter on a dry, mineral-matter-free basis. (See table 1)

marginal reserves.--Borders on being economic. See economic; general guideline no. 8; and subdivisions of reserves. (See also indicated marginal reserves and measured marginal reserves; and figs. 1, 2, and 3.)

measured.--The highest-degree of geologic assurance. Estimates of quantity are computed partly from dimensions revealed in outcrops, trenches, workings, and drill holes and partly by projection of thickness, sample, and geologic data not exceeding a specified distance and depth. Rank is calculated from the results of detailed sampling that may be located at some distance from this type of resource and may be on the same or other coal beds. The sites for thickness measurement are so closely spaced and the geologic character so well defined that the average thickness, areal extent, size, shape, and depth of coal beds are well established. However, a single measurement can be used to classify nearby coal as measured (fig4). Used as a modifier to resource terms.

measured reserves and measured marginal reserves.--See subdivisions of reserves. (See also measured reserves, measured marginal reserves; and figs. 1, 2, and 3.)

measured reserve base.--See reserve base. (See also measured reserve base; and figs. 2 and 3.)

measured resources.--See Measured Resources; (See also measured resources, and figs. 1, 2, and 3.)

medium-ash coal.--Coal containing 8 percent to 15 percent ash on an as-received basis. See ash content, low-ash coal, and high-ash coal.

medium-sulfur coal.--Coal containing more than 1 percent and less than 3 percent total sulfur on an as-received basis. See high-sulfur coal and low-sulfur coal.

medium-volatile bituminous coal.--A rank group of bituminous coal as defined by the American Society for Testing and Materials containing more than 69 percent and less than 78 percent fixed carbon and more than 22 percent and less than 31 percent volatile matter on a dry, mineral-matter-free basis. (See table 1)

metallurgical coal.--An informally recognized name for bituminous coal that is suitable for making coke by industries that refine, smelt, and work with iron. Other uses are space heating, blacksmithing, smelting of base metals, and power generation. Generally, metallurgical coal has less than 1 percent sulfur and less than 8 percent ash on an as-received basis. Most premium metallurgical coal is low- to medium-volatile bituminous coal.

metric ton, megagram, tonne, or millier.--A metric unit of weight equal to 1,000 kilograms; 1.1023113 short tons; 0.98420653 long ton; 2,204.6226 pounds. The metric ton is the preferred usage.

minable.--Capable of being mined under current mining technology and environmental and legal restrictions, rules, and regulations.

mineral-matter.--The solid inorganic material in coal.

mineral-matter-free basis.--A theoretical analysis calculated from basic analytical data expressed as if the total mineral-matter had been removed. Used in determining the rank of a coal.

mining.--All methods of obtaining coal or its byproducts from the Earth's crust, including underground, surface, and in situ mining.

moist, mineral-matter-free basis.--A theoretical analysis calculated from basic analytical data and expressed as if the mineral-matter had been removed and the natural moisture retained. Used in determining the rank of coal.

moisture, bed.--The percentage of moisture or water in a bed or sample of coal before mining.

moisture content.--The percentage of moisture (water) in coal. Two types of moisture are found in coal, namely, free or surface moisture removed by exposure to air, and inherent moisture entrapped in the coal and removed by heating to 220 degrees F.

noneconomic.--Not capable of profitable production or extraction. Coal classified as noneconomic may be reported in other occurrences. See other occurrences, noneconomic coal.

original.--The amount of coal resources in the ground before production.

original resources.--See Original Resources; and original resources. (See also figs. 1, 2, and 3.)

other occurrences.--Coal in the ground that is excluded from classification as coal resources. Includes anthracite and bituminous coal less than 14 inches thick, subbituminous coal and lignite less than 30 inches thick, and any coal more than 6,000 feet deep unless it is currently being mined. May include coal that contains more than 33 percent ash. (See other occurrences, noneconomic coal; and figs. 1 and 2.)

overburden.--Rock including coal and (or) unconsolidated material that overlies a specified coal bed. Overburden is reported in feet and (or) meters and used to classify the depth to an underlying coal bed.

partial or incomplete measurement of coal thickness.--A determination of an incomplete coal thickness at a point of measurement.

