Coal Resource Classification System 

of the U.S. Bureau of Mines and 

U.S. Geological Survey

GEOLOGICAL SURVEY BULLETIN 1450-B

A report published jointly by the 

l\S. Bureau of Mines and 

U.S. Geological Survey

Goal Resource Classification System 

of the U.S. Bureau of Mines and 

U.S. Geological Survey

MINERAL RESOURCE CLASSIFICATION SYSTEMS OF THE 

U.S. BUREAU OF MINES AND U.S. GEOLOGICAL SURVEY

GEOLOGICAL SURVEY BULLETIN 1450-B

A report published jointly by the 

UJS. Bureau of Mines and 

. Geological Survey

Definitions of coal resource classification 

terms used by the US, Bureau of Mines and 

US. Geological Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 76-600026

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-02814-0
FOREWORD

In order to use mineral resource terms with precision and common understanding and to compare resource data effectively, a 

joint U.S. Bureau of Mines and U.S. Geological Survey work 

group developed a standardized, definitive, broadly applicable 

classification system to derive uniform, coordinated resource estimates. The principles of the system are given in Chapter A of this 

series (Bulletin 1450-A). This chapter presents the classification 

system for coal resources. Future chapters will present classification terms for other specific commodities.

Thomas V. Falkie V. E. McKelvey

Director, Bureau of Mines Director, Geological Survey

III

CONTENTS

Page

Foreword _______________________________________________ III

Introduction ___________________________________________ Bl

Classification system _______________________________ 1

Glossary of coal classification terms _________-________ 1

Criteria for coal resource/reserve identification ____________________ 4

ILLUSTRATION

Page

FIGURE 1. Classification of coal resources _ ____ __________ ____ B2

TABLE

Page

TABLE 1. Coal resource/reserve criteria ___ ___ ______ B5

MINERAL RESOURCE CLASSIFICATION SYSTEMS OF THE 

U.S. BUREAU OF MINES AND U.S. GEOLOGICAL SURVEY

COAL RESOURCE CLASSIFICATION SYSTEM

OF THE U.S. BUREAU OF MINES AND

U.S. GEOLOGICAL SURVEY

INTRODUCTION

This method of classification is in conformity with the provisions of the Joint Geological Survey-Bureau of Mines Resource 

Classification Agreement of November 21, 1973, covering all mineral resources and will be used in future resource/reserve studies 

on coal conducted by agencies of the Department of the Interior. 

All resource and reserve estimates will be dated.

Within this system the term "coal resource" designates the estimated quantity of coal in the ground in such form that economic 

extraction is currently or potentially feasible. The "coal reserve" 

is that part of the resource for which rank, quality, and quantity 

have been reasonably determined and which is deemed to be minable at a profit under existing market conditions.

CLASSIFICATION SYSTEM

This system employs a concept by which coal beds are classified 

in terms of their degree of geologic identification and economic! 

and technologic feasibility of recovery. In the following conceptual 

diagram (fig. 1) showing the relationship of the various factors 

involved, coal resources are located on the horizontal scale, increasingly to the left, according to their degree of geological 

assurance of existence, and on the vertical scale, increasingly upward, according to their degree of economic and technologic feasibility of recovery.

The following general definitions of coal resource categories are 

amplified by the criteria for resource identification, which follow 

the Glossary. The criteria may be revised to reflect changing conditions without affecting the definitions.

GLOSSARY OF COAL CLASSIFICATION TERMS 

Resources. Concentrations of coal in such forms that economic 

extraction is currently or may become feasible.

Bl
B2 MINERAL RESOURCE CLASSIFICATION SYSTEMS

COAL RESOURCES 

As of January 1, __

Cblllion short tanas)

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nCMTIFIED

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RESERVE

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OVERED

SPECULATME

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-^ h>renwdC0<xaf<iuriUKa^nn: 1

factor

Includes billion tons of Reserve Ease

Total Remaining; Resources 

Cumulative Production 

Total Original Resources

coal that are mot currently minatole. 

(see recovery factor}.

Includes ___ billion tons of Reserve Base 

that are not currently minatole (see 

recovery factor).

Average production, (5 yr. period)_ 

Production, (most recent year)

million short tons 

million short tons

FIGURE 1. Classification of coal resources.

Identified Resources. Specific bodies of coal whose location, 

rank, quality, and quantity are known from geologic evidence 

supported by engineering measurements.

Undiscovered Resources. Unspecified bodies of coal surmised to 

exist on the basis of broad geologic knowledge and theory.

Reserve Base. That portion of the Identified Coal Resource from 

which Reserves are calculated.

Reserve. That portion of the Identified Coal Resource that can, 

be economically mined at the time of determination. The reserve is derived by applying a Recovery Factor to that component of the Identified Coal Resource designated as the 

Reserve Base.

