C.891

GEOLOGICAL SURVEY CIRCULAR 891

Coal Resource

Classification System

of the U.S. Geological Survey
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

1983
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

First printing 1983 

Second printing 1992

UNITED STATES GOVERNMENT PRINTING OFFICE: 1983

Free on application to 

Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225
FOREWORD

In order to use coal resource terms with precision and common understanding and to 

compare resource data effectively, the authors developed a standardized, definitive, 

broadly applicable classification system to derive uniform, coordinated coal resource 

estimates. The principles of the system for all mineral resources ape given in USGS Circular 831. Personnel engaged in coal resources work are to use the definitions, criteria, 

guidelines, and instructions incorporated in this coal resource classification system in all 

U.S. Geological Survey publications.

Director, Geological Survey
CONTENTS

Page

.introduction _ _ __ _ ___ __ j

History of the classification system 1

Coal resource classification system 2

Glossary of coal classification system and supplementary terms 4

Criteria for coal resource classification 23

Applications of criteria 23

opcciiic criteria _______ __________ _____ _________ _ ______« .___« ____________________ _^

Guidelines for estimating coal resources 32

General guidelines for classification of coal resources 32

Specific Instructions 33

1 1 > P JCVoIln anlfnf Ul r-nal IXKU j 33j

2 Overburden _____________________________________________ 33

3. Thickness of coal categories 34

T\ &ize or unit area *~ __________ ___ ____<__ ________ _______ _________ _____________ __ ^^

5. Major categories of resources 34

S* f'nsil VrftUcii Vu»r1 ucumana iiui|J3 __ » _________ __ »__ ________ __ _____ ________ _____ .j-j. ^4

7. Thickness of coal measurements 35

8. Distribution of coal bed thickness measurements 35

10. Weight of coal per unit volume 36

11. Calculation of coal resources 36

12. Rounding of tonnage estimates 36

13. Estimation of resources in the vicinity of where a coal bed bifurcates into two or more

tongues "~ ~~ » _ _ _ _ _ _ _ _ _ __ _ ___ _ ___ __ ___ __ __ __ __ __ _ _ __ _ JQ

Estimation of hypothetical resources 37

Extrapolated bed map method 37

Extrapolated coal zone method 38

Examples illustrating the basic geometric principles of constructing coal resource bed maps 38

Geophysical logs as a source of coal bed data 46

Focusing-electrode and induction logs 51

.L/iscussion or electric logs ~» ~ ____ __ _ ~_ __ ___ _ __ __ ____ ___ QJ

\jrainfna ray log ~~~~~~~~ ~~~~ ~-~-~~~ _ » jj

uensity log *. »___ _________ ___________»___ _______ __________ __ ^^

Neutron loff 55

A.COUSQC velocity log ~ __________ ____«._ ________ __________________ ____________ ___ QQ

Measurement of the coal bed thickness 55

Composition and rank of coal 61

kMT3.ogra.jxry ano structure ~ __ ____ ________ ^______«.___ __________ _________________ ___ Q^

Summovnr UlllillcUy ______________________________________________________________________ _ _ \J£, fiO

xntenoeci audience ^ ^ » QJ
ILLUSTRATIONS

Page

FIGURE 1. Format and classification of coal resources by reserves and subeconomic resources categories 4

2. Format and classification of coal resources by reserve and inferred reserve bases and subeconomic and inferred subeconomic resources categories 4

3. Flow chart showing hierarchy of coal resources and criteria for distinguishing resource categories 6

4. Diagram showing reliability categories based solely on distance from points of measurement 11 

5-8. Maps showing:

5. Coal fields of the conterminous United States 13

6. Coal fields of Alaska 14

7. Coal regions of the conterminous United States 15

8. Coal provinces of the conterminous United States 16 

9-21. Diagrams showing:

9. Determination of areas of reliability using coal thickness data only at points of measurement along outcrop

line 39

10. Determination of areas of reliability from points of measurement on the outcrop line, supplemented by mine and

drill-hole data 41

11. Determination of areas of reliability using coal thickness measurements taken along a continuously exposed,

strip-mined bed, supplemented by drill-hole data 42

12. Determination of minimum thickness isoline 43

13. Determination of areas of reliability and overburden thickness 44

14. A ridge with construction of 14-inch minimum thickness limit 45

15. Same ridge as figure 14 with areas of reliability, coal thickness isopachs, and overburden contour 46

16. A simple valley reentrant with construction of 14-inch minimum thickness limit 47

17. Areas of reliability, overburden contours, and coal thicknessisopachs to accompany figure 16 48

18. A valley and ridge with part of coal bed less than minimum thickness but within determined 14-inch limit as

illustrated in figure 16 49

19. Areas of reliability, overburden contours, and coal thickness isopachs to accompany figure 18 50

20. Coal categories in plane of coal bed, a structure section, and coal categories as viewed projected to the

ground surface from plane of coal bed 52

21. Areas of resource categories for a fladying coal bed based on data only from underground mined areas and drill

hnW IILUC5 - _____ ___ ««««_« ___________ _ ____________________________________________ j.j. S4.

22-23. Maps showing:

22. A small coal basin divided into areas of coal resource categories 56

23. A large, simple coal basin divided into areas of coal resource categories 57

24. Suggested form for recording coal resource data and calculated tonnage estimates 58

25. Suggested format for tabulating data and calculating tonnages 59

26. Early conventional electric log (1957) of the Owanah Kendrick No. 1 well, Big Horn County, Montana 60

27. Induction and conductivity geophysical logs (1960) of the Hose-Austin Drilling Company from Bone Brothers No. 1

well, Rosebud County, Montana 61

28. Examples of oil- and gas-well geophysical logs from western Kentucky that can be used for coal bed correlations and

coal resource evaluations 62

29. SP, gamma ray, acoustic velocity, normal resistivity, and induction conductivity logs (1971) from Davis Oil Grady Fed

No. 1-2 well, Sweetwater County, Wyoming 63

30. SP, gamma ray, density, dual induction-lateral log, and neutron logs (1978) from Getty Tri-County No. 1 well, Rosebud County, Montana 64

31. Five geophysical logs of a coal exploration drill hole showing response to coal and limestone 65
TABLES

TABLE 1. Classification of coals by rank 17

2. Average specific gravity and average weight of unbroken coal per unit volume of different ranks 22

3. Summary of coal resource criteria 24

4. Areas of reliability from figure 13 listed by overburden and coal thickness categories, and by counties 45

5. Areas of reliability from figure 15 listed in proper overburden and coal thickness categories 47

6. Areas of resource categories from figure 17 listed in proper reliability and thickness of coal and overburden categories 49

7. Areas of resource categories from figure 19 listed in proper reliability and thickness of coal and overburden categories 51

vn
Goal Resource Classification System 

of the U.S. Geological Survey

By GORDON H. WOOD, JR., THOMAS M. KEHN, M. DEVEREUX CARTER, 

and WILLIAM C. CULBERTSON

INTRODUCTION

Coal is the most abundant fossil fuel in the United 

States. Knowledge of the size, distribution, and quality 

of the Nation's coal resources is important for governmental planning; industrial planning and growth; the 

solution of current and future problems related to air, 

water, and land degradation; and for meeting the shortto long-term energy needs of the country. Knowledge of 

the Nation's coal resources is also important in planning 

for the exportation and importation of fuel.

Many estimates have been made of the coal resources 

of the Nation, the resources of other nations, and for the 

world as a whole. Because of differing systems of resource classification, these estimates vary greatly in 

magnitude within this Nation and other nations; geologic analysis shows some of these estimates to be little 

more than educated guesses. The accompanying coal 

resource classification system of the U.S. Geological 

Survey is recommended as an aid in solving the problems caused by the differing systems.

A detailed resource classification system should identify deposits of coal by area! location, distance from 

points of information, thicknesses of coal and overburden, rank and quality, and estimates of quantity. 

Classes in the system, furthermore, should impart some 

idea as to economic, technologic, legal, and environmental factors affecting the availability of coal.

A coal resource classification system was published 

jointly by the U.S. Bureau of Mines and the U.S. Geological Survey in 1976 as Geological Survey Bulletin 

1450-B and was believed to be a successful modification 

of an already existing system into the ideal system. 

However, Bulletin 1450-B left unanswered many questions concerning how to estimate coal resources and did

not provide sufficient criteria, guidelines, and a methodology so that comparable estimates would be obtained 

by all workers using the same data.

The Survey and the Bureau of Mines decided in 1977 

that the coal resource classification system, as outlined 

in Bulletin 1450-B, should be revised and expanded to 

provide a more definitive and less ambiguous coal resource classification system. The revision was to document standard definitions, criteria, guidelines, and 

methods to be used in estimating coal resources. Standardization of the elements of the system should result in 

comparable estimates by different workers using the 

same data.

In 1980, the two agencies published Circular 831, 

"Principles of a Resource/Reserve Classification for 

Minerals" (U.S. Geological Survey, 1980). The circular, which outlines a classification system for all mineral commodities, filled the classification needs of the 

Bureau of Mines, which was no longer responsible for 

coal resource classification, and was the basis for this 

revision of the coal resource classification system by the 

Geological Survey. The revision, embodied in this 

report, has two main objectives: (1) to provide detailed 

information lacking in Bulletin 1450-B; and (2) to provide standard definitions, criteria, guidelines, and 

methods required for uniform application of the principles outlined in Circular 831.

HISTORY OF THE CLASSIFICATION SYSTEM

Almost since their inceptions in 1879 and 1920, 

respectively, the U.S. Geological Survey and the U.S. 

Bureau of Mines have conducted modest ongoing programs in coal resource estimation and analysis. Between
1909 and World War II, tonnage estimates of the coal 

resources and reserves of the United States were summary totals derived for areas from estimates that were 

calculated by gross statistical methods. These early estimates were inadequate for the needs of the 1940's 

because they did not separate thin from thick coal beds, 

distinguish shallow from deeply buried coal, separately 

quantify identified resoures and undiscovered resources, 

or discriminate the quality and rank of coal on the basis 

of physical and chemical criteria.

After World War II, requests from the public indicated the need for more detailed information about the 

occurrence, distribution, and availability of the Nation's 

coal resources. These requests indicated that a more detailed coal classification system was needed and that it 

should be based on bed-by-bed analysis of thicknesses of 

coal and overburden, reliability (distance from control 

points) of areal data, rank of coal, and several chemical 

parameters related to determining quality and usage. As 

a result, ongoing programs of the Geological Survey and 

the Bureau of Mines for geologic mapping and engineering evaluation were expanded, and programs for 

appraising the coal resources of the Nation on a Stateby-State and a bed-by-bed basis were initiated.

After much consultation with potential users, the 

Geological Survey and the Bureau of Mines revised 

their procedures and prepared new definitions, criteria, 

and guidelines to be followed in estimating coal 

resources. The main elements of the programs used 

after World War II to 1976 were as follows:

1. Estimates of resources and reserves were based on 

existing information. Initially, attempts were 

not made to estimate the Nation's total coal resources or reserves; however, such estimates 

were long-term objectives.

2. Estimates of resources and reserves were prepared 

on a State-by-State basis.

3. Estimates of resources and reserves were divided 

into precisely defined categories such as rank, 

thicknesses of coal and overburden, and distance from points of information. 

The estimates of coal resources and reserves from 

World War II to 1976 were prepared in formats suitable 

for use by geologists and engineers, coal specialists, and 

economists working for the coal industry and government. These formats included geologic maps, coal bed 

maps, tables, and diagrams of resource and reserve data 

segregated into categories suitable for comparison with 

similarly categorized data from other sources.

Experience with the classification system utilized from 

World War II until 1976 gradually showed the need for

still greater detail. It also showed the need for rigidly enforced standards that would lessen individual geologic 

and engineering judgments in the interpretation of data 

and methods. Adoption of such standards should result 

in reproducible and comparable estimates from the 

same data and would allow adoption of computer 

technology.

In 1976, the Geological Survey and the Bureau of 

Mines adopted a modification of the 1944-75 system by 

publishing U.S. Geological Survey Bulletin 1450-B 

tided "Coal Resource Classification System of the U.S. 

Bureau of Mines and U.S. Geological Survey." This 

bulletin has been the standard reference for coal 

resource/reserve work by many Federal and State 

agencies.

COAL RESOURCE CLASSIFICATION SYSTEM

The classification system presented herein is an expansion of the system adopted in 1976. It employs a 

concept by which coal is classified into resource/reserve 

base/reserve categories on the basis of the geologic 

assurance of the existence of those categories and on the 

economic feasibility of their recovery. Categories are 

also provided for resources/reserve base/reserves that 

are restricted because of legal, environmental, or technologic constraints. Geologic assurance is related to the 

distance from points where coal is measured or sampled; 

thicknesses of coal and overburden; knowledge of the 

rank, quality, depositional history, areal extent, and 

correlations of coal beds and enclosing strata; and 

knowledge of the geologic structure. Economic feasibility of recovery is affected not only by such physical and 

chemical factors as thicknesses of coal and overburden, 

quality of coal, and rank of coal, but also by economic 

variables such as price of coal, cost of equipment, mining, labor, processing, transportation, taxes, and interest rates, demand for and supply of coal, and weather 

extremes and by environmental laws, restrictions, and 

judicial rulings. For example, the Clean Air Act of 1970 

issued standards that severely limited the emission of 

sulfur by new coal-burning powerplants and, as a result, 

made the low-sulfur, low-rank coal deposits of the 

Northern Great Plains economically competitive. Similarly, environmental restrictions on the surface mining 

of coal and the need for adequate reclamation of mined 

areas has adversely affected the economic and technologic feasibility of extracting coal from some nearsurface deposits.

The classification system is designed to quantify the
total amounts of coal in the ground before mining began 

(original resources) and after any mining (remaining resources). It is also designed to quantify the amounts of 

coal that are known (identified resources) and the 

amounts of coal that remain to be discovered (undiscovered resources). The system also provides for recognizing amounts of coal that are (1) standard distances from 

points of thickness measurements measured, indicated, inferred, and hypothetical; (2) similar to coal currently being mined (reserve base and inferred reserve 

base); (3) economically recoverable currently (reserves 

and inferred reserves); (4) potentially recoverable with a 

favorable change in economics (marginal reserves and 

inferred marginal reserves); and (5) subeconomic 

because of being too thin, too deeply buried, or lost-inmining. Finally, the system allows tabulation of coal 

amounts that are restricted from mining by regulation, 

law, or judicial ruling.

Two factors have created difficulties in categorizing 

resources and reserves in all classification systems. First, 

most geologists and engineers who classify resources and 

reserves are not experts in the economics of mining, 

transportation, processing, and marketing. Second, 

economic conditions change with time, so that the 

economic viability of coal is relatively fluid. For example, subeconomic resources of today can become reserves of tomorrow as the price of coal rises; conversely, 

reserves can become subeconomic resources as the price 

of coal drops. Finally, changing regulations, laws, and 

judicial rulings can affect mining, transportation, processing, and marketing, and thus the classification of coal 

resources. The concept of a reserve base was developed 

to alleviate these difficulties (U.S. Geological Survey, 

1976, p. B2).

The reserve base is identified coal defined only by 

physical and chemical criteria such as thicknesses of coal 

and overburden, quality, heat value, rank, and distance 

from points of measurement. The criteria for thickness 

of coal and for overburden have been selected so that the 

reserve base includes some currently subeconomic coal. 

The concept of the reserve base is to define a quantity of 

in-place coal, any part of which is or may become economic depending upon the method of mining and the 

economic assumptions that are or will be used. An additional purpose is to aid in long-range public and commercial planning by identifying coal suitable for 

economic recovery.

