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

SPECIFIC CRITERIA

anthracite and bituminous coal reserves.--Tonnage estimates for these classes of coal are determined by summing the recoverable quantities of coal in the reserve base and are assigned to the following categories: (a) thickness of coal--28 to 42 inches (70 to 105 cm), 42 to 84 inches (105 to 215 cm), 84 to 168 inches (215 to 430 cm), more than 168 inches (>430 cm); and (b) thickness of overburden--O to 500 feet (0 to 150 m) and 500 to 1,000 feet (150 to 300 m). Tonnage estimates for the bituminous coal class may be divided into low-volatile, medium-volatile, high-volatile A, high-volatile B, and high-volatile C groups. Similarly, tonnage estimates for the anthracite class may be divided into meta-anthracite, anthracite, and semianthracite groups. Reserves assigned to these coal classes must be considered as economically producible at the time of classification, but facilities for extraction need not be in place and operative. See specific instruction No. 3.

anthracite and bituminous coal inferred reserves.--Tonnage estimates for these classes of coal are determined by summing the recoverable quantities of coal in the inferred reserve base and are assigned to the same coal thickness and overburden thickness categories as anthracite and bituminous coal reserves. Inferred reserves must be considered as economically producible at the time of classification. Facilities for extraction need not be in place and operative.

anthracite and bituminous coal marginal and inferred marginal reserves.--Tonnage estimates for these classes of coal are determined by summing the marginally recoverable quantities of coal in the reserve base and in the inferred reserve base and are assignable to the same categories of thickness of coal and overburden described for anthracite and bituminous coal reserves. These classes of coal may be divided into the same rank groups as described for anthracite and bituminous coal reserves. Marginal and inferred marginal reserves must be considered uncertain as to economic producibility at the time of classification. Facilities for extraction need not be in place and operative.

anthracite and bituminous coal reserve base and inferred coal reserve base.--See reserve base for thickness of coal and thickness of overburden criteria.

anthracite and bituminous coal resources.--Tonnage estimates for these classes of coals are determined by summing the estimates for anthracite and bituminous coal identified and undiscovered resources (fig. 3). They are assignable to the same thickness categories as for anthracite and bituminous coal reserves with the addition of a 14-28 inch (35-70 cm) category (see specific instruction No. 3, p. 34), and the following overburden categories are to be recognized: 0 to 500 feet (0 to 150 m); 500 to 1,000 feet (150 to 300 m); 1,000 to 2,000 feet (300 to 600 m); 2,000 to 3,000 feet (600 to 900 m); and 3,000 to 6,000 feet (900 to 1,800 m). Tonnage estimates for the bituminous coal class may be divided into the low-volatile, medium-volatile, high-volatile A, high-volatile B, and high-volatile C groups, and tonnage estimates for the anthracite class may be divided into the meta-anthracite, anthracite, and semianthracite groups.

cumulative depletion.--Cumulative depletion is summed from all coal extracted and lost-in-mining prior to the date of the estimate, which may be subdivided on the basis of rank and subrank (class and group) of coal, overburden class, thickness class, mining method, heat value, usage, time, cokeability, chemical constituents, and area of production.

cumulative production.--Cumulative production is summed from production from a mine, field, basin, region, province, state, or nation prior to the date of the estimate, which may be subdivided on the basis of rank and subrank (class and group) of coal, overburden class, thickness class, mining method, heat value, usage, time cokeability, chemical constituents, and area of production.

demonstrated reserves and demonstrated marginal reserves.-- Tonnage estimates for these categories of coal are the sum of the estimates for measured and indicated reserves and marginal reserves, respectively, which are the preferred usages. See Reserves and Marginal Reserves.

demonstrated reserve base.--Tonnage estimates for this category of coal are determined by summing the estimates for the measured and indicated reserves bases. The demonstrated reserve base is the same as the "reserve base," which is the preferred usage. See reserve base.

demonstrated resources.--Tonnage estimates for this category are the sum of the estimates for the reserve base and subeconomic resources.

economic resources.--An informal term used by geologists to indicate their estimates of the coal resources that are potentially economic.

hypothetical resources.--Tonnage estimates for this category of resources are for (1) extensions of inferred resources (coal beyond a radius of 3 miles or 4.8 km from a point of measurement), and (2) regions where tonnage estimates are based on a knowledge of the geologic character of coal. Hypothetical resources include coal that is 14 inches (35 cm) or more thick (anthracite and bituminous coal) and 30 inches (70 cm) or more thick (subbituminous coal and lignite) to a depth of 6,000 feet (1,800 m). (See section on "Extrapolated Bed Map Method," and "Extrapolated Coal Zone Method," see also fig. 4.)