Discussion: Measurements of coal thicknesses that are incomplete because of (1) near surface slumping of coal and overlying beds, (2) weathering, (3) a drill hole not penetrating the entire coal bed, (4) identified planar erosion of top part of coal bed, or (5) removal of most of a coal bed by a stream channel are to be treated as points of measurement from which circles of reliability are to be constructed. A geologist must decide whether each measurement is complete or incomplete. The thickness of coal at places where a measurement is deemed incomplete shall be located on the coal bed map by the number of feet and inches actually measured followed by a plus sign to indicate that only a part of the bed was measured. Thus, incomplete measurements define measured coal of a stated minimum thickness. If other thickness data are available to show by isopaching that a coal thickness is incomplete at a point of measurement, the isopached total thickness at the point of measurement should be used to determine the average thickness for the tonnage estimates of measured, indicated, and inferred categories. In those places where the coal bed cannot be isopached, the partial thickness of coal should be used as the thickness for estimating tonnages. See point of measurement.

parting.--A layer or stratum of non-coal material in a coal bed which does not exceed the thickness of coal in either the directly underlying or overlying benches. (See specific instruction No. 3.)

parts per million (ppm).--A method of stating content of a substance in coal. One ppm equals 0.001 percent, or 0.000001.

point of measurement.--The exact location on an outcrop, in a trench, in mine workings, or in a drill hole where a coal bed is measured for thickness and (or) sampled for analysis. The surface position of a point of measurement must be located precisely on a map so that its geodetic position can be determined. The altitude of a subsurface point of measurement can be determined from cores, lithologic logs, mine workings, and also can be determined from a geophysical log of a drill hole or well if, in the opinion of a geologist or geophysicist, the log is of good quality. See partial or incomplete measurement of coal thickness.

point of observation.--Place on an outcrop where a coal bed is visible or where evidence indicates that a coal bed could be measured or examined by trenching or digging a pit. Points of observation are used to verify the existence of a coal bed, and apparent similarity and (or) difference of a coal bed's thickness as to thickness at points of measurement. They also can be used to confirm the position of a coal outcrop on a geologic map and to support the measured, indicated, and inferred classification of a coal bed; however, these points cannot be used without actual measurements to classify a resource body.

production.--The coal that has been extracted from a mine for a specified period. Production may be reported for a mine or larger area such as a coal field, region, province, basin, township, quadrangle, state, nation, and (or) the world. Production in the United States is usually reported in short tons; most other nations report production in metric tons.

proximate analysis.--In coal, the determination by prescribed methods of moisture, volatile matter, fixed carbon (by difference), and ash. Unless specified, proximate analyses do not include determinations of sulfur or phosphorous or any determinations other than those named. Proximate analyses are reported by percent and on as-received, moisture-free, and moisture- and ash-free bases.

quality--An informal classification of coal relating to its suitability for use for a particular purpose.

Discussion: Most coal is used as a source of heat or energy, but coal is or will be used in making petrochemicals, metallurgical coke, synthetic gas, and synthetic liquid fuel. Factors considered in judging a coal's quality are based on, but not limited to, heat value; content of moisture, ash, fixed carbon, phosphate, silica, sulfur, major, minor and trace elements; coking and petrologic properties; and organic constituents considered both individually and in groups. The individual importance of these factors varies according to the intended use of the coal. Therefore, any designation of "high-quality coal," "moderate-quality coal," or "low-quality coal" should plainly indicate the intended or optimum use or uses and is inappropriate without such documentation.

quantity.--Refers to the amount or tonnage of coal. Quantity should be reported in short or metric tons.

rank--The classification of coals according to their degree of metamorphism, progressive alteration, or coalification (maturation) in the natural series from lignite to anthracite.