Thus, resource specialists need not expend their time 

identifying the component parts of coal deposits that are 

currently economically recoverable (reserves) because 

the reserve base category contains much of the coal that

will be classed as reserves in the foreseeable future. 

Those required to classify coal as being economically recoverable, marginally recoverable, or subeconomic can 

examine reserve base estimates to locate such coal.

Figures 1 and 2 are conceptual diagrams modified 

from Circular 831 (U.S. Geological Survey, 1980) that 

show the relationship of the various classes of coal resources, the reserve base, and reserves. The classes are 

categorized in both figures according to their degree of 

geologic assurance (geologic assurance or proximity to 

points of control increases to the left), and according 

to their degree of economic feasibility of recovery 

(economic feasibility of mining increases upward). The 

resource/reserve base/reserve categories (classes) that 

can be used are not limited to those shown in figures 1 

and 2 nor to the categories described in succeeding 

pages. For example, a particular bed of coal may be 

identified as being low-sulfur (0-1 percent), low-ash 

(0-8 percent), high-volatile A bituminous, and premium coking coal; other beds of coal may be identified 

as medium-sulfur (1.1-3.0 percent), high-ash (> 15 percent), high-volatile bituminous, surface-minable cannel 

coal, and so forth. The ability of the classification system 

to precisely describe the characteristics of a body of coal 

allows the coal resources of the United States to be divided into many hundred resource classes or categories.

The hierarchy of coal resources shown in figure 3 

illustrates the conceptual relationships between the 

classes of resources as distinguished by their definitions 

and criteria. Examination of figures 1,2, and 3 makes 

clear that each succeeding class in the hierarchy from 

original and remaining resources to reserves is included 

in the overlying classes. Original resources include remaining resources and cumulative depletion. Remaining resources include identified and undiscovered 

resources (divisible into hypothetical and speculative 

resources). Identified resources include measured, indicated, inferred, and demonstrated resources. Measured and indicated resources contain coal classed as reserve base, and inferred resources contain coal classed as 

inferred reserve base. Some measured, indicated, and 

inferred resources are subeconomic because they are too 

thin to mine or are buried too deeply to be mined by 

current extraction techniques; furthermore, parts of the 

reserve base and inferred reserve base are potentially 

subeconomic because they will be lost-in-mining. Reserves and inferred reserves are economically minable as 

of the time of classification. The reserve base and inferred reserve base also contain some coal that is believed to be potentially economic and which is classed as 

marginal and inferred marginal reserves.
RESOURCES OF COAL 

AREA: (MINE,DISTRICT,FIELD.STATE. ETC.) UNITS: ISHORT TONS)

CUMULATIVE 

PRODUCTION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

DEMONSTRATED

MEASURED INDICATED

RESERVES

MARGINAL RESERVES

SUBECONOMIC 

RESOURCES

INFERRED

INFERRED 

RESERVES

INFERRED 

MARGINAL 

RESERVES

INFERRED 

SUBECONOMIC 

RESOURCES

UNDISCOVERED RESOURCES

PROBABILITY RANGE

HYPOTHETICAL | SPECULATIVE

 f 

-1-

OTHER 

OCCURRENCES

INCLUDES NONCONVENTIONAL MATERIALS 

__________I_________________I 

BYrCAUTHOR) DATE: 

A PORTION OF RESERVES OR ANY RESOURCE CATEGORY MAY BE RESTRICTED FROM EXTRACTION BY LAWS OR REGULATIONS.

FIGURE 1. Format and classification of coal resources by reserves and subeconomic resources categories.

RESOURCES OF COAL

AREA: (MINE.DISTRICT,FIELD,STATE, ETC.3 UNITS: CSHORT TONS)

CUMULATIVE 

PRODUCTION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

DEMONSTRATED

MEASURED INDICATED

.-* 

/

SUBECONOMIC RESOURCES

INFERRED

/

AV

/ 

/

^r

INFERRED 

SUBECONOMIC 

RESOURCES

UNDISCOVERED RESOURCES

PROBABILITY RANGE

HYPOTHETICAL j SPECULATIVE

+

+-

OTHFH 

OCCURRENCES INCLUDES NONCONVENTIONAL MATERIALS 

_________I__________________I_________

BY.(AUTHOR) DATE. 

A PORTION OF RESERVES OR ANY RESOURE CATEGORY MAY BE RESTRICTED FROM EXTRACTION BYLAWS OR REGULATIONS.

FIGURE 2. Format and classification of coal resources by reserve and inferred reserve bases and subeconomic and inferred subeconomic resources categories

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, beginning on p. 24, 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; 102.7903 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 nonagglomerating 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 semianthracite, 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, mineralmatter-free basis. This class may be either agglomerating or nonagglomerating 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,"
HIERARCHY OF COAL RESOURCES 1

ORIGINAL RESOURCES 2

CRITERIA

< 6000 ft 3

AnehracieeS. Bituminous coal4 >t4 in.

Subbuuminous coal & Lignite >30in.

CUMULATIVE DEPLETION = CUMULATIVE PRODUCTION + LOST-IN-MINING

REMAINING RESOURCES

Same criteria as Original Resources

IDENTIFIED RESOURCES

CRITERIA

teOOO ft., <3 mi.6 

Anthracite & Bituminous coal 

Subbilummous coal & Lignite

>14 m. 

>30 in.

DEMONSTRATED RESOURCES

CRITERIA

<6000 It. < 3/4 mi.

Anthracite & Bituminous coal >14m. 

Subbituminous coal & Lignite >30 in.

MEASURED RESOURCES

CRITERIA

<6000 ft., <1/4 mi. 

Anthracite & Bituminous coal 

Subbiluminous coal & Lignite

> 14 in. 

>30in.

INDICATED RESOURCES

CRITERIA

<6000fl. / >1/4mi. - <3/4 mi 

Anthracite !> Bituminous coal 

Subbituminous coal & Lignite

> 14 in. 

>30 in.

INFERRED RESOURCES

<6000 ft. >3/4mi.-<3mi

Anthracite & 

Bituminous coal

Subbilummous coal

81 Lignite >30 in.

SUBECONOMIC & 

INFERRED SUBECONOMIC RESOURCES

CRITERIA

Anthracite & >14-<28 in. < 1000 f I 

Bituminous coal

Subbituminous coal > 2.5-<s ft y <iooO fl 

Lignite >2.5-<5 fl.,<500 ft

AH coal >100O II. - <6000 fl.

except lignite >500-<1000 fl

RESERVE BASE

CRITERIA 

<3/4 mi

B^u coa, > 28 ,n.. 0000ft.

Subbiluminous coal >5 ft., <tOOO ft.

Lignite >5 fl , <500 ft

MEASURED RESERVE BASE

CRITERIA 

<1/4 mi.

Anthracite &

Bituminous coal

Subbituminous coal >5 ft., <1000 ft.

Lignite >5 II., <500 ft.

>28 in ,<1000 fl.

INDICATED RESERVE BASE

CRITERIA 

>t/4 ml- <3/4mi

Bilummous coal

Subbiluminous coal

Lignite

>28 in. ,<1000 ft.

>5 ft., <1000 l|.

>5 fl. <500 fl.

RESERVES & MARGINAL RESERVES

Anthracite & - O ooo fi Bituminous coal >28 '"- <100° fl

Subbiluminous coal >5 fl. <1000 ft.

Lignite >5 ft.,<500'ft

MEASURED &

MEASURED MARGINAL RESERVES

CRITERIA

<1/4 mi.

>2S in,, a000 ft Anthracite &

Bituminous coal

Bubbituminous coal >5 fl. ,<1000 ft.

Lignite >5 ft., <500 ft.

INDICATED & 

INDICATED MARGINAL RESERVES

CRITERIA 

>1/4 mi. ' <3/4 mi.

Subbiluminous coal 

Lignite

>« ,a, <1000 ,,.

>5 ft., <lOOOfl. 

>5 fl., <500 ft.

SUBECONOMIC RESOURCES

INFERRED RESERVE 

BASE

CRITERIA 

>3/4 mi. - <3 mi.

Antnracile & 

Bituminous coal >28 in. ,0000 ft.

Subbitummous >5 f, <tooo f,

>5 fl.,<500 ft.

INFERRED &

INFERRED MARGINAL

RESERVES

CRITERIA 

>3/4 mi.- <3 n

Anthracite & 

Bituminous coal

Subbilummous 

coal

>28 in., <1000 ft.

>5 fl. ,<1000 It. 

>5-fl ,<500 II.

INFERRED , 

SUBECONOMIC RESOURCES

FIGURE 3. Heirarchy of coal resources and
UNDISCOVERED RESOURCES

CRITERIA

< 6000 ft. , >3 mi.

Anthracite 4 Bituminous coal >14 in.

Subbituminous coal > Lignite >30 in.

HYPOTHETICAL RESOURCES

CRITERIA

<6000 ft., >3 mi.

Anthracite & Bituminous coal

Subbituminous coal & Lignite

> 14 in. 

>30 in.

SPECULATIVE RESOURCES

.CRITERIA 

<6000 ft.

Other criteria not set

1 Coal resource terms are defined in glossary.

2. Resources before mining.

3. 500 ft.. 1000 ft., and 6000 ft. Depth of burial or overburden thickness.

4. Anthracite, bituminous, Subbituminous, and lignite are ranks of coal. See Table t.

5. 14 in., 26 in.. 30 in., and 5 ft. are minimum thicknesses of coal.

6. 1/4 mi, 3/4 mi, and 3 mi. are distances from points of measurement of coal thickness.

7. Includes coal left in room and pillar mining, in property barriers, coal too thick to be 

	recovered completely by conventional mining, and mine end preparation plant waste.

NOTE

OTHER OCCURRENCES

Includes coal:

a) less than minimum thickness at any depth

b) containing more than 33 weight percent ash on dry ba»i»

c) buried at depths of more than 6.000 feet

Estimsted tonnage, where-calculated, is to be reported as 'Other Occurrences" Mid 

not as resources, unless mined. Where mined, tonnage quantity Is included in reterve 

base and reserve estimates.

criteria for distinguishing resource categories.
"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 (°F) at, or near, its point of maximum density of 39.1° 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 bum 

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° to 16° 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 cod 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 coed 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 figs. 5 and 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. (See also reserves, p. 30.) 

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 coed zone are assigned to the same geologic 

member or formation.

coke. A gray, hard, porous, and coherent cellularstructured 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

8
specified area or a specified coal bed. (See cumulative 

depletion, p. 25; 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, p. 25 ; and 

figs. 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 figs. 1,2, and 3.)

demonstrated reserves. Same as reserves. (See also,

demonstrated reserves, p. 25; and figs. 1, 2, and 3.) 

demonstrated reserve base. Same as reserve base. 

(See also demonstrated reserve base, p. 25 ; and figs. 1 , 

2, and 3.) 

demonstrated resources. See resources. (See also,

demonstrated resources, p. 25; and figs. 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, p. 25; 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 

(900-1,800m). See overburden. 

Discussion: The depth categories or overburden categories (see table 3, and specific instruction No. 

2, p. 33) 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-Jree 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/700r 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, area! 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, area! 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/kgtoBtu/lb, multiply by 1.8.

hectare (ha) or square hectometer (hm2). 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, p. 20; (See also hypothetical resources, p. 25; 

and figs. 1, 2, and 3.)

identified resources. See Identified Resources, 

p. 19; (See also identified resources, p. 25; 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 deanable 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 dassed 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 dassify coal lying beyond measured as 

indicated and to assign such coal to resource and 

reserve base categories (fig. 4). 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, p. 25; and figs. 

1,2, and 3.)

indicated reserves base and indicated marginal reserve base. See reserve base. (See also indicated 

reserve base, p 26, and figs. 1, 2, and 3.)

indicated resources. See Indicated Resources, p. 19. 

(See also indicated resources, p. 26; 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 dassed as indicated. 

There are no sample and measurement sites in areas 

of inferred coal. However, a single measurement can 

be used to dassify 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, p. 26 , and inferred marginal reserves, p. 26 ; 

and figs. 1,2, and 3.)

inferred reserve base. See reserve base. (See also 

inferred reserve base, p. 26; and figs. 1, 2, and 3.)

inferred resources. See Inferred Resources, p. 19. 

(See also inferred resources, p. 26; and figs. 1, 2, 

and 3.)

10
.DRILL HOLE \ 

'(POINT OF THICKNESS MEASUREMENT)

MEASURED COAL 

0- TO 1/4-MILE 

(0- TO 0.4-KM) RADII

-INDICATED COAL* 

1/4- TO 3/4- MILE / 

(0.4- TO 1.2- KM) RADII

-INFERRED COAL 

3/4- TO 3-MILE 

v(1.2- TO 4.8-KM) RADII

HYPOTHETICAL COAL 

BEYOND 3 MILES 

(4.8 KM)

"MEASURED AND INDICATED 

COAL CAN BE SUMMED 

TO DEMONSTRATED COAL

INFERRED COAL

3/4- TO 3-MILE

(1.2- TO 4.8-KM) RADII

INDICATED COAL* 

1/4- TO 3/4-MILE 

(0.4- TO 1.2-KM) RADII

MEASURED COAL*

0- TO 1/4-MILE

(0- TO 0.4-KM) RADII

POINT OF THICKNESS 

MEASUREMENT

0

I .',.'.'

0 1

1 2

2 3 KILOMETERS

3 MILES

FIGURE 4. Diagram showing reliability categories based solely on distance irom points of measurement.

11
inferred subeconomic resources. See Inferred 

Subeconomic Resources, p. 20. (See also inferred subeconomic resources, p. 31; 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 107 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 1010 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 gramcalories.

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, lowrank 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, highash 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, p. 32; and subdivisions of reserves. (See also indicated marginal 

reserves and measured marginal reserves, p. 25 and 27; 

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 (fig. 4). 

Used as a modifier to resource terms. 

measured reserves and measured marginal reserves. See subdivisions of reserves. (See also 

measured reserves, p. 27; measured marginal reserves; 

p. 27; and figs. 1,2, and 3.)

measured reserve base. See reserve base. (See also 

measured reserve base, p. 27; and figs. 2 and 3.)

12
NORTHERN GREAT 

PLAINS REGION r\

San Francisco

EXPLANATION

Anthracite andMmianthraciU

BS3

Low-volatile bituminoua coal

Medium- and hi(h-v*latile 

bituminoua coal

EZ2

Subbituminoua coal EH3 400 600 KILOMETERS

Prepared by Paul Averitt, 1975

FIGURE 5. Coal fields of the conterminous United States (from Averitt, 1975).

measured resources. See Measured Resources, p. 19; 

(See also measured resources, p. 27; 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 asreceived 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

13
vBarrow EXPLANATION

EAGLE 

FIELD"Si

RVIS CREEK 

PASS FIELD 

FIELD

Subbituminout 

coal and lignite

x

Isolated occurrence of coal of 

unknown extent 

A. Antkraeitt 

8, Bituntinout 

S, SuMttumimnu 

L. Lignite

0 100 200 300 MILES

cB rA«?^ER ° 100 200 300 KILOMETERS

Prepared by F F Barnes. 1959

FIGURE 6. Coal fields of Alaska (from Averitt, 1975).

coal, namely, free or surface moisture removed by exposure to air, and inherent moisture entrapped in the 

coal and removed by heating to 220°F.

noneconomic. Not capable of profitable production 

or extraction. Coal classified as noneconomic may be reported in other occurrences. See other occurrences, 

noneconomic coal, p. 27.

original. The amount of coal resources in the ground 

before production.

original resources. See Original Resources, p. 19; 

and original resources. (See also original resources, 

p. 27; and 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, p. 27; 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

14
500 MILES

0 500 KILOMETERS

EXPLANATION

1 Rhode Island Meta-anthracite 14 North Central

15 Tertiary lake beds

16 Bighorn

17 Wind River

18 Hams Fork

19 Uinta

20 Southwestern Utah

21 San Juan River

22 Raton Mesa

23 Denver

24 Green River

2 Pennsylvania Anthracite

3 Atlantic coast

4 Appalachian

5 Northern

6 Eastern Interior

7 Western Interior

8 Southwestern

9 Mississippi-Alabama

10 Texas-Louisiana

11 Fort Union

12 Powder River

13 Black Hills

25 Black Mesa

26 Pacific

FIGURE 7. Coal regions of the conterminous United States (modified from Trumbull, 1960.)

estimates of measured, indicated, and inferred either the directly underlying or overlying benches.