identified resources.--Tonnage estimates for this category of resource include all bituminous coal and anthracite 14 inches (35 cm) or more thick and all subbituminous coal and lignite 30 inches (75 cm) or more thick from the surface to a depth of 6,000 feet (1,800 m) whose location, rank, quality, and quantity have been determined within specified degrees of reliability as demonstrated, measured, indicated, and inferred.

indicated.--Virgin coal that lies between 1/4 mile (0.4 km) and 3/4 mile (1.2 km) from a point of thickness of coal measurement. (See fig. 4.) indicated marginal reserves.--Tonnage estimates for this category of reserves include those parts of an indicated reserve base that at the time of determination border on being economically producible assuming certain projected economic or technologic changes. The assumed changes and the specific criteria suggesting potential economic profitability should be documented. (See indicated and fig. 4.)

indicated reserves.--Indicated reserves are estimated from an indicated reserve base by subtracting the assumed tonnage of coal that will be lost-in-mining and indicated marginal reserves. The remaining tonnage --the coal that is assumed will be extracted--is the indicated reserves, which must be considered as economically producible at the time of classification. However, facilities for extraction need not be in place and operative. (See indicated and fig. 4.)

indicated reserve base.--An indicated reserve base is determined by projection of thicknesses of coal and overburden, rank, and quality data from points of measurement and sampling on the basis of geologic evidence using the following criteria: (a) individual points of measurement are bounded by measured coal for 1/4 mile (0.4 km) succeeded by indicated coal from 1/4 mile (0.4 km) to 3/4 mile (1.2 km); and (b) indicated reserve base includes anthracite and bituminous coal 28 inches (70 cm) or more thick and subbituminous coal 60 inches (150 cm) or more thick to depths of 1,000 feet (300 m) and lignite 60 inches (150 cm) or more thick to depths of 500 feet (150 m). (See indicated)

indicated resources.--Tonnage estimates for indicated resources are computed by projection of thicknesses of coal and overburden, rank, and quality data from points of measurement and sampling on the basis of geologic evidence and are assigned to the following categories: (a) individual points of measurement are bounded by measured coal for 1/4 mile (0.4 km) succeeded by 1/2 mile (0.8 km) of indicated coal; and (b) indicated resources include anthracite and bituminous coal 14 inches (35 cm) or more thick and lignite and subbituminous coal 30 inches (75 cm) or more thick to a depth of 6,000 feet (1,800 m). The quantity of coal estimated as indicated resources is the same as the sum of the indicated reserve base, and indicated subeconomic resources. (See indicated and fig. 4.)

inferred.--Virgin coal that lies between 3/4 mile (1.2 km) and 3 miles (4.8 km) from a point of thickness of coal measurement. (See fig. 4.)

inferred marginal reserves.--Tonnage estimates for this category of reserves include those parts of an inferred reserve base that at the time of determination border on being economically producible assuming certain projected economic or technologic changes. The assumed changes and the specific criteria suggesting potential economic profitability should be documented. (See inferred and fig. 4.)

inferred reserve base.--An inferred reserve base is determined by projection of thicknesses of coal and overburden, rank, and quality data from points of measurement and sampling on the basis of geologic evidence using the following criteria: (a) individual points of measurement are bounded by measured and indicated coal for 3/4 mile (1.2 km) succeeded by inferred coal from 3/4 mile (1.2 km) to 3 miles (4.8 km); and (b) inferred reserve base includes anthracite and bituminous coal 28 inches (70 cm) or more thick, subbituminous coal 60 inches (150 cm) or more thick, all to depths of 1,000 feet (300 m), and lignite 60 inches (150 cm) or more in thickness to depths of 500 feet (150 m).

inferred reserves.--Inferred reserves are estimated from the inferred reserve base by subtracting the inferred marginal reserves and the coal that is estimated will be lost-in-mining.Inferred reserves must be considered as economically producible at the time of determination considering environmental, legal, and technologic constraints. Extraction facilities need not be in place and operative.