Discussion: Classification is made on the basis of analysis of coal in accordance with table 1. The rank of coal can be used to infer the approximate dry, mineral-matter-free heat value, fixed carbon, and volatile matter in a coal, because the amounts of the constituents vary little within each coal rank. (See table 1 and rank calculation.)

rank calculation.--The determination of the rank of a coal. Such determination must use the instructions given under rank calculation.

recoverable coal.--The coal that is or can be extracted from a coal bed during mining. The term "recoverable" should be used in combination with "resources" and not with "reserves".

recovery percent.--The percentage of coal extracted from a bed where the total tonnage originally in the bed is equal to 100 percent.

recovery factor.--The estimated or actual percentage of coal that can be or was extracted from the coal originally in a bed or beds for an area, mine, district, field, basin, region, province, township, quadrangle, county, state, political province, nation, and (or) the world. See recovery factor method.

reliability categories.--Categories based on distance from points of measurement and (or) sampling. The measured, indicated, inferred, and hypothetical resource categories, as defined, indicate the relative reliability of tonnage estimates as related to distance from points of thickness control of particular parts of a coal deposit. The reliability categories are not indicative of the reliability of the basic data (that is, the accuracy of coal measurements, or the accuracy of location of the coal outcrop). It is assumed that all basic data used in resource estimation have been judged reliable by the estimator and that unreliable data have been discarded. (See fig. 4.)

reserves.--Virgin and (or) accessed parts of a coal reserve base which could be economically extracted or produced at the time of determination considering environmental, legal, and technologic constraints. The term reserves need not signify that extraction facilities are in place or operative. Reserves include only recoverable coal; thus, terms such as "extractable reserves" and "recoverable reserves" are redundant and are not a part of this classification system. (See figs. 1, 2, and 3; and reserves.)

Discussion: Reserves can be categorized as measured and indicated, as underground or surface minable, by thickness of overburden, by thickness of coal in the bed, and by various quality factors. The term "economic reserves" is not to be used because reserves by definition are economic. Reserves, which are derived from reserve base coal, exclude coal thinner or deeper than that classified as reserve base unless such coal is currently mined. See general guideline No. 7.

MANDATORY SUBDIVISIONS

  1. Indicated Reserves and Indicated Marginal Reserves.--Categories of virgin reserves having a moderate degree of geologic assurance. See indicated and marginal reserves. (See also reserves; and figs. 1 and 3.)
  2. Inferred Reserves and Inferred Marginal Reserves.--Categories of virgin reserves having a low degree of geologic assurance. See inferred reserves and marginal reserves. (See also reserves; and figs. 1 and 3.
  3. Measured Reserves and Measured Marginal Reserves.--Categories of accessed and virgin coal reserves having the highest degree of geologic assurance. See measured reserves and marginal reserves. (See also figs. 1 and 3

OPTIONAL SUBDIVISIONS

A. Reserves and Marginal Reserves.--Reserves may be divided into subcategories other than those heretofore defined. These subcategories may be differentiated, for example, by ash and sulfur content, and heat value; by types or varieties of coal such as boghead or cannel coal; by usage such as metallurgical, petrochemical, and synthetic fuel types; by mineral ownership such as State, Federal, Indian, or private ownership; by Federal coal underlying private surface ownership; and by reserves and restricted reserves underlying State or national parks, monuments, forests, grasslands; military and naval reservations, alluvial valley floors, steep slopes, lakes and large rivers, and environmentally protected areas.

Restricted Reserves and Restricted Marginal Reserves.-- Those parts of any reserve category that are restricted or prohibited by laws or regulations from extraction by underground and (or) surface mining.

Discussion: For example, coal in a national park may meet all the physical, chemical and economic requirements of a reserve but is prohibited from extraction. The assignment to a restricted category may be either temporary or permanent; however, because laws and regulations can be repealed or changed, such coal should be separately distinguished, and tonnage estimates recorded as a restricted reserve. Locally, a specific regulation or law might prohibit one method of mining and allow or not specify other methods. In such a circumstance, the coal would be restricted from mining by the prohibited method and tonnage estimates would be so recorded. In other circumstances, other methods would be unrestricted, and tonnage estimates would be reported accordingly.
The separation of coal reserves into the many different subcategories listed above and other subcategories not listed in this text is desirable and encouraged. All subcategories not listed should be defined clearly and explicitly so that other resource specialists and the public will not be confused.