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

(See specific instruction No. 3, p. 34.)

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

15
10°

500 KILOMETERS

EXPLANATION

1 Eastern

2 Gulf

3 Interior

4 Northern Great Plains

5 Pacific Coast

6 Rocky Mountains

FIGURE 8. Coal provinces of the conterminous United States (modified from Trumbull, 1960.)

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 geophysist, 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

16
TABLE 1. Classification of coals by rank A, l

Class Group

1. Meta-anthracite 

I. Anthracite* 2. Anthracite 

3. Semianthracitec

1 . Low volatile

bituminous coal

2. Medium volatile 

bituminous coal 

3. High volatile A 

II. Bituminous bituminous coal 

4. High volatile B 

bituminous coal

5. High volatile C 

bituminous coal

1 . Subbituminous A

coal

III. Subbituminous 2. Subbituminous B

coal 

3. Subbituminous C 

coal

 , T . . 1 . Lignite A IV. Lignite 6 

2. Lignite B

Fixed Carbon

Limits, percent 

(Dry, MineralMatter-Free Basis)

Equal 

or

Greater

Than

98 

92 

86

78

69

 

 

_

Less

Than

98 

92

86

78 

69

 

 

_

Volatile Matter

Limits, percent 

Dry, MineralMatter-Free Basis

Greater

Than

2 

8

14

22 

31

 

 

~

Equal 

or

Less

Than

2 

8 

14

22

31

 

 

_

Calorific Value

Limits BTU per 

pound (Moist, 8 

Mineral-Matter-Free Basis)

Equal 

or

Greater

Than

________

___________

14000° 

13000°

11 500

10500

10500

9500 

8300

6300

Less

_____

_______

14000

13000

11 500

11 500

10500 

9500

8300 

6300

Agglomerating Character

/ nonagglomerating

commonly agglomerating

agglomerating

nonagglomerating

AThis 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 bitununous 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.

BMoist refers to coal containing its natural inherent moisture but not including visible water on the surface of the coal.

clf agglomerating, classify in low-volatile group of the bituminous class.

°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.

EIt 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.

'ASTM, i98i, p. 215.

 Modified from ASTM, 1981.

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,

17
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, p. 28 .) 

rank calculation. The determination of the rank of

a coal. Such determination must use the instructions

given under rank calculation, p. 28.

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 recoveryfactor method, p. 28.

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 and 3; and reserves, p. 30-)

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, p. 32. 

MANDATORY SUBDIVISIONS: 

A. 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, p. 30; and figs. 1 and 3.) 

B. 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; p. 30; and figs. 1 and 3.) 

C. 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. 

1. 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

18
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 dearly 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, p. 29; 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, p. 30 ; and figs. 1, 2, 

and 3.)

MANDATORY SUBDIVISIONS:

A. Hypothetical Resources. See Undiscovered 

Resources (p. 20). (See also hypothetical 

resources, p. 25; 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, p. 25). 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, p. 25; 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, p. 26; 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, p. 26; and figs. 1,2, and 3.) 

E. Measured Resources. Accessed and virgin 

demonstrated resources having a high degree of 

geologic assurance. (See measured resources, 

p. 27; 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, p. 27; and figs. 1, 2, and 3.) 

G. Remaining Resources. The resources in the

19
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, p. 31; and figs, 

land 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, 

p. 31; and figs. 1 and 2.)

J. Speculative Resources. See Undiscovered 

Resources (below). (See also speculative 

resources, p. 30; 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, p. 31.) 

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 analagous 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 ye,t 

unrecognized for their economic potential. Jf 

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 SUBDIVISONS:

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.

20
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, 

p. 2).

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 powerplant 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; 

0.892857 Hong ton.

specific gravity of coal. The ratio of the mass of a 

unit colume of coal to the mass of an equal volume of 

water at 4°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

21
TABLE 2. Average specific gravity and average weight of unbroken coal per unit volume of different ranks

Weight of unbroken coal per unit volume

Rank Specific 

gravity

Anthracite and 

semianthracite 1 .47 

Bituminous coal 1.32 

Subbituminous coal 1.30

T iemi<-*» ______ _ 1 OQ

Short tons 

per acrefoot

2,000 

1,800 

1,770 

1,750

Short tons 

per acreinch

166.6 

150 

147.5 

145.8

Metric tons 

per acrefoot

1,814 

1,633 

1,605 

1,588

Short tons 

per square 

mile-foot

1,280,000 

1,152,000 

1,132,800 

1,120,000

Metric tons 

per square 

mile-foot

1,160,960 

1,045,120 

1,027,200 

1,016,320

Metric tons 

per square 

hectometermeter

14,700 

13,200 

13,000 

12,900

Metric tons 

per square 

hectometer 

meter

1,470,000 

1,320,000 

1,300,000 

1,290,000

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, p. 20 ; 

Speculative Resources, p. 20; and Undiscovered Resources, p. 20. (See also figs. 1, 2, and 3.)

speculative resources. See Speculative Resources, 

p. 20. (See also speculative resources, p. 30; 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 (km2-m). The volume of 

coal (1,000,000 cubic meters; 100 square hectaremeter-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, p. 31; 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 cool for resource calculations, p. 31; and specific instruction No. 3, p. 34.)

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°C, and moisture-and ash-free. 

Discussion: The principal reason for the ultimate analysis is the classification of coals by rank, although it

22
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 y p. 31; 

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, p. 184, 

D2361-66, D3761-79, D3175-77, D3176-74, 

D3178-73,andD3179-73).

CRITERIA FOR COAL RESOURCE 

CLASSIFICATION

Although not specifically noted, coal resources are 

classified in figures 1, 2, and 3 according to geologic 

assurances of existence and to the economic feasibility of 

recovery.

The degree of geologic assurance in this system of coal 

classification is determined from the interrelations of 

(1) proximity to or closeness of spacing of points where a 

coal bed is measured or sampled (reliability); (2) concepts, ideas, and models of the depth, rank, quality, 

thickness of coal, area! extent, depositional patterns and 

correlations of coal beds and enclosing rocks; and 

(3) knowledge of associated structural features as they 

control the distribution, extent, thickness, depth of 

burial, and metainorphism of coal resources. An understanding of these elements as they relate to the three

dimensional configurations of stratigraphic sequence is 

necessary to provide the highest degree of geologic assurance as to the existence and continuity or lack of continuity of specific coal beds.

The degree of economic feasibility is determined by 

interrelating the (1) thickness of coal (see specific instruction No. 3, p. 34); (2) thickness of overburden; 

(3) the rank and quality of coal as ascertained from analyses that may be from the same bed or adjacent beds 

and which may be projected on geologic evidence for 

several miles; (4) costs of mining, processing, labor, 

transportation, selling, interest, taxes, and demand and 

supply; (5) expected selling price; and (6) expected 

profits.

The thickness of overburden and the thickness of a 

coal bed are the primary factors controlling the feasibility of mining. Knowledge of the quantity of coal and 

rock that must be removed per unit of recovered coal, of 

the roof and floor conditions, and of the difficulty of 

separating coal from rock determine the mining method 

and the equipment chosen for the mining operation. 

The rank, purity, heat value, and selling price of the 

coal commonly dictate usage and marketability. Higher 

rank coals generally are judged more valuable than 

lower rank coals owing to greater heat values and chemical characteristics that are sought currently by the 

metallurgical and petrochemical industries. Economic 

variables that influence feasibility are price of coal, cost 

of equipment, mining, labor, processing, transportation, interest rates, and taxes. Supply and demand for 

coal also influence feasibility as do environmental laws, 

restrictions, judicial ratings, and political considerations. The relative value of coals may change markedly 

in the near future as the result of utilizing new techniques for converting coal to gas and or liquid fuels. 

Low-rank coals and coals containing pyrite that are currently of lower economic value may in the future be considered premium fuels for conversion processes.

The criteria for the principal classes of coal resources 

described hereafter are summarized in table 3 and are to 

be used in preparing all U.S. Geological Survey coal resource estimates from January 1, 1983, until further 

revised.

APPLICATIONS OF CRITERIA

The criteria are to be applied only to those deposits of 

coal that are currently or potentially feasible for 

economic extraction by underground mining, surface 

mining, and (or) in situ gasification methods. Coal beds 

that are thinner than 14 inches (35 cm; anthracite and

23
TABLE 3. Summary of coal resource criteria SPECIFIC CRITERIA

Depth' Thickness 1 

Feet (Meters) Inches (Centimeters)

Identified and undiscovered resources: 

Anthracite and bituminous

coal <6,000 (1,800) > 14 (35) 

Subbituminous coal and

lignite <6,000 (1,800) >30 (75)

Reserve base and inferred reserve base: 

Anthracite and bituminous

coal <; 1,000 (300) >28 (70) 

Subbituminous coal < 1,000 (300) > 60 (150) 

Lignite < 500 (150) > 60 (150)

Reserves2 , marginal reserves, and inferred reserves2 :

(Criteria same as reserve base and inferred reserved base but with 

factors based on engineering and economic analysis applied.)

Subeconomic resources: 

Anthracite and

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

coal 1,000 (300)-6,000( 1,800) > 14 (35)

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

coal 1,000 (300)-6,000 (1,800) >30(75)

Lignite 0-500(150) 30 (75)-60 (150) 

500 (150)-6,000 (1,800) >30(75)______

'The metric thicknesses shown in parentheses are not exact equivalents of the thicknesses 

shown in feet; for convenience, they have been rounded to the amounts shown.

2 Includes some beds that are thinner and (or) deeper than the general criteria permit, but 

that are being mined currently.

bituminous) and 30 inches (75 cm; lignite and Subbituminous) generally are excluded from resource consideration unless currently being mined. All coal beds deeper 

than 6,000 feet (1,800 m) are excluded. These limits are 

imposed as the result of consultations with geologists 

and mining engineers throughout the international coal 

community. In the United States, beds that contain 

more than 33 percent ash also are excluded; because of a 

shortage of energy in some countries, however, coal 

containing more than 33 percent ash is being mined and 

is classified as reserves.

Coal beds thinner or more deeply buried than the imposed limits have been mined locally at several places in 

the United States and are mined in other parts of the 

world; however, their extraction in the United States 

has generally not proven economic. Where such mining 

is taking place1 , the coal should be classed as a reserve 

and recorded at the time of assessment in the coal resource figures. With the few exceptions owing to current 

mining and similar future exceptions, the imposed limits 

should be adhered to. See general guideline No. 7, 

p. 32.

Note. Specific criteria and glossary terms are crossreferenced within this report. To aid the reader, specific 

criteria, beginning below, are printed in boldface italics, 

and glossary items, beginning on p. 5, are printed in 

boldface type.

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 0 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 lowvolatile, medium-volatile, high-volatile A, high-volatile 

B, and high-volatile C groups. Similarly, tonnage estimates for the anthracite class may be divided into metaanthracite, 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, p. 34.

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.

24
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 lowvolatile, 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.

cummulative production. Cummulative 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, repectively, which are 

the preferred usages. See Reserves and Marginal Reserves, p. 18.

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, 

p. 19).

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," p. 37; and "Extrapolated Coal Zone 

Method," p. 38; 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, p. 10; 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, 

p. 10 ; and fig. 4.)

25
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) suceeded 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, p. 10.)

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, p. 10; 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, p. 10; 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 0 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 

ground 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 surfaceminable 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, p. 34.)

26
lignite marginal and inferred marginal reserves. Tonnage estimates for these dasses 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, p. 12; 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, cummulative production, and coal lost-inmining. 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, p. 12.)

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

27
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 PanFormula.

8.2 Calculation to Mm-free basis: 

Parr Formulas:

Dry, Mm-free FC (FC - 0.15S)x 100 (1)

[100 - (M + 1.08A + 0.55S)] 

Dry, Mm-free VM = 100 - Dry, Mm-free FC (2) 

(Btu - 50S) Moist, Mm-free Btu

[100 - (1.08A + 0.55S)]

x 100 (3)

Note The above formula for fixed carbon is derived from the 

Parr formula for volatile matter. 

Approximation Formulas:

FC Dry, Mm-free FC x 100 (4)

[100 - (M + 1.1A + 0.1S] 

Dry, Mm-free VM = 100 - Dry, Mm-free FC (5) 

Btu

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 x 100

Moist, Mm-free Btu

[100 - (1.1A + U1S)]

x 100 (6)

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

28
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 otherwise, 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 = 100

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 factors 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-inmining such as (1) the coal that will not be mined between properties, and (2) coal in overlying and underlying beds rendered usuitable 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 factors 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

29
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 

0 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,

30
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

31
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).

GUIDELINES FOR ESTIMATING 

COAL RESOURCES

These guidelines were prepared so that coal-resource 

workers will have logical and uniform procedures to 

follow. They combine the best features of the procedures 

used in preparing many previously published estimates 

of State and national coal resources with additions and 

modifications from numerous conferences with experienced coal resource estimators.

These guidelines are aimed at producing a uniformity 

of procedures so that coal-resource estimates prepared 

by various individuals and groups can be compared and 

(or) combined into meaningful totals for a quadrangle, 

township and range, coal field, basin, region, province, 

county, state, nation, continent, and (or) the world.

Some statements in the guidelines obviously are not 

rigidly applicable to all coal beds or coal-bearing areas. 

Where such shortcomings are perceived, a logical procedure is to develop new guidelines to effect appropriate 

changes in methodology so that they can be studied, 

criticized, and accepted or rejected by others.

The following statements are to be strictly adhered to 

by all coal resource specialists in the U.S. Geological 

Survey: (1) All tables of coal resource or reserve estimates must 

contain a date and appropriate authorship information; and 

(2) each table of coal resource or reserve estimates must show 

where supporting basic data were located at the time of estimation.

GENERAL GUIDELINES FOR 

CLASSIFICATION OF COAL RESOURCES

These general guidelines are required for uniform 

classification and reporting of coal into the different 

resource categories. They are modeled after the guidelines in Circular 831 (U.S. Geological Survey, 1980, 

p. 3-4) for all minerals.

1. All naturally occurring concentrations of coal can 

be distributed into one or more of the classification 

categories.

2. Where the term "reserves" is used without a 

modifying adjective, for example, marginal, indicated, restricted, low-sulfur, or inferred, it is to be 

considered synonomous with the demonstratedeconomic category.

3. Quantities and qualities of coal may be expressed 

in a variety of terms and units to suit different purposes but must be clearly stated and defined.

4. A reserve base is a resource category delineated 

only by physical and chemical criteria. A major purpose for the recognition and discrimination of a reserve base is to aid in long-range public and commercial planning. A reserve base estimate for specific 

rank, thicknesses of coal and overburden, quality, 

usage, geologic formation, age, depositional environment, and many other factors can be specified for any 

given deposit or area, or for the Nation. The position 

of the lower boundary of a reserve base is intended to 

extend into the subeconomic category. The intention 

of this extension is to define quantities of in-place 

material, parts of which may become economic 

depending on the extraction plan finally utilized. As a 

result of any given extraction plan, the reserve base 

can be subdivided into component parts reserves, 

marginal reserves, and a remnant of subeconomic 

resources. For the purpose of Federal (USGS) assessment, criteria for the reserve base are listed on page 

29 of this report.