inferred resources.--Inferred resources are determined by projecting the thicknesses of coal and overburden, rank, and quality data from points of measurement and sampling on the basis of geologic knowledge and are assigned to the following categories: (a) individual points of measurement are bounded by measured and indicated coal for 3/4 mile (1.2 km) succeeded by 2-1/4 miles (3.6 km) of inferred coal; (b) inferred resources include anthracite and bituminous coals 14 inches (35 cm) or more thick and lignite and subbituminous coal 30 inches (75 cm) or more thick to depths of 6,000 feet (1,800 m). The quantity of coal estimated as an inferred resource is the same as the sum of the inferred reserve base and inferred subeconomic resources.

lignite reserves.--Tonnage estimates for this class of coal reserves are determined by summing the recoverable quantities of coal in the reserve base and are assigned to the following categories: (a) thickness of coal--5 to 10 feet (1.5 to 3.0 m), 10 to 20 feet (3 to 6 m), 20 to 40 feet (6 to 12 m), and more than 40 feet (> 12 m); and (b) thickness of overburden--O to 500 feet (0 to 150 m). Tonnage estimates for lignite reserves may be divided into the lignite A and B groups. Reserves assigned to the lignite class must be considered as economically producible at the time of classification, but facilities for extraction need not be in place and operative.

Discussion: Currently, there are few underground lignite mines in the United States. In addition, there are few, if any, lignite strip mines that are removing as much as 300 feet (100 m) of overburden. In order to provide for future technologic and surface mining advances, the maximum thickness of overburden for surface-minable lignite reserves and reserve base is set at 500 feet (150 m). It is deemed inexpedient to estimate underground-minable lignite reserves and reserve base at depths greater than 500 feet (150 m). (See specific instruction No. 3.)

lignite marginal and inferred marginal reserves.--Tonnage estimates for these classes of coal reserves are determined by summing the marginally recoverable quantities of coal in the reserve base and inferred reserve base and are assignable to the same categories of thicknesses of coal and overburden described for lignite reserves; coal may be divided into the lignite A and B groups. Lignite marginal and inferred marginal reserves must be considered as uncertain as to their economic producibility at the time of classification. Facilities for extraction need not be in place and operative.

lignite inferred reserves.--Tonnage estimates for this class of reserves are determined by summing the recoverable quantities of coal in the inferred reserve base and are assignable to the same categories of thicknesses of coal and overburden described for lignite reserves. Also, this class of coal may be divided into the lignite A and B groups. Such inferred reserves must be considered economically producible at the time of classification; facilities for extraction need not be in place and operative.

lignite reserve base and inferred reserve base.--See Reserve base for thickness of coal and thickness of overburden criteria.

lignite resources.--Tonnage estimates for this class of coal are determined by summing the estimates for lignite identified and undiscovered resources. The same thickness categories as for lignite reserves are to be used with the addition of a 30-60 inches (75-150 cm) category (see specific instruction No. 3, p. 34 ), and the following overburden categories are to be recognized: 0 to 500 feet (0 to 150 m); 500 to 1,000 feet (150 to 300 m); 1,000 to 2,000 feet (300 to 600 m); 2,000 to 3,000 feet (600 to 900 m); and 3,000 to 6,000 feet (900 to 1,800 m). The tonnage estimates for this class of coal may be divided into lignite A and B groups.

measured.--Accessed and virgin coal that lies within a radius of 1/4 mile (0.4 km) of a point of thickness of coal measurement. (See fig. 4.)

measured marginal reserves.--Accessed and virgin coal that lies within a radius of 1/4 mile (0.4 km) of a point of thickness of coal measurement. Tonnage estimates for this category of reserves includes those parts of a measured reserve base that at the time of determination border on economic producibility assuming certain projected economic or technologic changes. The assumed changes and the specific criteria suggesting potential economic producibility should be documented. (See measured and fig. 4.)

measured reserves.--Measured reserves are estimated from a measured reserve base by subtracting the sum of the assumed tonnage of coal that will be lost-in-mining and measured marginal reserves. The remaining tonnage--the coal that is assumed will be extracted--is measured reserves which must be considered as economically producible at the time of classification; however, facilities for extraction need not be in place and operative.

measured reserve base.--A measured reserve base is determined by projection of thicknesses of coal and overburden, rank, and quality data from points of measurement and sampling on the basis of geologic evidence for a radius of 1/4 mile (0.4 km). A measured reserve base includes anthracite and bituminous coal 28 inches (70 cm) or more thick and subbituminous coal 60 inches (150 cm) or more thick to depths of 1,000 feet (300 m) and lignite 60 inches (150 cm) or more thick to depths of 500 feet (50 m).