reserve base.--Those parts of the identified resources that meet specified minimum physical and chemical criteria related to current mining and production practices, including those for quality, depth, thickness, rank, and distance from points of measurement. (See reliability categories; and figs. 2 and 3.) The reserve base is the in-place demonstrated (measured plus indicated) resource from which reserves are estimated. The reserve base may encompass those parts of a resource that have a reasonable potential for becoming economically available within planning horizons beyond those that assume proven technology and current economics. The reserve base includes those resources that are currently economic (reserves), marginally economic (marginal reserves), some of those that are currently subeconomic (subeconomic resources), and some of the resources that have been or will be lost-in-mining but whose attributes indicate possible future recovery. The term "geologic reserve" has been applied by others to the reserve base category, but it also may include the inferred reserve base category; it is not a part of this classification system. (See reserve base; and figs. 2 and 3.)

reserve base, inferred.--The in-place part of an identified resource from which inferred reserves and inferred marginal reserves are estimated. Quantitative estimates are based largely on knowledge of the geologic character of a coal deposit for which there are no samples or measurements. The estimates are based on an assumed continuity beyond the reserve base for which there is geologic evidence. (See figs. 2 and 3.)

resources.--Naturally occurring concentrations or deposits of coal in the Earth's crust, in such forms and amounts that economic extraction is currently or potentially feasible. (See resources; and figs. 1, 2, and 3.)
MANDATORY SUBDIVISIONS:

A. Hypothetical Resources.--See Undiscovered Resources. (See also hypothetical resources; and figs. 1, 2, and 3.)
B. Identified Resources.--Resources whose location, rank, quality, and quantity are known or estimated from specific geologic evidence. (See identified resources). Identified coal resources include economic, marginally economic, and subeconomic components. To reflect varying distances from points of control or reliability, these subdivisions can be divided into demonstrated and inferred, or preferably into measured, indicated, and inferred. (See identified resources and figs.
1, 2, and 3.)

Discussion: Identified resources may be accessed and (or) in bodies of virgin coal which are assigned to resource and reserve base subcategories on the basis of geologic evidence from maps, samples, drill holes, wells, mine records, and fieldwork. Specific evidence must include data on the location, thickness of overburden, distance from points of measurement or sampling, and extent and thicknesses of the resource bodies. Evidence about quality and rank may be determined from analyses of samples collected from the resource bodies or may be inferred by projection of analytical data obtained elsewhere in the body or from adjacent bodies. An identified resource body may contain reserves, marginal reserves, inferred reserves, inferred marginal reserves, reserve base, inferred reserve base, demonstrated resources, measured resources, indicated resources, inferred resources, subeconomic resources and inferred subeconomic resources. (See figs. 1, 2, and 3.)

C. Indicated Resources.--Identified bodies of virgin coal having a moderate degree of geologic assurance. See indicated. (See also indicated resources; and figs. 1, 2, and 3.)
D. Inferred Resources.--Identified bodies of virgin coal having a low degree of geologic assurance. See inferred resources. (See also inferred resources and figs.
1, 2, and 3.)
E. Measured Resources.--Accessed and virgin demonstrated resources having a high degree of geologic assurance. (See measured resources and figs.
1, 2, and 3.)
F. Original Resources.--The amount of coal inplace before production. Where mining has occurred, the total of original resources is the sum of the identified resources, undiscovered resources, coal produced, and coal lost-in-mining. (See original resources and figs.
1, 2, and 3.)
G. Remaining Resources.--The resources in the ground in a mine, area, field, basin, region, province, county, state, and (or) nation after some mining. The term does not include coal lost-in-mining unless such coal can be considered potentially recoverable. Remaining resources may be divided into categories such as remaining economic, marginally economic, subeconomic, measured, indicated, inferred, identified, and undiscovered (hypothetical and speculative) resources or other types of resources. (See optional subdivisions, below; and figs.
1, 2, and 3.) The total remaining resources are the sum of the remaining identified and undiscovered resources as of the date of the estimate.
H. Subeconomic Resources.--That part of identified (demonstrated) resources that does not meet the economic criteria of reserves and marginal reserves. See resources and economic. (See also subeconomic resources and figs.
1 and 2.)
I. Inferred Subeconomic Resources.--That part of identified (inferred) resources that does not meet the economic criteria of inferred reserves or inferred marginal reserves. See resources and economic. (See also subeconomic resources and figs.
1 and 2.)
J. Speculative Resources.--See Undiscovered Resources (below). (See also speculative resources and figs.
1, 2, and 3.)
K. Undiscovered Resources.--Undiscovered resources, the existence of which is only postulated, comprise deposits that are either separate from or are extensions of identified resources. Undiscovered resources may be postulated in deposits of such quality, rank, quantity, and physical location as to render them economic, marginally economic, or subeconomic. To reflect varying degrees of geologic certainty, undiscovered resources may be subdivided into two parts as follows. (See undiscovered resources.)