5. Undiscovered resources should be subdivided in accordance with the definitions of hypothetical and 

speculative resources or they may be subdivided in 

terms of relative probability of occurrence (see figs. 1 

and 2).

6. Inferred reserves and the inferred reserve base represent postulated extensions of reserves and the reserve base. They are identified resources but are 

quantified with a relatively low degree of certainty. 

Postulated quantities of resources based on geologic 

inference alone should be assigned to the undiscovered categories.

7. Locally, limited quantities of coal may be produced 

from beds that are of insufficient thickness or are too 

deeply buried to be classified as reserves. This situation arises when production facilities are already established or when favorable local circumstances, such 

as particular coal qualities or removal of overburden 

for other purposes, make it possible to produce coal 

that otherwise could not be extracted profitably. 

Where such production is occurring, the quantity of 

in-place coal (including coal for in situ gasification) 

shall be included in a reserve base and the quantity 

that is potentially producible shall be documented as 

reserves. The profitable production of such coal, however, should not be used as a rationale to assign a 

reserves classification to coal in other areas having 

similar overburden, thickness of coal, and qualities.

8. Coal resources classified as reserves must be considered as economically producible at the time of

32
classification. Conversely, coal not currently producible at a profit cannot be classified as reserves. However, there are situations in which mining plans are 

being made, lands are being acquired, or mines and 

plants are being constructed to produce coal that does 

not meet economic criteria for reserve classification 

under current costs and prices but that would do so 

under reasonable future projections and expectations. 

The marginal reserve category applies to such situations. When economic production appears certain, 

coal classified as marginal reserves will be reclassified 

as reserves. Marginal reserves also may include any 

other coal whose economic producibility borders on 

being profitable.

9. Tonnage of coal that is too impure, too thin, too 

deep, or for other reasons not considered to be potentially economic may be estimated, but is not to be 

classified as a resource. These tonnages may be 

classified in the box labeled "other occurrences" in 

figures 1, 2, and 3.

10. Rank classes of coal, for example, bituminous coal 

as distinct from lignite, should be separately 

quantified.

11. The amount of cumulative production is not a part 

of the remaining coal resources. However, an understanding of what has been produced in the past is important to resource understanding, both in terms of 

the amount of production and the amount of coal remaining in place. Separate accounting for cumulative 

production should be made for each report area, 

county, State, and the Nation.

12. By-passed coal in large blocks left in the ground 

during mining, or planned to be left in the ground 

during current or future extraction, should be recorded in an appropriate resource category depending upon its economic recovery potential. Coal lostin-mining should also be recorded in an appropriate 

resource category if there is a potential for further 

recovery.

13. In classifying reserves and resources it may be necessary to recognize locally that some coal deposits 

derive their economic viability from coproduct or 

byproduct relationships with other commodities.

14. Factors other than economic and geologic considerations, including legal, regulatory, environmental, 

and political concerns, may restrict or prohibit the 

utilization of all or part of a coal deposit. Reserve and 

resource quantities known to be restricted should be 

recorded as such in an appropriate category.

15. This classification system allows for the presentation of more subdivisions than will commonly be reported or for which data are available. Where appropriate, subdivisions may be aggregated or omitted.

16. Data supporting resource estimates are to be documented and preserved.

SPECIFIC INSTRUCTIONS

1. RANK OF COAL

Where coal of more than one rank class or rank group 

is covered by an individual report, the resource data 

shall be reported separately for each major rank class 

and when possible for each rank group listed below:

Class Rankgmap Abbreviation Anthracite Meta-anthracite ma 

Do Anthracite an 

Do Semianthracite sa

Bituminous coal Low-volatile bituminous coal Ivb 

Do Medium-volatile bituminous coal mvb 

Do High-volatile A bituminous coal hvAb 

Do High-volatile B bituminous coal hvBb 

Do High-volatile C bituminous coal hvCb

Subbituminous coal Subbituminous A coal subA 

Do Subbituminous B coal subB 

Do Subbituminous C coal subC

Lignite Lignite A ligA 

Do Lignite B ligB

Abbreviations can be used wherever appropriate in 

reports and tables.

2. OVERBURDEN

Tonnage estimates shall be reported according to 

thickness of overburden:

Mandatory underground mining 

categories:

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)

3,000 to 6,000 feet 

(900 to 1,800 m)

Optional other occurrence category:

> 6,000 feet (> 1,800m)

Mandatory and optional surface 

mining categories1 :

0 to 500 feet (0 to 150 m) 

mandatory use

0 to 100 feet (0 to 30 m) 

optional use

100 to 200 feet (30 to 60 m) 

optional use

0 to 200 feet (0 to 60 m) 

optional use

200 to 500 feet (60 to 150 m) 

optional use

'Use of optional surface mining categories requires the complete coverage of the 0- 

500 feet (0-150 m) category. Other categories may be used if they are in increments of 100 feet 

(30 m) and do not exceed 500 feet (150 m).

33
Resources of currently and potentially strippable and 

underground minable coal beds and coal zones shall be 

computed for the 0-500 feet (0-150 m) overburden category. Use of the surface mining optional categories 

allows tonnage estimates to be related to overburden. 

When optional categories are used, the sum of their tonnage estimates must be equal to the tonnage estimate for 

the 0-500 feet (0-150 m) category.

In addition, other criteria such as the ratio of overburden to coal thickness or the ratio of cubic yards of 

overburden to tonnage of coal may be used to outline 

and evaluate strippable coal deposits. Such departures 

from the standard categories and criteria are advisable 

only where there is adequate data.

3. THICKNESS OF COAL CATEGORIES

Tonnage estimates shall be reported by rank and 

thickness of coal:

Anthracite and bituminous coal

14 to 28 inches (35 to 70 cm)1 

28 to 42 inches (70 to 105 cm) 

42 to 84 inches (105 to 210 cm) 

84 to 168 inches (210 to 420 cm) 

168 inches or thicker (420 cm +)

2.5 to 5 feet (75 to 150 cm)1 

5 to 10 feet (150 to 300 cm) 

10 to 20 feet (300 to 600 cm) 

20 to 40 feet (600 to 1,200 cm) 

40 feet or thicker (1,200 cm +)

'Centimeter thicknesses may be reported in meters.

4. SIZE OF UNIT AREA

For future planning by Federal and State governments, industry, and the public, coal resources such as 

identified resources, reserve base, and reserves, are to 

be estimated for relatively small areas. Unit areas the 

size of townships, 7.5-minute quadrangles, and 

15-minute quadrangles generally are satisfactory. For 

specific purposes such as tract assessment, evaluation, 

and management, however, it may be desirable and (or) 

necessary to report estimates for areas as small as an individual land section, or smaller.

In addition, there are many requirements for estimating the remaining tonnages and type of coal for each 

coal-bearing county, State, and the Nation. Although 

tonnage estimates for most currently known coalbearing counties and States exist, many are inadequate 

and out of date. Numerous requests for information emphasize the need for modern reliable county and State 

estimates. Such modern estimates are to be prepared as 

information becomes available and can be assembled. 

The unit area for such re-evaluations is logically a completed county, the smallest unit universally employed by

State and National agencies. Therefore, all resource 

estimates regardless of size of area (except for State and 

national estimates) must be reported by the county in 

which they occur.

5. MAJOR CATEGORIES OF RESOURCES

Estimates of the total coal tonnages in the following 

resource categories are required, where data are available, for adequate inventorying of county, State, and 

national coal resources: measured, indicated, inferred, 

and hypothetical resources; reserve base and inferred 

reserve base; and original resources and remaining resources. Additional divisions of total resource estimates 

are not required but are desirable to document more 

thoroughly the quantities of coal available for particular 

uses.

6. COAL BED MAPS

A map of each coal bed or coal zone known to contain 

coal resources must be prepared to document the estimate for coal resources. A coal bed map will show 

(1) the trace of a bed's outcrop; (2) all points where the 

thickness of coal was measured at outcrops and other 

surface exposures; (3) all wells, drill holes, and subsurface points where the thickness of coal was measured; 

(4) county, State, and national boundaries; (5) land 

lines (townships and ranges) and latitude and longitude 

coordinates; (6) boundaries of Federal and State Parks, 

forests, wildlife refuges, grasslands, military bases, and 

areas known to be environmentally or legally restricted 

from mining; (7) reliability categories; (8) isopachs of 

thicknesses of coal and overburden; (9) structure contours; (10) areas where coal is absent because of erosion 

or nondeposition; (11) structural features affecting coal, 

such as fold axes and traces of faults; and (12) boundaries of prior surface and subsurface mining. Information derived from proprietary data can be deleted where 

required by the author in preparing a bed map for publication. Such proprietary information may include, for 

example, points of thicknesses of coal beds and overburden, drill-hole locations, mine boundaries, and other 

data furnished by mining companies or land proprietors. Coal beds may be so numerous in some areas that 

only selected bed maps of the major deposits would be 

published, even though bed maps of all coal beds would 

be prepared in order to estimate the resources.

Figures 9 through 23 show how to prepare parts of a 

coal bed map. Tables 4 through 7 show listings of areas 

of reliability by thicknesses of coal and overburden categories derived from figures 9 through 23. The areal

34
interrelations of data depicted on a coal bed map define 

a series of areas, each of which represent a separate coal 

resource category. The areal extent, average thickness 

of coal, and the depth, rank, and weight of coal per unitvolume in each area must be determined in order to 

estimate the tonnages of coal underlying each area.

7. THICKNESS OF COAL MEASUREMENTS

The stratigraphic thickness of coal is measured at outcrops, in trenches and prospect pits, at mine faces either 

underground or in surface pits, and in drill holes by 

direct measurement or geophysical logging. (See "Geophysical Logs* * *," p.46.) Measurements may be made in 

inches or in feet and inches, in feet and tenths of feet, or in meters 

and centimeters. Partings in a coal bed greater than 3/8 

inch (1 cm) should be excluded from the thickness measurements of a coal bed when estimating coal resources 

(see thickness of coal for resource calculations, p. 31, and 

specific instruction No. 3, p. 34) but should be recorded 

as stratigraphic information. Field measurements of the 

thicknesses of coal beds should be made to the nearest 1 inch, tenth 

of a foot, or metric equivalent. Thicknesses determined from core 

and drill-hole logs and from geophysical logs should be reported to 

the smallest practical unit of measurement. The weighted average 

thickness of a coal bed based on field measurements in an area 

should be reported to the nearest inch, tenth of a foot, or metric 

equivalent. An average thickness should be determined by 

using all measurements of coal, by the thickness gradients between measurements, and by isopaching. The 

thickness of coal at specific points should be recorded on 

coal bed maps, and then the bed should be isopached using the points of measurements and gradients between 

the points. An isopach should be drawn along each of 

the standard coal thickness category limits pertinent to 

the rank and thickness of coal in the area to be 

estimated. The weighted average thickness of an area of 

coal is calculated by estimating the size of each isopach 

unit according to the percent or proportion of the area 

covered by the unit. Where mining has been extensive, 

the thickness of coal in unmined areas can be extrapolated from data on thicknesses of coal obtained 

from mine maps and from adjacent mines.

8. DISTRIBUTION OF COAL BED 

THICKNESS MEASUREMENTS

The distribution of data points is exceedingly important in the estimation of coal resources because distribution (including spacing) is universally considered as 

the ultimate control governing the reliability, accuracy,

and precision of any estimation. In most areas the coal 

resource worker must rely on existing outcrops, 

trenches, prospect holes, mine workings, and drill-hole 

data. If drilling is possible, a drilling pattern should be 

established, on the basis of geologic knowledge, to 

supplement the existing data so as to raise the assignment of coal resources to a higher degree of reliability.

9. MEASUREMENT OF AREAS

Coal-bearing areas, as determined from coal bed 

maps (see figs. 9-23) are to be measured to a precision 

of 2 percent or less. Such determinations may be made 

with a planimeter, with graph paper, with equally 

spaced dots, or with a computer. The most common 

instrument used for area determinations is the polar 

planimeter. In recent years the digital electronic 

planimeter has become increasingly popular. Prior to 

determining areas with a planimeter, a planimeter factor for acres or hectares, which depends upon the scale 

of the map used, must be ascertained. This is done according to instructions that accompany the planimeter. 

A similar factor must be determined if the graph paper 

technique of determining acreages is employed. After 

either factor is ascertained, the user is prepared to start 

measuring areas on the map and to convert the measurements using the appropriate planimeter or graph 

paper factor into acres or hectares.

On many 7.5-minute quadrangles, several dozen to 

several hundred areas must be measured that are based 

on the many parameters into which coal-bearing areas 

can be categorized. These parameters may include 

thicknesses of coal and overburden; distance from points 

of coal thickness measurements (reliability categories); 

quality; physical characteristics; rank, land ownership 

by Federal, State, Indian, and local governments, companies, individuals, and other nations; county, State, 

and townships and ranges of the land classification 

system; quadrangle, coal field, basin, region, and province; legally and environmentally restricted areas, and 

others as desired.

As the areas are measured, mean results ascertained 

by averaging the planimeter or graph paper readings are 

recorded so that each area's acreage can be computed.

A planimeter is accurate in measuring map areas 

ranging from several square inches to 20-30 square 

inches. Generally, planimetric measurements are 

repeated several times and then averaged. However, if 

readings are in disagreement by more than 2 percent, 

they should be repeated until an agreement of 2 percent, 

or less, is achieved. Map areas of less than 1 square inch

35
commonly are not as precisely measureable with a planimeter as are larger map areas and must be remeasured 

and the planimeter vernier read many times to obtain 

an agreement within a 2-percent error. Repetition of 

planimetric measurements with only two vernier readings, one at the beginning and one at the end, can be accomplished by continuously tracing the borders of the 

area being measured a predetermined number of times 

in a manner similar to the repetitive turning of angles 

with a transit by a surveyor. The difference between the 

two readings should be divided by the number of times 

the borders are traced; the answer multiplied by the 

planimeter factor is the size of the area. This repetitive 

measurement of an area is a good method of obtaining 

precise planimetric readings.

areas (acres or hectares), coal thicknesses, and weight of 

coal per unit volume and summed for each coal category. The tonnage estimates for each coal category are 

then rounded to significant numbers so as to not eliminate tonnage estimates for small areas of measured and 

indicated coal. For example, the tonnage estimates for 

small areas such as A, B, C, F, G, H, K, M, O, P, Q, 

and R in figure 17 and A, B, C, D, E, F, G, H, L, Q, 

W, Y, AA, HH, and OO in figure 19 are not rounded 

out of the totals and are included in the summing of the 

total coal resources for all areas in figures 17 and 19. 

After summing, the resultant total coal resources 

estimates for a whole map area (a quadrangle, county, 

or basin) are not rounded because their component 

parts have been rounded.

10. WEIGHT OF COAL PER UNIT VOLUME

A tonnage estimate for any coal deposit can be made 

if the thickness of coal, area! extent, and weight of coal 

per unit volume are known. The weight of coal per unit 

volume (density) or specific gravity varies with rank, ash 

content, and the amount of each macerals group (such 

as vitrinite, inertinite, and exinite) in the coal. Ideally 

the density of coal in a deposit should be determined 

from numerous specific gravity determinations on unbroken coal, but there are rarely sufficient determinations to characterize a deposit. Therefore, it is 

recommended that tonnage calculations be based either 

on the average specific gravity or the average weight of 

unbroken coal per unit volume of the different ranks 

shown in table 2.