measured resources.--Tonnage estimates for measured resources are computed by projection of thicknesses of coal and overburden, rank, and quality data for a radius of 1/4 mile (0.4 km) from a point of measurement. Measured resources include anthracite and bituminous coal 14 inches (35 cm) or more thick and lignite and subbituminous coal 30 inches (75 cm) or more thick to depths of 6,000 feet (1,800 m). The quantity of coal estimated as measured is the same as the sum of the measured reserve base and measured subeconomic resource.

original resources.--Tonnage estimates determined for coal in the ground prior to production. Where coal has been mined, estimates are made by summing remaining resources, cumulative production, and coal lost-in-mining. An estimate of total original resources is the sum of the original resources determined for many mines, fields, basins, regions, provinces, States, and the Nation. (See resources for thicknesses of coal and overburden; and lost-in-mining.)

other occurrences, noneconomic coal.--Such coal, except where mined locally, consists of anthracite and bituminous coal beds less than 14 inches (35 cm) thick; lignite and subbituminous coal beds less than 30 inches (75 cm) thick; and all coal beds that are buried by more than 6,000 feet (1,800 m) of overburden; coal containing more than 33 percent ash; and that coal lost-in-mining that is considered noneconomic. Tonnage estimates are optional for such coal. However, if estimates are made, they should be reported as "other occurrences" and not as resources. However, where currently mined, coal that is considered too thin or too high in ash and would normally be classed as "other occurrences" is to be classed as reserves.

rank assignments.--The assignment of rank is a necessary part of classifying a coal; however, data for determining rank are commonly sparse or are far-removed from the localities where the data required for rank assignment is needed. In general, rank gradually changes laterally over many miles or stratigraphically over hundreds to thousands of feet. Because of the lack of data in some areas, conclusions concerning rank assignments commonly must be derived from analytic or petrographic determinations made on coal that lies some distance from where the rank assignment is desired. Conclusions concerning rank where analytic or petrographic data are sparse must be viewed as tentative. However, if a geologist's understanding of the setting of the area sampled is adequate, the rank assignment probably will be correct even though the rank data are sparse.

rank calculation.--The rank of coal is to be calculated by using the following instructions which are quoted from the standard specifications for classification of coals by rank (ASTM Standards, 1981, p. 212-216):

8. Calculation to Mineral-Matter-Free Basis

8.1 Calculation of Fixed Carbon and calorific value: For classification of coal according to rank, fixed carbon and calorific value shall be calculated to the mineral-matter-free basis in accordance with either the Parr formulas, Eqs 1, 2, and 3, or the approximation formulas, Eqs 4, 5, and 6, that follow. In case of litigation use the appropriate Parr Formula.

8.2 Calculation to Mm-free basis: Parr Formulas:
Dry, Mm-free FC=(FC-0.15S)/{100-(M+1.08A+0.55S)}*100 (1)
Dry, Mm-free VM=100-Dry,Mm-free FC (2)
Moist, Mm-free Btu=(Btu-50S)/{100-(1.08A+0.55S)}*100 (3)

Note--The above formula for fixed carbon is derived from the Parr formula for volatile matter. Approximation Formulas. Dry, Mm-free FC=FC/{100-(M+1.1A+0.1S}*100 (4)
Dry, Mm-free VM=100-Dry,Mm-free FC (5)
Moist, Mm-free Btu=Btu/{100-1.1A+0.1S)}*100 (6)

where

Mm = Mineral matter,
Btu = British thermal units per pound (calorific value),
FC = percentage of Fixed carbon, VM = percentage of volatile matter,
M = percentage of moisture,
A = percentage of ash, and
S = percentage of sulfur.

Above quantities are all on the inherent moisture basis. This basis refers to coal containing its natural inherent or bed moisture but not including water adhering to the surface of the coal.

recovery factor method.--Only a part of the coal in any deposit can be extracted when mined. The coal not extracted during underground mining, strip mining, or auger mining; the coal that becomes a part of a underground or strip-mine waste pile; or the coal that is not removed adjacent to a strip-mine or underground-mine boundary is considered as lost-in-mining unless sufficient tonnages are left unextracted so that additional mining or recovery can be foreseen.

If it is not feasible or possible to calculate the reserves of an area using an economic analysis, a reasonable approximation of the reserves can be determined by using the recovery factor method described hereafter.