1. Hypothetical Resources.--A class of undiscovered resources that are either similar to known coal deposits which may be reasonably expected to exist in the same coal field or region under analogous geologic conditions or are an extension from inferred resources. In general, hypothetical resources are in the central parts of broad areas of coal fields where points of sampling and measurement and evidence for thickness and existence is from distant outcrops, mine workings, drill holes, and wells. If exploration confirms the existence of hypothetical resources and reveals enough information about their quality, quantity, and rank, they will be reclassified as identified resources.
2. Speculative Resources.--A class of undiscovered resources that may occur either in known types of deposits in favorable geologic settings where coal deposits have not been discovered or in types of deposits as yet unrecognized for their economic potential. If exploration confirms the existence of speculative resources and reveals enough information about their quality, quantity, and rank, they will be reclassified as identified resources.

OPTIONAL SUBDIVISIONS: Resources may be divided into subcategories, for example, on the basis of ash content, sulfur content, and heat value; type or variety of coal such as boghead or cannel coal; usage such as metallurgical, petrochemical, and synthetic fuel types; resources underlying specified lands owned by State governments, the Federal Government, or private interests; by restricted resources underlying State or national parks, monuments, forests, grasslands; military, naval, and Indian reservations; and alluvial valley floors, steep slopes, lakes and large rivers, and environmentally protected areas.
A. Restricted Resources.--Those parts of any resource category that are restricted or prohibited from extraction by laws or regulations.

Discussion: Restricted resources meet all requirements of coal classified as resources, except that they are restricted from extraction by law or regulation. The assignment to a restricted category may be either temporary or permanent, but, because laws and regulations can be repealed or changed, such coal should be separately distinguished and tonnage estimates recorded as restricted resources.
The division of coal resources into the many different categories described heretofore and into other categories not differentiated in the text is desirable and encouraged. Many requests for information about resources are received by coal resource specialists and are unanswerable because the scopes of the systems of classification used in the past were too limited. Persons and institutions classifying resources are, therefore, encouraged to use initiative in defining and developing additional classes of coal resources.

restricted reserves.--See optional subdivisions of reserves.

restricted resources.--See optional subdivisions of resources.

roof.--Stratigraphically, in underground mining the rock immediately overlying a coal bed. Where a bed is overturned, the stratigraphic roof is the mining floor.

sample.--A representative fraction of a coal bed collected by approved methods, guarded against contamination or adulteration, and analyzed to determine the nature; chemical, mineralogic, and (or) petrographic composition; percentage or ppm content of specified constituents; heat value; and possibly the reactivity of the coal or its constituents.

Discussion: Some samples are also collected so that fossil remains can be ascertained and physical, magnetic, or other geophysical properties can be determined, tested, observed, or analyzed. All samples should be accompanied by a description of the sample, including location, thickness of coal, and stratigraphic relationship to other rocks.

TYPES OF SAMPLES:
A. as-received sample.--A sample of coal as it is received at a laboratory.
B. bed or channel sample.--A sample of coal collected from a channel cut perpendicular to the stratification.

Discussion: This type of sample is used to ascertain the chemistry, rank of coal, mineralogy, petrography, and geophysical and physical properties of coal. Instructions for this type of sampling are contained in Geological Survey Circular 735 (Swanson and Huffman, 1976).