11. CALCULATION OF COAL RESOURCES

After the area underlain by coal, the average thickness of coal, and the weight of coal per unit volume for 

each category shown on a coal bed map have been determined, the tonnage can be estimated. The tonnage is 

estimated by the following formula:

A x B x C = tonnage of coal where

A - weighted average thickness of coal in inches,

feet, centimeters, or meters, 

E = weight of coal per appropriate unit volume in

short or metric tons, and 

C = area underlain by coal in acres or hectares.

12. ROUNDING OF TONNAGE ESTIMATES

The rounding of tonnage figures shall be done only 

after all calculations have been completed using data for

13. ESTIMATION OF RESOURCES IN THE VICINITY OF

WHERE A COAL BED BIFURCATES INTO

TWO OR MORE TONGUES

Resource estimation is difficult in those localities 

where a coal bed bifurcates or splits into tongues, each of 

which exceeds the minimum thickness for resource estimation. The difficulty is caused by the necessity to 

delineate a boundary between the area where the resources of the main coal bed are estimated and the areas 

where the resources of the tongues are individually 

estimated.

As stated in the glossary, a parting is "a layer or 

stratum of non-coal material in a coal bed which does 

not exceed the thickness of coal in either the underlying 

or overlying benches" of coal. Where the non-coal 

material exceeds the thickness of either the underlying 

or overlying parts of the coal bed, the coal bed is considered for the purpose of resource estimation to have split 

into two coal beds (each of which is depositionally a 

tongue from a thicker main coal bed).

In estimating resources in such a geologic situation, it 

is necessary to delineate on a coal bed map the areas 

where resources will be separately calculated; to do so, a 

line is used to connect all points where one of the 

tongues becomes thinner than the intervening parting. 

It is also necessary to locate on the coal bed map all 

points where the thickness of the main coal bed and the 

tongues were measured. Measured, indicated, and inferred reliability circles should be drawn on the bed map 

from each point of thickness of coal measurement on the 

main bed and the tongues. The circles should be drawn, 

according to the appropriate distance for each reliability 

category, across the boundary line for resource estimation. After the circles are drawn, the thickness variations 

of the main coal bed and each tongue (bed) are to be isopached separately. Tonnage estimates should then be

36
calculated for each tongue (bed) and the main bed using 

procedures outlined elsewhere in this report for separate 

coal beds.

ESTIMATION OF HYPOTHETICAL RESOURCES

The estimation of hypothetical coal resources in areas 

where geologic, thickness, rank, and areal size data are 

sparse or absent is necessary to promote exploration for 

poorly known and undiscovered coal areas. Currently 

about 2.24 trillion short tons of the Nation's 3.68 trillion 

tons of remaining coal resource inventory are classified 

as undiscovered (hypothetical) (Averitt, 1975). Much 

additional unknown coal may be concealed in the central parts of basins and is not as yet included in the Nation's coal inventory. This additional unknown coal 

must be identified, as must the 2.24 trillion tons currently remaining in the inventory, because knowledge of 

the quantity, quality, and rank of the unknown and 

hypothetical coal could influence the Nation's energy 

usage plans. Therefore, a question commonly asked by 

government and industry estimators responsible for inventorying coal resources is: How do you estimate resources where there are no nearby thickness, depth, and 

rank data? Another way of stating this question is: How 

do you estimate the resources of central parts of poorly 

explored or unexplored basins?

The problem of estimating hypothetical resources in 

the central parts of basins and elsewhere has been approached by (1) not attempting to calculate them; (2) assuming that the centers of the basins or coal-bearing 

areas where control is lacking are barren of coal; (3) assuming that the better known marginal areas of coalbearing basins are representative of the average quantity 

of coal per square mile in the central parts of the basins; 

(4) assuming an average thickness of coal and applying 

this factor to the volume of coal-bearing rocks in an 

area; and (5) making an outright guess. None of these 

solutions are satisfactory. A sixth solution has been to 

extrapolate measurement data from surrounding areas 

into the unknown area. This approach is better than the 

others but commonly is inadequate because it does not 

consider geologic factors that would control unsystematic distribution of coal in an unknown area.

The following discussion describes two methods, 

based on geologic principles, for estimating hypothetical 

resources. Both are acceptable if the supporting basic 

geologic and measurement data are documented and 

presented for evaluation. These alternative approaches 

can be termed the "extrapolated bed map method" and 

the "extrapolated coal zone method." Both methods are 

based on the extrapolation of geologic knowledge and

measurement data into an area of unknown coal resources. In some problem areas, a mix of the two methods may be desirable.

Much of the data called for in the descriptions of the 

extrapolated bed map and extrapolated coal zone 

methods will not be available in many areas where 

hypothetical resources must be estimated. The descriptions of the methods are written for the ideal amount of 

control adjacent to an unknown area; thus, the methods 

will have to be modified on the basis of the type, availability, and amount of control data.

EXTRAPOLATED BED MAP METHOD

Step 1. Assemble all available geologic data in the 

areas adjacent to the unknown area for the coal bed 

whose resources are to be estimated.

Step 2. Assemble and plot on a base map all pertinent data on thicknesses of coal and overburden, 

quality of coal, reports of coal in drill holes and 

wells, and rank of coal.

Step 3. Analyze all data collected in steps 1 and 2 to 

ascertain trends of coal deposition by (a) constructing isopach maps of the coal bed in known areas; 

(b) constructing isopleth maps of heat values, ash 

contents, and contents of trace elements that may 

be related to sources of sediment; and (c) identifying trends of persistent thick or thin coal, or other 

parameters of coal.

Step 4. Identify depositional and erosional trends 

and features in the known areas of rocks adjacent to 

the coal bed and in the coal bed itself that may indicate ancestorial through-flowing rivers in 

swamps, coastal environments, unconformities, 

directions of delta building, paleogeomorphic locations of swamps on a coastal plain or delta, and 

directions and distances to sediment sources. Plot 

all pertinent data on a map.

Step 5. Extend stratigraphy from known areas into 

and across the unknown area by utilizing all surface and subsurface information. Collect surface 

stratigraphic data and existing petroleum and 

water well information. Evaluate and analyze all 

data and determine best correlations so that coal 

bed extensions and correlations can be made. Determine from available data probable geologic 

model of deposition. Plot all data on the map of 

step 4.

Step 6. Construct structure sections across the unknown area so that changes in the thickness of overburden and rank of the coal bed can be considered 

and (or) postulated.

37
Step 7. Construct a structure contour map of the 

coal bed in the unknown area.

Step 8. Combine all pertinent data from steps 2, 3, 

4, 5, and 7 onto a single coal bed map.

Step 9. Fill in the unknown area by projecting pertinent coal and other data across or through the 

unknown area. This projection should include isopachs for thicknesses of coal and overburden, 

isopleths for rank and (or) heat values, predicted 

erosional channels cutting through the coal, and 

other stratigraphic, depositional, or structural features affecting the coal bed.

Step 10. Place land lines and boundaries of political 

subdivisions on the map.

Step 11. Define areas of hypothetical coal by plotting the outer limits of inferred coal as determined 

from points of thickness control in the known areas. 

Determine acreages of hypothetical coal for the 

following resource categories: thicknesses of coal 

and overburden; rank; quadrangle; townships and 

ranges; counties; and State.

Step 12. Determine the average coal bed thickness 

for each acreage unit in each category.

Step 13. Calculate and sum estimated tonnages for 

each category, round estimated tonnages to significant figures as per specific instruction No. 12, 

p. 36, and sum into a final hypothetical tonnage. 

These estimated tonnages and the total summed 

tonnage probably represent the best information 

that can be estimated for a bed that extends from a 

known area into an unknown area where control 

points are absent or sparse.

EXTRAPOLATED COAL ZONE METHOD

If the procedure for an extrapolated bed map analysis 

of hypothetical coal resources is impractical because of 

time limitations or sparsity of data it is suggested that 

the approach to estimating hypothetical resources be by 

an extrapolated coal zone method.

Step 1. Determine if a coal zone exists by analyzing

the available data.

Step 2. Assemble and plot on a base map all available geologic data in areas adjacent to the unknown 

area.

Step 3. Construct an isopach map of the cummulative thicknesses of all coal beds in the coal zone (less 

partings) that exceed 14 inches for anthracite and 

bituminous coal beds and 30 inches for lignite and 

subbituminous coal beds in known areas. 

Step 4. Identify trends and geologic features in the 

rocks of the coal zone in the known area that may 

indicate persistent ancient through-going rivers,

coastal environments, unconformities, directions of 

sediment transport, intertonguing sediments and 

coal beds, regressions and transgressions, and so 

on. Plot pertinent data on a map.

Step 5. Extend stratigraphy of the coal zone and 

related rocks into and across the unknown area on 

the map of step 4 by utilizing all surface and subsurface data. Determine geologic model of deposition of the coal zone. Construct stratigraphic 

diagram of the coal zone and fit diagrammatic data 

to the map.

Step 6. Construct structure sections across the unknown area so that thickness of overburden and 

rank changes of coal zone can be considered and 

(or) postulated.

Step 7. Construct a structure contour map of the 

coal zone in the unknown area.

Step 8. Combine all pertinent data from steps 2, 3, 

4, 5, and 7 onto a single coal zone map.

Step 9. Fill in unknown area by projecting pertinent 

coal zone and other data across or through unknown area.

Step 10. Place land lines and boundaries of political subdivisions on map.

Step 11. Define areas of hypothetical coal by plotting an outer limit of inferred coal as determined 

from points of thickness control in the known areas. 

Determine acreages of hypothetical coal for the 

following categories: thicknesses of overburden, 

cummulative thicknesses of coal, rank, quadrangle, 

townships and ranges, counties, and State.

Step 12. Calculate and sum tonnages for each category after determining a cummulative thickness of 

coal for each acreage unit in each category and calculating tonnage. Round tonnages to significant 

figures as per specific instruction No. 12, p. 36, 

and sum into a total hypothetical tonnage amount.

The extrapolated coal zone solution for hypothetical 

coal eliminates the estimation of tonnages for many individual beds and, in the view of some workers, provides a more valid estimate than the bed-by-bed approach. Despite this view, the bed-by-bed method is 

herein recommended for use where practicable and (or) 

feasible.

EXAMPLES ILLUSTRATING THE BASIC

GEOMETRIC PRINCIPLES OF 

CONSTRUCTING COAL RESOURCE BED MAPS

Coal resource estimation systems on a worldwide 

basis generally lack illustrations amplifying written instructions as to how to calculate coal resources.

38
The following illustrations are intended to bridge this 

gap and to show how coal resources data are utilized to 

construct bed maps. Many more illustrations could be 

prepared, but the accompanying figures are believed to 

be representative.

Figure 9 illustrates how to determine areas of reliability (areas based on distance of from points of thickness 

measurements measured, indicated, inferred, and 

hypothetical coal) where a coal bed has been mapped on 

the surface and the thickness of coal has been measured

AREA OF

HYPOTHETICAL COAL RESOURCES 

BEYOND 3-MILE (4.8-km) RADIUS

AREA OF

INFERRED COAL RESOURCES

BETWEEN 3/4- AND 3-MILE

(1.2- AND 4.8-km) RADII

1 MILE

1 KILOMETER

AREA OF

INDICATED COAL RESOURCES

BETWEEN 1/4- AND 3/4-MILE

(0.4- AND 1.2-km) RADII i-

*

AREA OF

MEASURED COAL RESOURCES 

N WITHIN 1/4-MILE 

(0.4-km) RADIUS

COAL ^POINT OF THICKNESS 

MEASUREMENT

FIGURE 9. Determination of areas of reliability using coal thickness data only at points of measurement along outcrop line. Radii origins

are at points of thickness measurement.

39
at five points. The solid outcrop line indicates where the 

bed has been mapped with assurance as to its intersection with the ground surface; the dashed line indicates uncertainty as to the exact location. Utilizing radii 

originating at the points of thickness measurements, 

arcs are constructed at appropriate distances according 

to the criteria for the measured, indicated, inferred, and 

hypothetical reliability categories. The arcs enclose 

areas theoretically underlain by coal for each reliability 

category.

Figure 10 uses the same coal bed outcrop as figure 9 

and shows the effect of additional thickness information 

from drill-hole measurements and a small mine. A comparison of figures 9 and 10 illustrates how the additional 

data derived from drill-holes and mining expand the 

areas designated in figure 10 as measured and indicated 

and modify the inferred and hypothetical areas.

The objective of figure 11 is to show how drill-hole 

data combined with outcrop data on a continuously exposed strip-mined coal bed result in the coalescing of 

measured and indicated coal and in an expansion of the 

area of the inferred reliability category. It also shows 

how a single point of thickness of coal measurement defines an area of measured, indicated, and inferred coal.

The primary objective of figure 12 is to show how to 

construct a minimum thickness-of-coal isopach 

(14 inches) based solely on outcrop data. A secondary 

objective is to illustrate how to locate a coal outcrop and 

boundaries such as county lines on a bed map from a 

geologic map. Generally, as a bed thins along an outcrop, it is increasingly more difficult to trace. Figure 12 

illustrates a simple, nearly straight coal bed outcrop with 

three thickness-of-coal measurements. The three measurements are all thicker than the 14-inch minimum 

thickness of coal considered when calculating anthracite 

and bituminous coal resources. The thickness gradients 

between the 26-inch and each of the 20-inch points of 

measurement of coal are extrapolated along the crop line 

to the east and west of the 20-inch measurements to 

points A and B where the coal gradients predict coal 

thicknesses of 14 inches. A straight line is then drawn 

between the predicted 14-inch points A and B on the 

crop line and subsequently is bisected at mid-point 0. 

An arc AB is drawn between the predicted 14-inch 

points using either AO or BO as a radius. The arc AB 

locates the 14-inch isopach according to the available 

data on thickness and length of outcrop.

Figure 13 displays areas of reliability, thickness-ofoverburden contours, political subdivisions, and a completed coal bed map ready for determinations of acreages and weighted average thickness of coal in each 

area. Areas A to P are ready for planimetry of acreages,

for calculation of coal tonnages after weighted average 

thickness of coal for the areas are determined, and for 

subsequent entry on tonnage tables according to their 

assignment to a thickness of coal, thickness of overburden, reliability category, and to a county. Table 4 

lists the areas of reliability by thickness of overburden 

category, by a single thickness of coal category, and by 

counties. To avoid confusion, figure 13 and table 4 were 

constructed with only one thickness of coal category, 14 

to 28 inches. Subsequent figures are more complicated 

because more thickness categories are introduced.

Figures 14 and 15 show a diagram of a ridge underlain by coal; they also show the geometric method of 

determining the 14-inch minimum thickness of bituminous coal isopach and the areas of reliability constructed 

from the points of thickness of coal measurement. The 

28- and 42-inch coal isopach lines were constructed by 

using the gradients between the various thicknesses of 

coal measurements. This method of determining the 

area of coal under a ridge is commonly necessary when 

thickness measurements are sparse and an estimation of 

coal resources beneath a ridge is required. Table 5 ennumerates the areas assigned to the varous thicknesses 

of coal and overburden and reliability categories.

Figures 16 and 17 and table 6 show how to construct 

the 14-inch minimum thickness limit of anthracite or 

bituminous coal resources, other coal isopachs, and the 

areas of reliability and overburden isopachs for a simple 

valley reentrant in a coal bed outcrop. Figures 16 and 17 

and table 6 are important because they show how to 

treat a valley reentrant in a coal outcrop characterized 

by minimal thickness data.

Figures 18 and 19 and table 7 show how to determine 

the location of the minimum coal isopach (14 inches for 

bituminous coal) in a valley and ridge underlain by a 

coal bed that is locally less than the minimum thickness. 