Each operating mine has a unique percentage of coal that is recovered. This percentage is termed the recovery factor of the mine and is obtained from the following formula:

RF= (Y * 100)/X

where

RF= Recovery factor or percent coal estimated extractable during mining,
X = The total tonnage of coal estimated in the ground,
Y = The tonnage of coal estimated to be recoverable during mining.

A recovery factor can be applied to a reserve base to obtain an estimate of the reserves of an area. Such use of a recovery factor is appropriate when there is a paucity of geologic data for estimating the tonnage of potentially extractable coal.

It is difficult to estimate accurately the recoverable coal in a very large area such as a field, region, province, basin, State, or the Nation because it is impossible to determine how much coal in the area will not be mined for legal or environmental reasons, what method or methods of mining will be used, and what the average recovery factor will be for all mining methods.

A reserve base and reserves have been estimated by industry for most operating mines in the United States. Generally, data that can be used to compute recovery factors for individual mines are closely held by the operators; therefore, there is little publicly available information to guide estimators in determining local, regional, and national recovery factors. Commonly, estimators must extrapolate recovery factors from experience gained in a few mines by assuming that (1) geologic conditions controlling mining will be similar, and (2) success in the recovery of coal in unmined areas will be similar to that of mined areas utilizing the same mining method. Such extrapolation of recovery factors from a few well known mined areas to less well known or unknown areas requires experience regarding the geology, the mining method or methods to be employed, and an awareness of the difficulties, geologic and other-wise, that affect the estimation of reserves. Area, quadrangle, township, field, basin, province, county, State, and national recovery factors can be determined by using formulas after determining the mean recovery factor in percent for many mines, ascertaining the quantity of reserve base coal in the area of study, and ascertaining the total quantity of coal that is restricted from mining for any legal, environmental or technologic reason. These formulas are:

Z=100X/Y NRF=W(100-Z)

where

X = tonnage of coal restricted from extraction for any legal, environmental, or technologic reason,
Y = tonnage of coal included in the reserve base category of a large area,
Z = restricted coal (percent),
W = recovery factor percent obtained from local mines, and
NRF = National, State, or large area recovery factor in percent applied to all coal including restricted.

In the United States, recovery factors for underground mining as determined from mine maps of abandoned and operating mines generally range from about 35 to about 70 percent and average about 50 percent. Similarly, recovery factor for abandoned and operating surface mines range from about 70 to 95 percent and average about 80 percent. These local recovery factors are valid for individual mines but are not valid for large areas because they fail to consider the coal lost-in-mining such as (1) the coal that will not be mined between properties, and (2) coal in overlying and underlying beds rendered unsuitable for future mining by past underground mining. Further, the local recovery factors do not consider the coal that is restricted or prohibited from mining, such as the coal underlying national parks and wild life sanctuaries; coal that is too deep and too thin to be mined because of excessive costs; and coal that cannot be mined because of unsolved technologic, geologic, or engineering problems.

The authors recommend applying a recovery factor of 50 percent to the reserve base when computing underground and surface mining reserves of large areas. However, if actual local recovery factor have been calculated, the procedure outlined with the two formulas should be implemented for smaller areas.

remaining resources.--The resources remaining in the ground after prior mining. These resources include identified and undiscovered resources and include coal lost-in-mining whose attributes indicate possible future recovery. See resources for thickness of coal and overburden criteria, and figure 3.

reserve base.--A tonnage estimate for this category of coal consists-of the sum of the estimates for measured and indicated reserves, marginal reserves, and a part of the measured and indicated subeconomic resources (the coal that has or will be lost-in-mining). The reserve base is the same as the demonstrated reserve base. The term reserve base is preferred for reports of the U.S. Geological Survey. The criteria for the reserve base include bituminous coal and anthracite 28 inches (70 cm) or more thick, subbituminous coal 5 feet (1.5 m) or more thick that occurs at depths to 1,000 feet (300 m), and lignite 5 feet (1.5 m) or more thick that occurs at depths to 500 feet (150 m).

