U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2011–1148

A Critical Review of Published Coal Quality 

Data From the Southwestern Part of the 

Powder River Basin, Wyoming
Cover image. Shot coal from the highwall face at a mine in the Powder River Basin, Wyoming 

(photograph by Lames A. Luppens, U.S. Geological Survey).
A Critical Review of Published Coal Quality 

Data From the Southwestern Part of the 

Powder River Basin, Wyoming

By James A. Luppens

Open-File Report 2011–1148

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Luppens, J.A., 2011, A critical review of published coal quality data from the southwestern part of the Powder River 

Basin, Wyoming: U.S. Geological Survey Open-File Report 2011–1148, 23 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

In-Place Coal Quality.....................................................................................................................................1

Fort Union Formation Coal Quality .....................................................................................................4

Estimation of the Quality of Deep Fort Union Formation Resources ..........................................10

Wasatch Formation Coal Quality......................................................................................................14

Conclusions...................................................................................................................................................21

References Cited..........................................................................................................................................21

Figures

1. Location of Southwestern Wyoming Powder River Basin, Northern 

Wyoming Powder River Basin, and Gillette coal field, Wyoming.........................................2

2. Location of coal fields in the Southwestern Powder River Basin........................................3

3. Variation in moist, mineral-matter-free British thermal units in the 

Wyodak-Anderson coal zone in the Powder River Basin, Wyoming, 

showing a trend towards increasing apparent rank of the coal from 

subbituminous C at shallow depths to subbituminous B, and finally 

subbituminous A in the deepest part of the basin ..................................................................9

4. Estimated variability of as-received gross calorific values versus depth 

from table 3 data for the Wyodak coal beds in the Powder River Basin, Wyoming........13

5. Coal bed and coal zone names used in different publications in the 

Powder River Basin, including current SWPRB coal assessment area ...........................15

 6. Estimated variability of as-received and moist, mineral-matter-free 

gross calorific values versus depth from table 4 for the Wyodak coal 

beds in the Powder River Basin, Wyoming ............................................................................17

7. Core log for core hole USGS-1 in the Lake De Smet coal field, Wyoming, 

illustrating core intervals analyzed, shale intervals excluded from 

the analyses, and lost-recovery intervals ..............................................................................20

Tables

1. Available as-received coal quality data from the Fort Union 

formation in the SWPRB assessment area ..............................................................................5

2. A comparison of reported coal quality data in the Sussex 

coal field, Wyoming, from successive publications ...............................................................8

3. Properties and composition of coal beds cored in the 

Powder River Basin, Wyoming.................................................................................................11

4. Edited version of table 3 in which core holes with anomalous data were deleted.........16

5. Available as-received coal quality data from the Wasatch Formation 

in the Lake De Smet area and other Watsatch coal beds ...................................................18

6. Estimated impacts of compositing the originally excluded shale, pyrite, 

and other high-ash parting material with the coal analyses for the 

USGS-1 core hole from the Lake De Smet coal field, Wyoming .........................................21

A Critical Review of Published Coal Quality Data From the 

Southwestern Part of the Powder River Basin, Wyoming

By James A. Luppens

Abstract

A review of publicly available coal quality data during the 

coal resource assessment of the southwestern part of the Powder 

River Basin, Wyoming (SWPRB), revealed significant problems and limitations with those data. Subsequent citations of 

data from original sources often omitted important information, 

such as moisture integrity and information needed to evaluate 

the issue of representativeness. Occasionally, only selected data 

were quoted, and some data were misquoted. Therefore, it was 

important to try to resolve issues concerning both the accuracy 

and representativeness of each available dataset. The review 

processes demonstrated why it is always preferable to research 

and evaluate the circumstances regarding the sampling and 

analytical methodology from the original data sources when 

evaluating coal quality information, particularly if only limited 

data are available. 

Use of the available published data at face value would 

have significantly overestimated the coal quality for all the coal 

fields from both the Fort Union and Wasatch Formations in the 

SWPRB assessment area. However, by using the sampling and 

analytical information from the original reports, it was possible 

to make reasonable adjustments to reported data to derive more 

realistic estimates of coal quality. 