Figure 19 also illustrates the areas of reliability, overburden categories, and coal isopachs. The location of 

the minimum coal isopach (14 inches) on a ridge which 

has thickness data on only one side is a common problem facing geologists estimating coal resources.

One of the most difficult problems in estimating 

resources of coal is where the beds are moderately to 

steeply dipping or are highly deformed. A simplified 

problem is illustrated in figures 2QA , B, and C by a bed 

that dips uniformly 30° into the subsurface. The problem as illustrated can be solved by two methods. The 

first method is to estimate all areal resource tonnages in 

the plane of the coal bed and then to project all pertinent 

areal categories to the ground surface. For example, the 

measured, indicated, inferred, and hypothetical 

reliability circles, coal isopachs, overburden contours,

40
HYPOTHETICAL COAL

INFERRED COAL

1 MILE

I______I_____I

1 KILOMETER

COAL IN SUBSURFACE

INDICATED COAL

f___ S"

^-^-^K/iEASURED 

°x

,~ >»

/MEASURE^ V<COAL

POINT OF THICKNESS OF 

COAL MEASUREMENT

FIGURE 10. Determination of areas of reliability from points of measurement on the outcrop line, supplemented by mine and drill-hole data.

41
1/2-MILE- 

(0.8-km-) 

WIDE BELT

21/4-MILE- 

(3.6- km-) 

WIDE BELT

i/4-MILE- 

(0.4-km-) 

WIDE BELT

EXPLANATION

MEASURED COAL 

INDICATED COAL 

INFERRED COAL 

HYPOTHETICAL COAL 

DRILL HOLE

* POINT OF MEASUREMENT ON OUTCROP

 OUTCROP LINE

FIGURE 11. Detennination of areas of reliability using coal thickness measurements taken along a continuously exposed, strip-mined bed,

supplemented by drill-hole data.

and structure contours should be drawn in the plane of 

the coal bed (fig. 20/4). Then the areas of each of these 

categories are planimetered and coal tonnages estimated. This procedure provides an adequate estimate of 

coal tonnages for all values of dip. However, it is difficult to project the areas from the plane of the coal bed to 

the surface area overlying any particular category (fig. 

205). Although this method gives an adequate tonnage 

estimate, it distorts the ground surface depiction of area! 

categories (fig. 20C).

The second method is to draw the area! boundaries of 

the various categories on a surface map, measure the 

areas underlain by the various categories, and estimate 

the coal tonnages for each category. After estimation of 

the resources as if they were flat-lying, the estimates are

divided by the cosine of the dip. The resultant answer is 

the tonnage in the plane of the bed. This method will 

provide correct tonnage estimates in each resource category underlying the surface but will horizontally exaggerate the areas of measured, indicated, and inferred 

coal and should be used only where the dip is more than 

10° and less than 30°. For dips above 30°, it is recommended that resource estimates be made by the first 

method. For dips less than 10°, all estimation of resources should be done as if the bed was flat-lying.

The authors suggest that geologists involved with estimating the coal resources of highly deformed areas consult with coal geologists who have worked on similar 

areas and estimation problems. Currently, resource 

problems in highly deformed areas have not been solved

42
S37S>

 oft

^

/

x'0

\<

COAL IN SUBSURFACE

OUTCROP 20V'OF~26"

EXPLANATION 

20"POINT OF MEASUREMENT OF THICKNESS OF COAL IN 

x INCHES

 14-INCH ISOPACH

A,B POINTS WHERE COAL BED IS 14" THICK AS DETERMINED 

BY EXTRAPOLATING FROM POINTS OF MEASUREMENT

O MIDPOINT OF LINE AB

FIGURE 12. Determination of minimum thickness isoline of 14-inch thick anthracite or bituminous coal using data from a nearly straight 

outcrop line. A and B are located on the outcrop line where the coal thickness as extrapolated from the points of measurement are predicted to be 14 indies. Line AB connecting the points of predicted 14-inch coal is bisected at 0. The radius AO or BO is then used to construct arc AB, the 14-inch isopach or predicted limit of 14-inch-thick coal. This figure also shows the location of the outcrop line and a 

county boundary on a coal bed map as derived from a geologic map.

escept locally for small areas. Although methods have 

been developed, they are generally very time consuming 

and as yet unproven by mining.

Figure 21 illustrates a coal bed that has been mined 

from shafts at some distance from outcrops. All data are

derived from the mines or drill holes. The figure shows 

measured, indicated, inferred, and hypothetical coal; 

county lines; thickness of coal measurements; coal thickness categories; and the 0-200 feet, 200-300 feet, and 

the 300-500 feet overburden categories. It also shows a

43
0 3 MILES

0

1 I

2 3 KILOMETERS

S>* 20^-oJLJ

20- OF ~ 26" BE°

EXPLANATION

POINT OF COAL MEASUREMENT, IN INCHES 

AREAS OF RELIABILITY 

STRIKE AND DIP

COAL OVERBURDEN 

ISOLINES, IN FEET

FIGURE 13. Same basic diagram as figure 12. Shows determination of areas of reliability and 500-, 1,000- and 2,000-foot overburden contours which were obtained by subtracting elevations of bed from surface elevations. Bed elevations were obtained from a topographic 

map, from the outcrop of a coal bed, and from structure sections utilizing dips. Areas of reliability determined by using 1/4-mile radius 

(measured), 3/4~mile radius (indicated), 3-mile radius (inferred), and more-than-3-m'' radius (hypothetical) from measurement 

points. Individual areas of reliability are identified by letters A through U.

national park, a national bird refuge, a State park, and a 

large lake, all of which merit assignment of the underlying coal in those areas to restricted categories.

Figure 22 demonstrates a small coal basin where 

many resource classification categories are distinguished, including two large areas of hypothetical coal. 

The figure also shows two coal beds that intertongue 

with an intervening linear sandstone body. The coal

beds have been strip-mined at six localities and underground mined at three localities. The coal beds have 

been tested by 15 drill holes which revealed the existence 

of two coal beds separated by sandstone. Numerous 

surface measurements of coal thickness support the 

widespread extent of at least one bed, and three measurements revealed the relations of two coal beds with the 

sandstone body. A bed map of the basin revealed two

44
COAL IN SUBSURFACE 

AND THICKER THAN 14 INCHES \

FIGURE 14. A ridge showing construction of accepted 14-inch minimum thickness limit of a bituminous coal bed. Point C is the most distant point of outcrop on the ridge from end points A and B. Accepted 14-inch thickness of coal limit comprises an arc AB. Construction 

of arc AB is as follows: from midpoint 0 of line AB, draw line OC to furthest point on outcrop. Swing arcs from A and B with a radius of 

OC. The intersection of the arcs is point 0' . From 0' draw arc^45 using either AO' or BO' as the radius.

TABLE 4. Areas of reliability from figure 13 listed by overburden and coal 

thickness categories, and by counties

[Tonnage estimated for each area could be determined (1) by planimetering each area for 

acreage, and (2) by multiplying acreage by proper weight of coal per acre-inch by the 

average thickness of coal in the area. Average thickness of coal would be obtained by 

isopaching coal thickness as shown on figure 13 ]

Geologic 

assurance 

category

Coal thickness

14 to 28 inches 28 to 42 inches

0 to 500 feet overburden: 

Measured A, B, C None

Indicated D, E, F Do. 

Inferred G, H, I,J, K Do.

500 to 1,000 feet overburden: 

Indicated L, M, N None

Inferred 0, P Do.

1,000 to 2,000 feet overburden 

Inferred Q, R ________________ INO0C

2,000 to 3,000 feet overburden: 

Inferred S, T None

Hypothetical U Do.

Political division

Holmes County 

A, F, G, H, N, 

0, R, S, U

Dead Bull County 

B, C, D, E, I, J, 

K, L, M, P, Q, T

coal-bearing areas separated stratigraphically and 

areally by an intervening sandstone body. The completed bed map in figure 22 allows the segregation and 

subsequent estimation of the resources of the two coal 

beds into a large number of categories enumerated in 

the caption for the figure. The two coal beds are 

subdivided by their heat values on a moist, mineralmatter-free basis into lignite and subbituminous coal. 

The 8,300-Btu isolines that represent the boundary between subbituminous coal and lignite are drawn so 

that tonnage estimates can be made for proper rank 

classification.

Figure 23 depicts a large but simple subbituminous 

coal and lignite basin that contains predominantly hypothetical resources traversed by three synclines and two 

anticlines. The total thickness of coal at each point of 

surface measurement and in each drill hole penetrating 

the coal zone is obtained by summing the thicknesses of 

all coal beds that are thick enough to be considered as 

resources (2.5 feet, or 75 cm). These thicknesses are isopached at 40-foot thick increments, and the overburden, heat value, and reliability categories are plotted

45
3 MILES

ii n r

0 1 3 KILOMETERS

SMALL BUTTE CONTAINING 56" 

OF COAL, AREA TOO SMALL TO 

BE ECONOMIC FOR MINING

FIGURE 15. Same ridge shown in figure 14 with the addition of an isolated butte containing bituminous coal. Shows determination of areas 

of reliability; 14-, 28-, and 42-inch coal isopachs, and 500-foot overburden contour. Categories defined by intersections of lines 

separating areas of reliability, thickness of coal, and thickness of overburden are labelled A through R. Overburden contour was computed from dips and by subtracting bed elevations from surface topographic elevations.

on the map. Estimates of coal tonnage should be calculated for each rank category. The total tonnage 

estimate for hypothetical resources in the example is the 

sum of each of these estimates. Generally, the size of the 

thickness of coal increments is left to the geologist to 

determine. Figure 23 is appropriate to an analysis based 

on a coal-zone approach to estimation of hypothetical 

resources as outlined on p. 37-38.

Figures 24 and 25 show suggested forms for recording 

coal resource data. These forms, or similar forms, are 

necessary for recording coal bed names; overburden and 

thickness of coal categories; planimeter readings; calculated areas; and weighted average thickness of coal for 

the area being calculated. The forms also provide columns for tabulating tonnage estimates in the measured, 

indicated, and inferred reliability categories, by coal 

thickness categories, and miscellaneous information 

such as planimeter factor, persons responsible for steps 

of work, and total tonnages by thickness and overburden categories. The data recorded on such forms and 

the calculations made therefrom represent the culmination of coal resource work starting in the field and continuing through the preparation of bed maps such as 

those in figures 9 through 23. The forms for data entry 

and calculation should be designed with care so the data 

entered on the forms from accompanying bed maps contain the necessary information for estimating all types of 

coal resources and reserves. This information can be 

combined with other information such as pertinent data 

on costs, transportation, and marketing to provide a 

documented analysis of the coal resources/reserves in a 

coal bed, field, region, basin, province, county, State, 

and (or) the Nation.

GEOPHYSICAL LOGS AS A SOURCE OF 

COAL BED DATA

Geophysical well logs have been invaluable in the 

search for oil and gas because they provide rapid, economical, and detailed information on the thickness, 

lithologic characteristics, fluid content, correlation,

46
COAL IN SUBSURFACE 

AND THICKER THAN 14 INCHES

\-f

>

7 OUTCROP 

CALCULATED 

14" OF COAL

0 1

32"

- POINT OF THICKNESS 

OF COAL MEASUREMENT

i i i I

3 MILES

CALCULATED 14* 

OF COAL

1 2 3 KILOMETERS

FIGURE 16. A simple valley reentrant in a bituminous coal bed outcrop. Shows determination of 14-inch coal isopach and limit of area to 

be planimetered for underlying coal. The 14-inch coal thickness points were obtained by extrapolating data from points of measurement along outcrop line. Projected limits of coal resource isopach (14-inch coal) and area to be calculated are determined by constructing: (1) an arc BB' with origin at C drawn from B to the extension at B' of line AC and CB; and (2) final segment of arc AB' is drawn with 

origin at midpoint 0 of line AB'.

TABLE 5. Areas of reliability from figure 15 listed in proper overburden and 

coal thickness categories

Geologic 

assurance 

category

Measured -

Inferred 

Coal thickness

14 to 28 inches 28 to 42 inches

0 to 500 feet overburden:

A, D, E B, d 

j K J _____ / If ______ _

0 N 

42 to 84 inches

C, R

G,D

M

500 to 1,000 feet overburden: 

Inferred P None None

structure, and depth of strata penetrated by a well. Such 

logs have been run in many thousands of oil and gas exploratory, discovery, and producing wells that have 

penetrated coal-bearing strata in many regions of the 

country.

Geophysical logs can be used to identify coal beds and 

to quantify their resources because coal has several 

unique physical properties including low natural radioactivity, low density, and high resistance to electrical 

currents; these properties contrast with those of most

47
3 MILES

T r

3 KILOMETERS

2000===COAL | N SUBSURFACE

^=3B^: ;Jfe;; v:'->S

20 COAL THICKNESS 

POINT OF COAL _ 'SOLINES, IN INCHES

MEASUREMENT, 

IN INCHES _

__., «' OVERBURDEN 1000 A.B.C AREAS OF "I""",? CONTOURS,IN FEET INFERRED 

RESOURCE 2000J '

CATEGORIES ft STRIKE AND DIP

MEASURED 

INDICATED

HYPOTHETICAL

FIGURE 17. Same diagram as figure 16. Shows determinadon of areas of reliability; 500-, 1,000-, and 2,000-foot overburden contours; 

and 14-inch, 28-inch, and 42-inch coal isopachs. Overburden contours were determined from surface elevations on a topographic map 

and by subtracting bed elevations utilizing structure sections, and dips. Coal isopachs were determined from points of coal thickness 

measurement and 14-inch limit of coal. Areas of reliability were determined by using 1/4-mile radius (measured), 3/4-mile radius 

(indicated), 3-mile radius (inferred), and more-than-3-mile radius (hypothetical) from points of thickness of coal measurement. 

Individual areas of resource categories are identified by letters .4 through DD.

other rocks in the coal-bearing sequence. Thus, geophysical logs can provide information on the existence, 

continuity, thickness, and correlation of shallow to 

deeply buried coal beds in known coal-bearing areas 

that have not yet been fully explored and in the future 

may provide information in areas not previously 

thought to contain coal. Most of these logs are available 

to the public through commercial companies and may 

be studied at State geological surveys or at similar 

agency offices in some States.

Caution should be used in evaluating and interpreting the existence, thickness, and correlation of coal 

beds from the geophysical logs of oil and gas exploratory 

and production wells for two reasons. First, if only one 

type of log is available, other rock types may be misidentified as coal, for example, highly resistive limestone on 

a resistivity log or pure quartz sandstone on a natural 

radioactivity log. However, this problem can be mitigated by a thorough knowledge of the coal-bearing 

strata in the area under investigation and by an under48
ot

/

-y

3V

§/

COAL IN SUBSURFACE 

AND THICKER THAN 14 INCHES

\

^,

FIGURE 18. A valley and ridge with part of coal bed less than minimum thickness but within determined 14-inch limit as illustrated in figure 16. Determined limit comprises the outcrop line from A to B and from BtoD along the arc B through D to intersect projection of line 

AC at G. Radius CD is obtained from distance to nearest point of outcrop line, D, below minimum thickness. Arc BF using radius from 

line BC includes coal on outcrop line below minimum thickness and therefore is not acceptable. Arc AG is drawn from midpoint O' of 

line AG. Arc EDG is drawn from C to extend arc AG.