Recovery Factor. The percentage of total tons of coal estimated 

to be recoverable from a given area in relation to the total 

tonnage estimated to be in the Reserve Base in the ground.
COAL RESOURCE CLASSIFICATION SYSTEM B3

Identified Subeconomic Resources. The part of coal resources 

that occurs in Demonstrated and Inferred Resources that is 

not now minable economically.

Hypothetical Resources. Undiscovered Coal Resources in beds 

that may reasonably be expected to exist in known mining 

districts under known geologic conditions. In general, Hypothetical Resources are in broad areas of coal fields where 

points of observation are absent and evidence is from distant 

outcrops, drill holes, or wells. Exploration that confirms their 

existence and reveals quantity and quality will permit their 

reclassification as a Reserve or Identified Subeconomic 

Resource.

Speculative Resources. Undiscovered coal in beds that may occur 

either in known types of deposits in a favorable geologic setting where no discoveries have been made, or in deposits that 

remain to be recognized. Exploration that confirms their 

existence and reveals quantity and quality will permit their 

reclassification as Reserves or Identified Subeconomic 

Resources.

The following definitions are applicable to both the Reserve and

Identified Subeconomic Resource components.

Measured. Coal for which estimates of the rank, quality, and 

quantity have been computed, within a margin of error of less 

than 20 percent, from sample analyses and measurements 

from closely spaced and geologically well-known sample sites.

Indicated. Coal for which estimates of the rank, quality, and 

quantity have been computed partly from sample analyses' 

and measurements and partly from reasonable geologic projections.

Demonstrated. A collective term for the sum of coal in both 

Measured and Indicated Resources and Reserves.

Inferred. Coal in unexplored extensions of Demonstrated Resources for which estimates of the quality and size are based 

on geologic evidence and projection.

Rank. The classification of coals relative to other coals, according 

to their degree of metamorphism, or progressive alteration, 

in the natural series from lignite to anthracite (Classification 

of Coal by Rank, 1938, American Society for Testing Materials, ASTM Designation D-388-38, p. 77-84).

Quality or Grade. Refers to individual measurements such as 

heat value, fixed carbon, moisture, ash, sulfur, phosphorus, 

major, minor, and trace elements, coking properties, petrologic properties, and particular organic constituents. The indi-
B4 MINERAL RESOURCE CLASSIFICATION SYSTEMS

vidual quality elements may be aggregated in various ways 

to classify coal for such special purposes as metallurgical, 

gas, petrochemical, and blending usages.

CRITERIA FOR COAL RESOURCE/RESERVE 

IDENTIFICATION

Estimates of the different classes of coal resources and reserves 

are arbitrarily based upon three criteria: (1) thickness, rank, and 

quality of the coal bed, (2) depth of the coal bed, and (3) the 

proximity of the coal resource data upon which the estimate was 

based. Depth and thickness are criteria because they control economic and technologic feasibility of recovery- The criteria for each 

class are described below and summarized in table 1 and will be 

used in preparing all Department of the Interior coal resource/ 

reserve estimates from January 1, 1975, until further revised.

These criteria apply only to those coal bodies that are or will be 

economically extractable by underground, surface, and/or in situ 

methods. Coal thinner than 14 inches (35 cm) (anthracite and 

bituminous) and 30 inches (75 cm) (subbituminous and lignite) 

and all coal deeper than 6,000 feet (1,800 m) is excluded. These 

thinner and deeper coals will be considered at a later date. Coal 

containing more than 33 percent ash is excluded from resource 

and reserve estimates.

Identified Resources. Include beds of bituminous coal and anthracite 14 inches (35 cm) or more thick and beds of subbituminous coal and lignite 30 inches (75 cm) or more thick 

that occur at depths to 6,000 feet (1,800 m), and whose existence and quantity have been delineated within specified degrees of geologic assurance as measured, indicated, or inferred. Include also thinner and/or deeper beds that presently 

are being mined or for which there is evidence that they could 

be mined commercially.

Undiscovered Resources. Include beds of bituminous coal and 

anthracite 14 inches (35 cm) or more thick and beds of subbituminous coal and lignite 30 inches (75 cm) or more thick 

that are presumed to occur in unmapped and unexplored 

areas to depths of 6,000 feet (1,800 m).

Remaining Resources. Includes the sum of the Identified and 

Undiscovered Resources as of the date of the estimate.

Cumulative Production. Includes the sum of all production prior 

to the date of the estimate.
COAL RESOURCE CLASSIFICATION SYSTEM B5 

TABLE 1. Coal resource/reserve criteria

_.,_,,. x . Thickness, Inches 

_______ ____ ____ Depth, Feet (Metres) (Centimetres)

Total Resources and 

Undiscovered Resources.

Anthracite and ^6,000 (1,800) ^14 (35) 

bituminous 

coal.

Subbituminouis ^6,000 (1,800) ^30 (75) 

coal and 

lignite.

Identified Resources.1

Anthracite and ^6,000 (1,800) ^14 (35) 

bituminous 

coal.

Subbituminous ^6,000 (1,800) ^30 (75) 

coal and 

lignite.