Discussion: Individual reserve bases, where needed and appropriate, are to be determined by categories of reliability, thicknesses of coal and overburden; rank, chemical constituents, ash content, heat value, and potential usage. Additionally, estimated individual reserve base estimates are to be summed into totals for each township, quadrangle, coal field, basin, region, province, township and range, county, State, and the Nation. Assignment of coal to a reserve base is controlled by physical and chemical criteria such as categories of reliability, thicknesses of coal and overburden, rank of coal, and knowledge of depositional patterns of coal beds and associated structural features. Changing economic, technologic, and environmental considerations do not control assignment of coal to a reserve base. In contrast, the discrimination of reserves is largely controlled by economic factors such as judgments of cost, profit, and supply of and demand for coal. Reserve discrimination is controlled secondarily by advances or differences in mining, preparation and transportation technologies, and by environmental regulations, laws, and judicial rulings.

The physical and chemical criteria used to assign coal to a reserve base category have been used already to evaluate many coal beds that are currently mined in the United States. These evaluations indicate coal assigned to a reserve that is derived from a physically-chemically defined reserve base can be expected to be economically minable with a high degree of confidence. In a few places, however, where the thickness of a coal bed or associated rock conditions are exceptionally variable or severe, the varying tonnages of coal classified as a reserve from a physically and chemically defined reserve base may or may not prove to be extractable at a profit.

Changes in environmental laws and regulations generally affect the tonnages of coal assigned to the various categories of the reserve base; however, assessments of these changes have not been made.

reserves.--Reserve tonnage estimates are to be determined by summing the recoverable quantities of coal in the reserve base for each rank of coal and are assigned to the following categories: (1) thickness of overburden--O to 500 feet (0 to 150 m) and 500 to 1,000 feet (150 m to 300 m); and (2) thickness of coal--28 to 42 inches (70 to 105 cm), 42 to 84 inches (105 to 215 cm), 84 to 168 inches (215 to 430 cm), and more than 168 inches ( > 430 cm) for anthracite and bituminous coal; and 5 to 10 feet (1.5 to 3.0 m), 10 to 20 feet (3.0 to 6.0 m), 20 to 40 feet (6.0 to 12.0 m), and more than 40 feet ( > 12.0 m) for subbituminous coal and lignite. (See specific instruction No. 3, p. 34.) Reserves must be considered as economically producible at the time of classification, but facilities for extraction need not be in place and operative. In addition, categories based on potential mining methods (surface and underground); chemical constituents such as sulfur, phosphorous and ash content; heat value; and usage such as metallurgical, steam, petrochemical, gasification, and liquefaction are desirable. Reserves and marginal reserves are estimated by determining the amount of coal in each reserve base category that can be extracted at the time of classification (reserves), and the amount that borders on being extractable at a profit (marginal reserves). These two amounts and the amount that will be lost-in-mining, when summed, are equal to the reserve base. The estimates of each reserve category are to be totaled into quadrangle, township, field, basin, region, province, county, and State estimates, and into a national total, and then the various estimates for all categories are to be totaled similarly to reach an inclusive estimate of all reserves.
Reserves are derived from the reserve base, which includes bituminous coal and anthracite 28 inches (70 cm) or more thick, subbituminous coal 5 feet (1.5 m) or more thick that occurs at depths to 1,000 feet (300 m), and lignite 5 feet (1.5 m) or more thick that occurs at depths to 500 feet (150 m). Reserves also include thinner and (or) more deeply buried beds of these ranks of coal that are currently being mined.

resources.--Tonnage estimates for coal resources are determined by summing the estimates for identified and undiscovered deposits of coal that are 14 inches (35 cm) or more thick for anthracite and bituminous coal and under less than 6,000 feet (1,800 m) of overburden, and 30 inches (75 cm) or more thick for lignite and subbituminous coal and under less than 6,000 feet (1,800 m) of overburden.

speculative resources.--As of publication of this circular, there are no speculative resources of coal estimated for the United States. However, if it is desirable to make such estimates, the definition of Speculative Resources (p. 20) and the criteria for resources will be followed, and the geologic evidence supporting the estimates and methods of quantification will be made available publicly.

subbituminous coal inferred reserves.--Tonnage estimates for this class of coal are determined by summing the recoverable quantities of coal in the inferred reserve base and are assigned to the same categories of thickness of coal and overburden described for subbituminous coal reserves. This class of coal may be divided into the same rank groups as described for subbituminous coal reserves. Inferred reserves must be considered as economically producible at the time of classification. However, facilities for extraction need not be in place and operative.