TABLE 6. Areas of resource categories from figure 17 listed in proper 

reliability and thickness of coal and overburden categories

[Tonnage estimate for each area could be determined (1) by planimetering each area for 

acreage, and (2) by multiplying acreage by proper weight of coal per acre-inch by the 

average thickness of coal in the area. Thickness of coal for each area is obtained by referring 

to coal thickness isopachs]

Geologic 

assurance 

category

Measured 

1 

Inferred 

2

Hypothetical 

Coal thickness

14 to 28 inches 28 to 42 inches

0 to 500 feet overburden:

A f U BE1

/, N E, J, L, M 

O p __ _ p rt _

500 to 1,000 feet overburden: 

C T/ T D 77

W V - - Y

,000 to 2,000 feet overburden: 

Z AA 

DD None

,000 to 3,000 feet overburden: 

BB None 

c/c/ do.

42 to 84 inches

C

K

None

None 

Do.

None 

Do.

None 

Do.

standing of how these strata are recorded on a log. For 

example, if limestone beds are not present in the coalbearing sequence and if all sandstone beds are more 

radioactive than the coal beds, problems of lithologic 

identification are nonexistent. If several log types are 

available, the coal generally can be identified with confidence despite other strata with similar log characteristics in the sequence. Second, some oil and gas logs are 

not suitable for identifying coal beds for reasons that are 

not always clear because they give readings that either 

mask coal beds, indicate coal beds where none are present, or are ambiguous. Here again, a knowledge of the 

stratigraphic positions of coal beds in coal-bearing sequences will aid identificaiton of unsuitable logs. 

Because coal thicknesses interpreted from geophysical 

logs are considered as points of measurement for calculating coal resources, it is advisable to use only those 

coal thicknesses that are determined with confidence.

The geophysical log types generally used in coal bed 

recognition and stratigraphic identification and rank,

49
3 MILES

1 I

3 KILOMETERS

12

18" A

EXPLANATION

50"

POINTS OF COAL 

THICKNESS 

MEASUREMENT, 

IN INCHES

A,B,C AREAS OF RESOURCE 

CATEGORIES

 14--

 28"-|

 42*

 500' -

 1000' -

 2000' -

 3000' -

COAL THICKNESS 

1SOLINES, IN INCHES

OVERBURDEN 

> CONTOURS, 

IN FEET

MEASURED 

INDICATED 

INFERRED 

HYPOTHETICAL

FIGURE 19. Same diagram as figure 18. Shows determination of areas of reliability; 500-foot, 1,000-foot, 2,000-foot, and 3,000-foot 

overburden contours; and 14-inch, 28-inch, and 42-inch bituminous coal isopachs. Overburden contours are determined by subtracting bed elevations from surface topographic map elevations. Bed elevations are determined by utilizing structure sections and dips. 

Coal isopachs are obtained from points of coal thickness measurement and 14-inch limit of coal. Areas of reliability determined by 

using 1/4-mile radius (measured), 3/4-mile radius (indicated), 3-mile radius (inferred), and more-than-3-miles radius (hypothetical) 

from points of coal thickness measurement. Areas of resource categories are identified by letters A through 00.

quality, and thickness evaluations are the electrical, 

gamma ray, density, neutron, and acoustic velocity. 

The following discussion of log types is concerned principally with geophysical logs of oil and gas exploratory 

wells. It should be noted that coal exploration programs 

increasingly have become reliant on coal-oriented geophysical logs, which provide important data on thickness, depth, and correlation of coal beds, and locally on 

the composition of coal. A carefully chosen suite of coaloriented logs can provide positive recognition of coal, 

identification of specific coal beds, and precise coal 

thickness measurements (Vaninetti and Thompson,

1982). Such suites are used currently to supplement information obtained from core holes, driller's logs, and 

examination of drill cuttings. They are useful especially 

where core or sample recovery of coal was incomplete. 

The ease and accuracy of recognizing, identifying, and 

evaluating coal beds with coal-oriented logs strongly 

contrasts with the difficulty of performing the same interpretations using the higher speed and less accurate 

geophysical logs of oil and gas wells, which generally are 

run at different instrument settings and with different 

equipment. Most oil and gas geophysical logs are recorded at a scale of 1 inch equals 50 feet, with selected

50
sections recorded at the larger scale of 1 inch equals 

20 feet, which is reduced for commercial sale to 1 inch 

equals 100 feet and 1 inch equals 40 feet, respectively.

ELECTRIC LOGS

By far the most common geophysical logs run in oil 

and gas exploratory wells are electric logs. Prior to the 

1950's, conventional electrical logging surveys consisted 

of one measurement of the spontaneous potential (SP) 

and three measurements of apparent electrical resistivity 

of the rocks adjacent to the bore hole (fig. 26). These 

rock properties were measured only in the uncased part 

of a well that was filled with water or a water-based 

mud. The diameter of the well and the effect of adjacent 

rocks combined to give confusing curves on older electric logs. In solving this problem, a new family of resistivity curves, the focusing-electrode and the induction 

logs, came into use in the late 1950's. These logs provide 

better resolution of the coal beds than the older conventional logs and permit more accurate coal thickness 

measurements.

SPLOG

The spontaneous potential (SP) log measures the difference in electrical potential between rock types, and 

the resulting curve is recorded on the left-hand side of 

the log as a single trace. This curve generally reflects the 

invasion of drilling fluid into the rocks, so a permeable 

sandstone bed tends to record as a large deflection to the 

left of the log response for shale (figs. 27 and 28). There 

are many exceptions to this generalization as shown by 

the deflections caused by high-porosity coal beds (see 

fig. 29, SP curve). There are also many wells where the 

SP curve is nearly featureless in a coal-bearing section 

and the porosity is recorded the same as shale (fig. 28/4 

and B).

NORMAL AND LATERAL LOGS

Three types of resistivity curves are recorded on the 

right side of a geophysical log. These are the 16-inch 

normal (short normal); the 64-inch normal (long normal); and the 18-foot, 8-inch or 24-foot lateral (lateral); 

the names referring respectively to the spacing and to 

the configuration of the electrodes in the probe. These 

curves record the resistance of rock types to the flow of 

an electric current. Because most coal beds are highly

TABLE 7. Areas of resource categories from figure 19 listed in proper reliability

and thickness of coal and overburden categories 

[Average thickness of coal in each area can be determined by utilizing coal isopachs]

Geologic 

assurance 

category

Coal thickness

14 to 28 inches 28 to 42 inches 42 to 84 inches

0 to 500 feet overburden:

Measured A, F,G, H B, E C, D 

Indicated /, MM /, J, P K

Inferred CC, Y, S AA, Z W

Indicated 

Inferred -

500 to 1,000 feet overburden:

 L, R, M N, Q 

 U, BB V,X 

1,000 to 2,000 feet overburden:

Inferred DD, FF EE None 

Hypothetical GG,JJ None Do.

2,000 to 3,000 feet overburden: 

Inferred LL, II HH 00

Hypothetical NN None None

3,000 to 6,000 feet overburden: 

Hypothetical KK None None_____

resistant to the flow of an electric current compared with 

most adjacent rocks, resistivity curves generally show a 

large deflection opposite a coal bed. In the short and 

long (16-inch and 64-inch, respectively) normal curves, 

however, coal beds thinner than the electrode spacing 

show a "reverse" (low resistivity) curve bounded by two 

small peaks (fig. 2&4 and B, No. 6 coal). The lateral 

curve shows a large deflection opposite thin coal beds 

and a low deflection below the bed. The lateral curve is 

of little value in the measurement of the thickness of coal 

beds because it is asymmetric and generally offset from 

the coal bed. Nevertheless, this curve can be useful in 

correlating coal beds (fig. 28/4 and B).

FOCUSING-ELECTRODE AND INDUCTION LOGS

Focusing-electrode logs (for example, lateral logs) use 

special electrodes to send a narrow focused electric current horizontally into adjacent rocks. This results in a 

resistivity curve that has good resolution of thin resistive 

beds such as coal (figs. 29 and 30). These focusing and 

lateral logs measure the conductivity (inverse of resistivity) and the resistivity of rocks by means of induced 

alternating currents. Commonly, an induction log is 

run in conjunction with a SP and 16-inch normal log 

(fig. 27). The induction log is recorded simultaneously 

as two curves, conductivity and resistivity. The conductivity curve is hyperbolic, which compresses the 

parts of the curve characterized by low conductivity.

51
NOT CALCULATED

= 6000=

14" COAL O 34 OUTCROP 14"

B

x

'*** V'V WV/ I ^>

NOT CALCULATED

2000^

14" COAL O 34 OUTCROP 14"

FIGURE 20. Diagrams showing A, Coal categories in plane of coal bed. B, Structure section OC showing relations between features measured in plane of bed and ground surface when bed dips 30°. C, Coal categories as viewed projected to the ground surface from plane of coal bed. Note contraction of category areas towards the outcrop as compared to plane of coal bed; contraction is due to dip.

52
The resistivity curve, however, is not compressed, so it 

can be compared directly to the short normal curve and 

can be used for measurement of the thickness of a coal 

bed. Combinations of induction and focusing-electrode 

logs are also common.

DISCUSSION OF ELECTRIC LOGS

With the exception of some high-moisture-content 

lignites, most coal beds are responsible for highresistivity deflections on resistivity curves. However, 

some other rock types such as limestone or resistive 

sandstones also show high-resistivity deflections and 

may be mistaken for coal. Limestone is indistinguishable from coal on most electric logs. Fortunately, 

limestone beds are absent in many regions. In the Midcontinent region, however, limestone is abundant in the 

coal-bearing sequence and generally can be differentiated from coal only by use of supplementary logs (see 

fig. 31) or by examination of closely spaced samples of 

drill cuttings.

Resistive sandstone beds that are permeable generally 

can be differentiated from coal beds because of their 

large deflections on a SP log (figs. 26, 27, and 28). 

Where a SP log is featureless or ambiguous, a knowledge of the lithology of a coal-bearirig sequence can help 

differentiate coal from sandstone. For example, in the 

Powder River basin of Montana and Wyoming, many 

sandstone beds are gradational with adjacent low 

resistivity shale beds, whereas the shale beds are in sharp 

contrast with adjacent high-resistivity coal beds. As a 

result, the resistivity curves delineating coal beds are 

more nearly parallel than the curves representing the 

contacts of sandstone beds.

GAMMA RAY LOG

The gamma ray log records the natural gamma radiation from rocks adjacent to a drill hole. Coal generally 

has low natural radioactivity as compared with other 

rocks, particularly shale, in a coal-bearing sequence 

(figs. 29, 30, and 31). In some coal-bearing regions, 

limestone and sandstone may have similar low natural 

radioactivity (for example, Midcontinent or Appalachian regions) so that in those regions supplemental logs 

such as density or acoustic velocity are needed to identify coal. In other regions, gamma ray logs alone are sufficient to identify coal beds as, for example, in the 

Fruitland Formation (Fassett and Hinds, 1971) and in 

the Northern Great Plains where no other rock types in

the Tertiary coal-bearing sequence, including limestone, are known to have as low a natural radioactivity 

as coal. Even in areas where a gamma ray log is diagnostic of coal, a few oil and gas well gamma ray logs are 

useless because their time constant is so long and their 

sensitivity is so low that a coal bed either cannot be detected or its boundaries are obscured. Locally some coal 

beds are uraniferous; consequently, a high radioactivity 

is recorded on gamma ray logs. These uranium-bearing 

coal beds usually can be identified by using other logging methods.

A gamma ray log is the most versatile of the geophysical logs for the following reasons: (1) it does not require 

fluid in the hole; (2) it is not sensitive to small variations 

in hole diameter; and (3) it can be used to detect coal 

beds through well casing. In fact, near-surface gamma 

ray logs of cased oil and gas wells are a prime source of 

data for identifying and measuring the thickness of 

shallow coal beds in the Northern Great Plains region.

The gamma ray logs can detect shale partings in a 

coal bed, but generally the thickness of thin partings is 

exaggerated. In the Powder River basin of Montana, 

there is an example where the gamma ray log records a 

0.3-foot shale parting at the same thickness as a 2-foot 

parting.

DENSITY LOG

A gamma-gamma density log records the bulk density 

of rocks adjacent to a drill hole by measuring the induced gamma rays emitted by the rocks after bombardment by a gamma ray source encased in a probe and 

lowered into the drill hole. The denser the adjacent 

rocks, the more gamma rays are absorbed and not returned to a detector in the probe where they are measured in grams/cm3 . Most ranks of coal are low in density 

(about 0.7 to 1.8 grams/cm3) compared to adjacent 

rocks; therefore, a density log is an excellent tool for 

coal-bed evaluation. The density log is capable of identifying detailed variations in the density of rocks. Density 

logs from oil and gas wells are commonly run at 30 feet 

per minute, are recorded at 1 inch equals 20 feet, and 

then are reduced to 1 inch equals 100 feet. At this scale 

and speed, thin coal beds and partings can be detected 

and their thicknesses accurately measured (fig. 30). Unfortunately, density logs must be run in uncased holes 

and are strongly affected by differences in hole diameter; thus, the curve recorded for a caved or enlarged 

part of a well may simulate the curve of a coal bed. A 

caliper log, which measures the diameter of a well, is 

generally run in conjunction with these logs so that the 

proper interpretation and correction can be made.

53
WILDLIFE

R 5 E R 6 E R 7 E

0

0

r 

5

5

1

10 MILES

10 KILOMETERS

FIGURE 21. Flat-lying bituminous coal bed with data only from underground mined areas and drill holes. Note pattern areas of reliability around each drill hole and paralleling the boundaries of mined areas and the tunnel that connects the mines. Numerous 

thickness of coal measurements along tunnel and mine boundaries are not shown. Coal tonnages and acreages should be reported 

separately for each resource category, as determined by areas of reliability; the 0-200 foot, 200-300 foot, 300-500 foot, and > 500-ft

54
Density logs should be used in conjunction with other 

logs to avoid mistaking spurious low-density readings 

for coal. These logs are generally recorded simultaneously with a gamma ray log, which resolves most uncertainties of rock identification.

NEUTRON LOG

The neutron log, which is similar to the density log, 

records the varying intensity of gamma rays resulting 

from inducing neutrons into rocks adjacent to a drill 

hole by a probe containing a radioactive source. In 

general, the number of gamma rays and neutrons detected as they return to a detector in the probe are inversely proportional to the hydrogen ion content of a 

particular rock type. The curve recorded by the detector 

of returning gamma rays and neutrons can be interpreted to indicate the relative fluid content of the rocks 

and therefore their porosity and permeability. This log 

is commonly called the porosity index. The curve of the 

neutron log records high readings adjacent to permeable 

fluid-filled rocks because of their high hydrogen contents, but it also records high adjacent to a coal bed 

because of its high carbon content (fig. 31). A clay with 

high-moisture content will also record high reading on 

this curve. Therefore, a high-moisture clay adjacent to a 

coal bed can obscure the contact between the clay and 

coal and record a spurious thickness of coal. Other types 

of logs such as the density or gamma ray will allow a 

more accurate measurement of the thickness of coal. 

Neutron logs are also strongly affected by caved or oversized diameter holes; therefore, neutron logs should be 

used in conjunction with caliper logs.

ACOUSTIC VELOCITY LOG

Acoustic velocity logs record the velocity of pulsed 

sonic waves generated in a probe, transmitted into rocks 

surrounding the probe in a drill hole, and reflected to 

receivers in the probe. The results are recorded as the 

inverse of velocity that is, the time in microseconds for 

the waves to travel 1 foot (interval transit time). The 

velocity of the sonic wave depends both upon the 

lithology and the porosity of the rock type being penetrated. If the rock type is known, therefore, the acoustic

velocity log can be used as a measure of porosity. A decrease in velocity (increase in interval transit time) can 

be interpreted to be the result of an increase in porosity. 