Reserve Base.2

Anthracite and ^1,000 (300) ^28 (70) 

bituminous 

coaL

Subbituminous ^1,000 (300) ^60 (150)

coal.

Lignite ________ ^120 (40) ^60 (150) 

Reserves.

Criteria same as Reserve Base but with Recovery Factor applied. 

Subeconomic Resources?

Anthracite and 0-1,000 (300) 14 (35)-28 (70) 

bituminous 1,000 (300)-6,000 (1,800) ^14 (35) 

coal. 

Subbituminous 0-1,000 (300) 30 (75)-60 (150)

coal. 1.000 (300)-6,000 (1,800) =5=30 (75) 

Lignite __________ 0-120 (40) 30 (75)-60 (150)

120 (40)-6,000 (1,800) ^30 (75)

^Identified Resources are classified as Measured, Indicated, and Inferred according to the 

degree of geologic assurance as described in the text.

LThe Reserve Base includes some beds that are thinner and/or deeper than the general 

criteria permit, but that presently are being mined or are judged to be minable commercially at this time.

'Also includes currently nonrecoverable part of Reserve Base.

Total Original Resources. Includes the sum of the Remaining 

Resources and Cumulative Production as of the date of the 

estimate.

Reserve Base. Includes beds of bituminous coal and anthracite 

28 inches (70 cm) or more thick and beds of Subbituminous. 

coal 60 inches (150 cm) or more thick that occur at depths to 

1,000 feet (300 m). Includes also thinner and/or deeper beds 

that presently are being mined or for which there is evidence 

that they could be mined commercially at this time. Includes
B6 MINERAL RESOURCE CLASSIFICATION SYSTEMS

beds of lignite 60 inches (150 cm) or more thick which can be 

surface mined generally those that occur at depths no 

greater than 120 feet (40 m).

Reserve. Includes that portion of the Reserve Base that can be 

mined at the time of classification (See Recovery Factor).

Recovery Factor. On a national basis the estimated Recovery 

Factor for the total Reserve Base is 50 percent. More precise 

recovery factors can be computed by determining the total 

coal in place and the total coal recoverable in any specific 

locale.

Subeconomic Resources, Include all Identified Resources that do 

not fall into the Reserve category. Include in this category 

beds of bituminous coal and anthracite 14 inches (35 cm) to 

28 inches (70 cm) thick and beds of subbituminous coal 30 

inches (75 cm) to 60 inches (150 cm) thick that occur at 

depths to 1,000 feet (300 m). Include also 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 (300 m) and 6,000 feet (1,800 

m). Include lignite beds 30 inches (75 cm) or more thick that 

cannot be surface mined generally those that occur at 

depths greater than 120 feet (40 m), and lignite beds 30 

inches (75 cm) to 60 inches (150 cm) thick that can be surface mined. Include the currently nonrecoverable portion of 

the Reserve Base.

The following criteria are applicable to both the Reserve and 

Subeconomic Resources components:

Measured. Resources are computed from dimensions revealed in 

outcrops, trenches, mine workings, and drill holes. The points 

of observation and measurement are so closely spaced and the 

thickness and extent of coals are so well defined that the tonnage is judged to be accurate within 20 percent of true tonnage. Although the spacing of the points of observation necessary to demonstrate continuity of the coal differs from region 

to region according to the character of the coal beds, the 

points of observation are no greater than i/2 mile (0.8 km) 

apart. Measured coal is projected to extend as a 14-mile (0.4- 

km) wide belt from the outcrop or points of observation or 

measurement.

Indicated. Resources are computed partly from specified measurements and partly from projection of visible data for a 

reasonable distance on the basis of geologic evidence. The
COAL RESOURCE CLASSIFICATION SYSTEM B7

points of observation are Y> (0.8 km) to I 1/-) miles (2.4 km) 

apart. Indicated coal is projected to extend as a ^-mile (0.8- 

km) wide belt that lies more than 1/4 mile (0.4 km) from the 

outcrop or points of observation or measurement. 

Inferred. Quantitative estimates are based largely on broad 

knowledge of the geologic character of the bed or region and 

where few measurements of bed thickness are available. The 

estimates are based primarily on an assumed continuation 

from Demonstrated coal for which there is geologic evidence. 

The points of observation are iy~> (2.4 km) to 6 miles (9.6 

km) apart. Inferred coal is projected to extend as a 2 1/4-mile 

(3.6-km)widebelt that lies more than % mile (1.2 km) from 

the outcrop or points of observation or measurement.

Hypothetical Resources. Quantitative estimates are based on a 

broad knowledge of the geologic character of a coal bed or 

region. Measurements of coal thickness are more than 6 miles 

(9.6 km) apart. The assumption of continuity of a coal bed 

is supported only by geologic evidence.

Speculative Resources. Quantitative estimates are based on geologic assumptions that undiscovered coal may occur in known 

types of deposits or in favorable geologic settings.