C. bench sample.--A sample of a subdivision and (or) layer of a coal bed separated from other subdivisions by partings of non-coal rock.

Discussion: The term bench sample does not apply to coal lithotypes such as vitrinite and exinite as used by petrologists.

D. blend pile sample.--A sample of coal collected from the blend-pile of a processing plant or a utilization facility such as a power plant or steel mill.
E. breaker sample.--A sample of coal broken or crushed in a breaker plant. A breaker sample is usually collected prior to cleaning of coal.
F. cleaned coal sample.--A sample of coal collected after use of a cleaning procedure.
G. core sample.--A sample of coal recovered from a core which was obtained at depth by a coring device in a drill hole.
H. cutting sample.--A sample of coal taken from the cuttings returned during drilling. Discussion: Cutting samples are not recommended because many comparisons with properly or conventionally collected samples indicate they are rarely representative.
I. delivered coal sample.--A sample of coal collected from a shipment that is being or will be delivered to a user.
J. grab sample.--A sample, commonly a single piece, selected from a coal bed, tipple, preparation plant, blend pile, conveyor belt, or coal car.

Discussion: Grab samples are not recommended because many comparisons with properly collected samples indicate they are rarely representative.

K. mine sample.--A sample of coal collected from a mine, generally from an underground working face or from a strip-wall face.
L. run of mine or mine run sample.--Generally the same as a tipple sample.
M. tipple sample.--A sample of coal collected at a mine tipple.

seam.--A bed of coal lying between a roof and floor. This term is not to be used in place of "coal bed" in reports of the U. S. Geological Survey.

short ton.--A unit of weight equal to 2,000 pounds; 0.9071847 metric ton, tonne, or megagram; 0.8928571 long ton.

specific gravity of coal.--The ratio of the mass of a unit column of coal to the mass of an equal volume of water at 4 degrees C.

Discussion: The specific gravity of coal varies considerably with rank and with differences in ash content. The values shown in table 2 are close to the average specific gravities of unbroken or unmined coal in the ground (in situ) for the four major rank categories and are to be used in preparing U.S. Geological Survey estimates of coal resources and reserves.
Persons associated with individual mining operations sometimes use lower specific gravity factors to allow for anticipated losses in extraction. Such usage may be suitable for specific mine areas but is not recommended for use in general reports because the recoverability of coal varies greatly between areas, beds, mining methods, and mine operators.

speculative.--Lowest degree of geologic assurance. Estimates of rank, thickness, and extent are based on assuming the existence of known types of coal deposits in favorable geologic settings or on assuming the existence of unknown types of deposits as yet unrecognized for their economic potentials. Tonnages are estimated by assuming thickness of coal, overburden, extent, and rank to a specified depth. There are geologic evidence sites but no measurement sites in areas of speculative coal. Used as a modifier to resource terms. See Hypothetical Resources, Speculative Resources, and Undiscovered Resources. (See also figs. 1, 2, and 3.)

speculative resources.--See Speculative Resources. (See also speculative resources and figs. 1, 2, and 3.)

square hectometer-meter (hm2-m).--A metric unit of the volume of coal that covers 1 square hectometer at a thickness of 1 meter; 10,000 cubic meters; 10 cubic dekameters; 0.010 square kilometer-meter; 13,079.51 cubic yards; 8.107132 acre-feet; 0.0126674 square mile-foot. The weight of coal in this volume varies according to rank. (See table 1.)

square kilometer.--1,000,000 square meters; 100 hectares; 247.10538 acres; 1,195,990 square yards; 10,763,910 square feet.

square kilometer-meter (km**2-m).--The volume of coal (1,000,000 cubic meters; 100 square hectare-meter-meters or 100 hectare-meters; 1,307,950.6 cubic yards; 35,314,667.0 cubic feet) that covers 1 square kilometer at a thickness of 1 meter. The weight of coal varies according to the rank. (See table 2.)

square mile.--27,878,400 square feet; 3,097,600 square yards; 2,589,988.1 square meters; 258.99881 hectares; 640 acres; 2.5899881 square kilometers.

square mile-foot.--The volume of coal (27,878,400 cubic feet; 789,428.38 cubic meters; 1,032,533.33 cubic yards) that covers 1 square mile to a thickness of 1 foot. The weight of coal varies according to the rank. (See table 2.)

strip or stripping ratio.--The amount of overburden that must be removed to gain access to a unit amount of coal.