Coal generally has a longer interval transit time (lower 

velocity) than adjacent rocks. Because an acoustic velocity log can record velocity changes in great detail, it is 

commonly recorded on an expanded scale (1 inch = 20 

feet) along with a gamma ray and caliper log. However, 

its value as a tool for identifying coal beds is dependent 

on the nature of the coal-bearing sequence. An acoustic 

velocity log is an excellent tool in deeply buried rocks 

such as the Tertiary rocks of southwest Wyoming. 

Acoustic velocity logs can be used to delineate coal beds 

with precision (see fig. 29). They are of lesser value 

where the same rocks are near the surface, are poorly 

consolidated, or are fractured because the many 

spurious low-velocity deflections are recorded. Such logs 

are everywhere useful in distinguishing coal from limestone, which has a much shorter interval transit time.

MEASUREMENT OF THE COAL BED THICKNESS

The measurement of thickness of coal beds on geophysical logs requires the identification of the top and 

base of the coal beds by either of the following methods: 

(1) point of inflection method (the points where curves 

change direction), or (2) midpoints of inflection method 

(arbitrary points located midway between the points of 

inflection). The points of inflection method is used for 

measuring the thickness of coal on gamma ray logs, on 

resistivity logs, and for thin beds on density, neutron, 

and acoustic velocity logs. It is important to understand 

that the thickness of coal measured on the short normal 

and long normal resistivity curves is less than the true 

thickness of the coal bed by the amount of the electrode 

spacing; that is, 16 inches less for the short normal 

curve, and 64 inches less for the long normal curve. The 

midpoints of inflection method is used to identify the top 

and base of thick coal beds on the density, neutron, and 

acoustic velocity logs (see figs. 26, 27 29, and 30 for examples). In general, the SP, long normal resistivity, 

lateral resistivity, and induction curves are not suitable 

for accurate measurement of the thickness of coal beds.

The precision and accuracy of coal bed thickness 

measurements on geophysical logs are dependent on 

several factors, such as the speed of logging, the scale at

overburden categories; counties; areas underlying the lake; State and national parks and the National Wildlife Refuge; and each 

township and range. The 200-, 300-, and 500-foot overburden contour lines and the coal isopachs were constructed to define depth 

of burial and average thickness of coal in each areal category. Explanation for patterns of resource categories is in figure 19.

55
EXPLANATION

JADE COAL BED

DEPTH TO COAL BED 

THICKNESS AT DRILL HOLE

DIRT COAL BED

THICKNESS AT DRILL HOLE 

DEPTH TO COAL BED

B MINE SHAFT 

----- LIMIT OF STRIP MINING

D

D

MINED OUT AREA 

STRIP MINE

Lv^ii SANDSTONb

x 42" THIChNESS OF COAL BED AT

POINT OF MEASUREMENT ON

OUTCROP 

8250 Btu HEAT VALUE AT POINT OF SAMPLING

500'^

1000'> 

2000'J

AND ON HEAT VALUE ISOLINE 

OVERBURDEN ISOLINES

10 hILDMCTI RS

FIGURE 22. A small coal basin containing two coal beds that thin out into sandstone by intertonguing. Area of intertonguing 

sandstone and coal beds trends slightly west of north and is marked by absence of coal beds or by coal too thin to mine. Note 

8,300 Btu isolines trending about east-west. Isolines mark approximate areal boundaries between lignite A and subbituminous C coal as denned by heat value on a moist, mineral-matter-free basis. Coal tonnages should be computed as follows: by demonstrated, inferred, and hypothetical categories; by mined out (underground and strip); by 0-500 feet

56
STRIP MINES ALLUVIAL VALLEY

15 

OUTCROP DRILL HOLE /

MEASURED AND INDICATED RESOURCES

INFERRED RESOURCES

CZD HYPOTHETICAL RESOURCES

EXPLANATION 

MEASURED THICKNESS Of COAL - - - -200- - - -

1000 DEPTH TO TOP OF COAL ZONE 

IN FEET

ANTICLINE

SYNCLINE

ISOLINE OF COAL THICKNESS 

IN FEET. INTERVAL 40 FEET

ISOLINE OF OVERBURDEN, 

IN FEET

COUNTY tME

STATE LIME

BOUNDARY OF AREAS OF 

RESTRICTED RESOURCES

20 30 40 SO MILES

0 10 20 30 40 50 KILOMETERS

FIGURE 23. A large, simple coal basin containing a large quantity of subbituminous coal and lignite. Coal occurs in a zone as thick as 

500 feet. Information on coal zone is derived from two strip mines, 20 exposed localities where coal-bearing rocks can be measured, described, and correlated, and numerous oil and gas wells for which there are drill cuttings, cores, and (or) geophysical logs. Demonstrated, inferred, and hypothetical areas of reliability are based on points of control from the surface exposures and wells. Townships 

and ranges and rank differences are not shown on map because of density of detail. Coal should be categorized and tonnages estimated 

by States; counties; townships and ranges; rank; demonstrated, inferred, and hypothetical areas of reliability; 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) 

categories of overburden; thickness of coal in coal zone; and coal underlying Indian reservations, military reservations, national 

grasslands, national mineral reservations, and State park categories. Coal beneath Indian reservations, towns, lakes, and alluvial 

valleys could be tabulated separately from other coal resources and those that are restricted so labeled. Cumulative total thickness of 

coal at a point of surface measurement or well is the summing of all coal in beds greater than 2.5 feet thick.

which the log is recorded, the type of log, the type of 

equipment, and the instrument settings as well as the 

ability of the user to pick the correct top and base of a 

coal bed.

Density, neutron, gamma ray, and acoustic velocity 

logs in oil and gas wells are recorded at speeds of 30 to 

60 feet per minute and at a scale of 1 inch equals 20 feet. 

Such logs generally permit the measurement of the 

thickness of coal beds within an error of ± 1 foot and 

allow identification of coal beds as thin as about 2 feet. A 

suite of logs run in a coal exploratory drill hole at slow 

speeds of 15 feet or less per minute and recorded at 1

inch equals 10 feet or less, with a standard mineral 

probe and recording instruments set for coal identification, can measure coal beds as thin as ±0.5 foot. The 

use of special equipment and slower logging speeds can 

improve the precision of thickness measurements.

Resistivity logs of oil and gas wells generally are run 

at higher speeds of about 100 feet per minute and are 

recorded at 1 inch equals 50 feet, wkh the result that 

coal beds thinner than about 2 feet cannot be identified 

or measured as to thickness. Some of the focusing electrode logs recorded at 1 inch equals 20 feet may permit 

greater precision. In contrast with oil and gas logs, logs

(0-150 m), 500-1,000 feet (150-300 m), 1,000-2,000 feet (300-600 m), and 2,000-3,000 feet (600-900 m) overburden 

categories; by 30-60 inches (75-160 cm), 60-96 inches (150-240 cm), 96-120 inches (240-300 cm), and 120-240 inches 

(300-600 cm) thickness of coal categories; by rank of coal (lignite A and subbituminous C), and by townships and ranges, 

counties, coal field, basin, and State.

57
Coal reserves or 

resources of Towns hi p, Range , Quadrangle , Field , 

Formation , Member , Rank of Coal , Sulfur Content , Ash Content 

Name 

of 

Bed

Overburden 

Range 

(feet)

Planimeter 

Reading

Area 

(Acres)

Weighted 

Average 

Thickness 

(inches)

Remaining or Original Tonnage (millions 

Measured (inches)

14-28 28-42 42-84 84-168

Indicated (inches)

14-28 28-42 42-84 84-

Column total

Planimeter factor

Short or metric tons

per acre-inch

Planimetered by

Calculated by

Checked by

Total Measured

Total 14-28" Total 28-42"

Total 0-500'

Total 2,000-3,000'

Total Indicated

Total 42-84"

Total 500-1,000'

Total 3,000-6,000

* Categories for hypothetical and speculative coal may be added.

FIGURE 24. Suggested type of form for recording data and computed tonnages for a mine, area, coal bed, quadrangle, township and range, coal 

or political province, and the Nation.
<£>

FIGURE 25. Suggested format for tabulating data and calculating tonnages with headings left to judgment of esti
SP

KH|

MV

RESISTIVITY Cohms m2/m) 

NORMAL 16" NORMAL 64" LATERAL 18'8"

EXPLANATION

Coal

Limestone

Sandstone

Points of inflection or 

midpoints of inflection 

curve

FIGURE 26. Early conventional electric log (1957) of the Owanah Kendrick No. 1 well in Big Horn County, Montana, showing 

response of subbituminous coal beds in the Paleocene Fort Union Formation. Short normal peak at 320 feet (1) looks like a 

coal bed, but is identified as limestone on the acoustic velocity log of a nearby hole. Noteworthy are long normal reversals 

(2) where the resistive bed is thinner than the electrode spacing of 64 inches, and the high lateral resistivity response opposite 

a thin resistive bed (3), with low response just below bed, and at where the long normal curve indicates that the bed is 4 feet 

thinner than does the short normal curve (4) because of the difference in electrode spacing. Note also the large SP response of 

the sandstone beds compared to coal.

60
SP NORMAL 16' 

RESISTIVITY

INDUCTION 

RESISTIVITY CONDUCTIVITY

250m mho/m 0

EXPLANATION 

POINT OF COAL INFLECTION 

USANDSTONE

FIGURE 27. Induction and conductivity geophysical logs (1960) of the Hose-Austin Drilling Company, Bones Brothers No. 1 

well, Rosebud County, Montana, showing response of subbiruminous coal in Paleocene Fort Union Formation.

of the "single point" resistivity type in coal exploration 

drill holes can provide precision similar to those of 

radiation-type logs.

COMPOSITION AND RANK OF COAL

Geophysical logs can also provide information on the 

composition and rank of coal. High-ash or shaly zones 

(partings) in coal generally are recorded on logs by

curves indicating that they are more radioactive, more 

dense, and less resistive than purer coal. High-rank coal 

is more dense and has a shorter interval travel time in 

sonic logs than low-rank coal. Efforts have been made to 

quantify compositional factors of coal, such as moisture 

and ash content, using geophysical logs. Bond and 

others (1971) report that in the Illinois basin an experimental program using computer processed data from 

logs of coal exploratory drill holes produced excellent 

determinations of moisture and ash in coal. However,

61
FIGURE 28. Examples of oil- and gas-well geophysical logs from western Kentucky that can be used for coal bed correlations and coal resource evaluations.

attempts to quantify coal composition using geophysical 

logs from wells in the Northern Great Plains have had 

erratic results.

STRATIGRAPHY AND STRUCTURE

Geophysical logs that have been properly related to a 

known stratigraphic section can be used to correlate coal 

beds and determine their structure. In many areas a coal 

bed and adjacent rocks are recorded on geophysical logs 

as a unique curve, or sets of curves, that can be recognized on logs of drill holes throughout a large area (see 

fig. 28, No. 9 coal bed). Recognition of "signature" 

curves permits correlation of stratigraphic units across 

any area where the lithology remains reasonably constant and in some places allows correlation throughout a 

coal basin. Properly correlated logs can provide the necessary data to construct coal bed maps that show structure, thickness of coal, and thickness of overburden.

Geophysical logs are also useful in studying the ancestoral sedimentary environments of coal beds. In the Appalachian coal region, Wanless and others (1963) used 

electric logs of oil and gas wells augmented by drill-hole

and outcrop data to identify and map sedimentary environments of some coal-bearing rocks of Pennsylvanian 

age. Daniels and Scott (1980; 1982) in Kentucky and in 

Montana used geophysical logs of coal drill holes to 

refine their interpretations of drillers' lithologic logs in a 

study of ancestoral sedimentary swamp environments of 

Pennsylvanian and Tertiary rocks.

SUMMARY

Geophysical logs of oil and gas wells are the source of 

abundant data on coal beds, but the value of any of 

these logs in any particular coal area or basin depends in 

large part on a knowledge of the coal-bearing rocks and 

of how the data are recorded on the curves of the geophysical logs. Consequently, it is advisable for those 

planning to use geophysical logs for coal interpretation 

to study the logs of drill holes in an area where the 

lithology is known to determine the reaction of logging 

equipment to various rock types. Use of a suite of 

several types of logs is recommended for the best results.

It is important to recognize and discard poor quality 

or ambiguous logs so that erroneous data is not used in 

calculating coal resources.

62
GAMMA RAY ACOUSTIC 

VELOCITY

O tij CACOUSTLOG")

NORMAL INDUCTION

RESISTIVITY 16" CONDUCTIVITY

10 100 10 100

70 20° ohmsm2/m 1000 100 ohms m2

EXPLANATION 

* Points of inflection or midpoints of inflection curve Coal

FIGURE 29. SP, gamma ray, acoustic velocity (sonic), normal resistivity, and induction conductivity logs (1971) from the Davis Oil Grady 

Fed No. 1-2 well, Sweetwater County, Wyoming, showing response of subbituminous(?) coal beds in the Paleocene Fort Union 

Formation.

INTENDED AUDIENCE

This circular was prepared for two audiences: (1) coal 

resource specialists, coal geologists, and coal-mining 

engineers; and (2) the public, Congress, and many State 

and Federal agencies who desire more than a casual 

knowledge about the need for and procedures of coal resource classification.

The definitions, criteria, guidelines, and illustrations 

are designed to aid coal geologists and engineers in preparing their reports by providing guidance in calculating, categorizing, and describing the coal resources/ 

reserve base/reserves they are evaluating. The description of geophysical log usage also should provide aid in 

obtaining and using subsurface information on the area! 

extent, thickness, and correlation of coal beds and

should open up a vast and generally untapped data 

source for most coal geologists. Adherence to the categories of resource assignment, to the definitions, and to 

the criteria can result in more accurate, precise, explicit, 

and consistent reports.

The classification system and terminology described 

in this circular are equally applicable to hand and computer system determinations of resources and reserves. 

The illustrations are applicable to manual methods of 

coal resource/reserve base/reserve tonnage estimations. 

Computer programs have been designed that will yield 

similar tonnage estimates of coal resources. Those wishing to inquire about the computer software and methodologies should write to Chief, Branch of Coal Resources, 

956 National Center, U.S. Geological Survey, Reston, 

Virginia 22092.

63
SP GAMMA RAY DENSITY

COMPENSATED

NEUTRON

LATEROLOG POROSITY 

3.0 8 (SANDSTONE)

EXPLANATION

COAL SANDSTONE LIMESTONE POINT OF INFLECTION OR 

MIDPOINT OF INFLECTION 

CURVE

FIGURE 30. SP, gamma ray, density, dual induction-lateral log, and neutron logs (1978) from Getty Tri-County No. 1 well, Rosebud 

County, Montana, showing response of subbituminous coal in the Paleocene Fort Union Formation.

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64
LATEROLOG 

mmhos/m

GAMMA RAY 

API Units

DENSITY 

grams/cc

.0 3.0

SONIC 

JJsec/ft

150 50

NEUTRON LITHOLOGIC LOG

Limestone 0 %

FIGURE 31. Five geophysical logs of a coal exploration drill hole in the Illinois basin (Midcontinent region) showing response of bituminous coal and limestone of Pennsylvanian age on gamma ray, density, neutron, acoustic velocity (sonic), and lateral logs. Modified 

from Bond, Alger, and Schmidt (1971).

Wanless, H. R., Tubb, J. B., Jr., Gednetz, D. E., and Weiner, Wood, G. H., Jr., Culbertson, W. C., Kehn, T. M., and Carter,

J. L., 1963, Mapping sedimentary environments of Pennsylvanian cycles: The Geological Society of America Bulletin, v. 74, 

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264.

M. D., 1982, Coal resources classification system of the United 

States Geological Survey, 1982, in Gurgel, K. D., editor, Proceedings, Fifth Symposium on die geology of Rocky Mountain 

coal 1982: Utah Geological and Mineral Survey, Bulletin 118, 

p. 233-238.

65