subbituminous coal marginal and inferred marginal reserves.--Tonnage estimates for these classes of coal are determined by summing the marginally recoverable quantities of coal in the reserve base and inferred reserve base, respectively, and are assigned to the same categories of thicknesses of coal and overburden described for subbituminous coal reserves. These classes of coal may be divided into the same rank groups as described for subbituminous coal reserves. Marginal and inferred marginal reserves must be considered uncertain as to their economic producibility at the time of classification. Facilities for extraction need not be in place and operative.

subbituminous coal reserves.--Tonnage estimates for this class of coal are determined by summing the recoverable quantities of coal in the reserve base and are assigned to the following categories: (a) thickness of coal--5 to 10 feet (1.5 to 3.0 m), 10 to 20 feet (3.0 to 6.0 m), 20 to 40 feet (6.0 to 12.0 m), and more than 40 feet ( > 12.0 m) (see specific instruction No. 3, p. 34 ); and (b) thickness of overburden--0 to 500 feet (0 to 150 m) and 500 to 1,000 feet (150 to 300 m). Such reserve estimates may be divided into subbituminous A, B, and C rank groups. Reserves assigned to this coal class must be considered as economically producible at the time of classification, but facilities for extraction need not be in place and operative.

subbituminous coal reserve base and inferred reserve base.--See reserve base for thickness of coal and thickness of overburden categories.

subbituminous coal resources.--Tonnage estimates for this class of coals are determined by summing the estimates for identified and undiscovered subbituminous coal resources. The same thickness categories as for subbituminous coal reserves are to be used with the addition of a 30 inches-5 feet (75 cm-1.5 m) category (see specific instruction No. 3, p. 34), and the following overburden categories are to be recognized: 0 to 500 feet (0 to 150 m); 500 to 1,000 feet (150 to 300 m); 1,000 to 2,000 feet (300 to 600 m); 2,000 to 3,000 feet (600 to 900 m); and 3,000 to 6,000 feet (900 to 1,800 m). Such resource estimates may be divided into subbituminous coal A, B, and C rank groups.

subeconomic resources and inferred subeconomic resources.-- Tonnage estimates for these classes of coal are determined by summing the estimates for measured, indicated, and inferred resources that do not meet the criteria for assignment to the reserve base or inferred reserve base because they are too thin to mine, are too deeply buried to mine, or are those parts of the reserve base or inferred reserve base that have been or will be lost-in-mining but whose attributes indicate future recovery may become feasible. Included are all measured, indicated, and inferred reliability categories of bituminous coal and anthracite beds 14 to 28 inches (35 to 70 cm) thick, all subbituminous coal beds 30 to 60 inches (75 to 150 cm) thick that are less than 1,000 feet (300 m) below the surface, and all lignite beds 30 to 60 inches (75 to 150 cm) thick that are less than 500 feet (< 150 m) below the surface, unless the coal in these beds will be recovered in the process of extracting coal from thicker beds. Also included are all beds of bituminous coal and anthracite 14 inches (35 cm) or more thick and beds of subbituminous coal 30 inches (75 cm) or more thick that occur at depths between 1,000 and 6,000 feet (300 and 1,800 m) and lignite beds 30 inches (75 cm) or more thick and more than 500 feet (> 150 m) below the surface.

thickness of coal for resource calculations.--The thickness of coal used for resource calculations is the net thickness of coal in a bed excluding all partings more than 3/8 inch (> 1 cm) thick. Beds and parts of beds made up of alternating layers of thin coal and partings are omitted from calculations if the partings comprise more than one-half of the total thickness. Also, benches of anthracite and bituminous coal less than 14 inches (35 cm) thick and benches of subbituminous coal and lignite less than 30 inches (75 cm) thick are omitted from calculations if they lie above or below partings that may deter their mining. Coal and coaly material containing more than 33 percent ash is excluded from resource and reserve estimates unless the ash is largely in associated partings so that the coal is cleanable to less than 33 percent ash. (See parting, p. 15, and specific instruction No. 13, p. 36.)

undiscovered resources.--Tonnage estimates for this category of resources are based (1) on knowledge of the geologic character, habit, and pattern of a coal bed or coal zone in an area or region or (2) on speculative geologic data. Estimates are made by summing the tonnage estimates for coal assigned to the hypothetical and speculative resources reliability categories. Included are hypothetical and speculative resources of bituminous coal and anthracite in beds 14 inches (35 cm) or more thick and hypothetical and speculative resources of subbituminous coal and lignite in beds 30 inches (75 cm) or more thick presumed to occur in mapped areas and in unmapped or unexplored areas to depths of 6,000 feet (1,800 m).

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