Discussion: A stripping ratio may be expressed as (1) thickness of overburden to thickness of coal, (2) volume of overburden to volume coal, (3) weight of overburden to weight of coal, or (4) cubic yards of overburden to tons of coal. A stripping ratio commonly is used to express the maximum thickness, volume, or weight of overburden that can be profitably removed to obtain a unit amount of coal.

strip or surface mining.--The extraction of coal by using surface mining methods such as area strip mining, contour strip mining, or open-pit mining. The overburden covering the coal is removed and the coal extracted using power shovels, front end loaders, or similar heavy equipment.

subbituminous coal.--A rank class of nonagglomerating coals having a heat value content of more than 8,300 Btu's and less than 11,500 Btu's on a moist, mineral-matter-free basis. This class of coal is divisible on the basis of increasing heat value into the subbituminous C, B, and A coal groups. (See table 1.)

subeconomic resources.--See resources and economic. (See also subeconomic resources and figs. 1, 2, and 3.)

sulfur content.--The quantity of sulfur in coal expressed in percent or parts per million. May be divided into the quantities occurring as inorganic (pyritic) sulfur, organic sulfur, and sulfate sulfur.

thickness categories.--The categories of thickness of coal beds employed in calculating, estimating, and reporting coal resources and reserves. (See thickness of coal for resource calculations; and specific instruction No. 3.)

ultimate analysis.--In coal, the determination by prescribed methods of the ash, carbon, hydrogen, nitrogen, oxygen (by difference), and sulfur contents. Quantities of each analyzed substance are reported by percentage for the following conditions: as-received, dried at 105 degrees C, and moisture-and ash-free.

Discussion: The principal reason for the ultimate analysis is the classification of coals by rank, although it is often used for commercial and industrial purposes when it is desirable to know the sulfur content. The ultimate analysis also is known as the "total analysis." This, however, is a misnomer because substances other than those noted above are not identified and quantified, such as trace elements, oxides, and rare gases.

underground mining.--The extraction of coal or its products from between enclosing rock strata by underground mining methods, such as room and pillar, longwall, and shortwall, or through in situ gasification.

undiscovered.--A category of virgin resources of coal having the lowest degree of geologic assurance. Category is divisible into the hypothetical and speculative categories. (See hypothetical and speculative.) Estimates are quantitative. There are no sample or measurement of coal thickness sites in areas of undiscovered coal. Used as a modifier to resources.

undiscovered resources.--See mandatory subdivisions of resources. (See also undiscovered resources and figs. 1, 2, and 3.)

vein.--A bed of coal lying between a distinct roof and floor. Term is not to be used in place of "coal bed" in reports of the U.S. Geological Survey.

virgin coal.--Coal that has not been accessed by mining. See accessed.

volatile matter.--In coal, those products, exclusive of moisture, given off as gas and vapor, determined by definite prescribed methods (ASTM, 1981, D2361-66, D3761-79, D3175-77, D3176-74, D3178-73, and D3179-73).

**A This classification does not include a few coals, principally nonbanded varieties, which have unusual physical and chemical properties and which come within the limits of fixed carbon or calorific value of the high-volatile bituminous and subbituminous ranks. All of these coals either contain less than 48 percent dry, mineral-matter-free fixed carbon or have more than 15,500 moist, mineral-matter-free British thermal units per pound.
**
B Moist refers to coal containing its natural inherent moisture but not including visible water on the surface of the coal.
**C If agglomerating, classify in low-volatile group of the bituminous class.
**D Coals having 69 percent or more fixed carbon on the dry, mineral-matter-free basis shall be classified according to fixed carbon, regardless of calorific value.
**E It is recognized that there may be nonagglomerating varieties in these groups of the bituminous class, and there are notable exceptions in the high-volatile C bituminous group.
**1 ASTM, 1991, p.215. *Modified from ASTM, 1981.

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