Introduction

A review of publicly available coal quality data during the 

2010 coal resource assessment of the southwestern part of the 

Powder River Basin, Wyo. (SWPRB; fig. 1), revealed problems 

and limitations with those data. Because a detailed discusion of 

the coal quality data issues exceeded the scope of the assessment, a more thorough discussion of the coal quality data 

review is presented separately in this report.

Coal quality is one of the major factors in the marketability 

of coal and is an important parameter in economic evaluations. 

Coal quality directly impacts the cost of generating electric 

power. Quality parameters such as increased ash content, which 

lowers the gross calorific value (British thermal units per pound 

[Btu/lb]), increase the operating and maintenance costs at coalfired powerplants. Coals with lower ash content, and therefore 

higher gross calorific values, command a premium selling price.

Furthermore, the Clean Air Act Amendments (CAAA) 

have enforced emission limits to reduce certain air pollutants such as sulfur dioxide. The sulfur dioxide emissions 

limits for all new powerplants built after 1976 were capped 

at 1.2 pounds of sulfur dioxide per million Btu (U.S. Environmental Protection Agency, 1980). Any coal that could be 

burned and meet the sulfur dioxide emission standards for 

air quality (emitting less than 0.6 pound of sulfur per million 

Btu or 1.2 pounds of sulfur dioxide per million Btu) without 

the need for flue-gas desulfurization was designated “compliance coal,” also known as low-sulfur coal (U.S. Energy 

Information Administration, 2010). Subsequent phases of the 

CAAA have further restricted sulfur dioxide emissions. Currently, New Source Performance Standards (NSPS) establish 

uniform national U.S. Environmental Protection Agency air 

emission standards that limit the amount of pollution allowed 

from new sources or from modified existing sources. Under 

NSPS, Best Available Control Technology (BACT) emission 

limitations are based on the maximum degree of reduction of 

each pollutant subject to regulation under the Clean Air Act 

(U.S. Environmental Protection Agency, 2006). The minimum 

BACT standard for sulfur dioxide emissions is 90 percent 

regardless of the sulfur content. The abundance of low-ash and 

low-sulfur coal in the Powder River Basin (PRB) has been a 

significant factor in the steadily growing demand for coal from 

this region.

In-Place Coal Quality

There are currently no active mines in the SWPRB 

assessment area, but historically, very limited production from 

coal beds in both the Fort Union and Wasatch Formations has 

occurred. Lacking the relatively thick, shallow coal beds found 

in the Gillette coal field (fig. 1) on the eastern flank of the 

PRB, mining has not been widespread. As a result, publicly 

available coal quality information from coal exploration cores 

and mines is sparse, with the notable exception of the now 

abandoned Dave Johnston Mine near Glenrock, Wyo. 

(figs. 1 and 2). Limited coal quality data from deeper coal beds 

have been generated from a few wells during recent coal bed 

methane development (Stricker and others, 2007).
2 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

106 o

104 o

MONTANA

WYOMING

MONTANA

WYOMING

Fallon

Carter

Custer

Powder River

Rosebud

Sheridan

Treasure

Musselshell

Big Horn

Big Horn

Washakie Johnson

Natrona Freemont

Hot Springs

Campbell

Converse

Platte

Niobrara

Weston

Crook

0

0 20 40 MILES

20 40 60 KILOMETERS Glenrock

Sheridan

Buffalo

Gillette

Decker

Birney

Miles City

Douglas

Moorcroft

Casper

INDEX MAP

WYOMING

MONTANA

Powder River Basin

90

90

94

25

43 o

44 o

46 o

45 o

o

108

Tertiary limit of the Powder

 River Basin

Gillette coal field boundary

County line

Main railroad line

Interstate highway

City or town

25

Northern Wyoming Powder River

Basin assessment area

Southwestern Powder River

Basin assessment area

EXPLANATION

Figure 1. Location of Southwestern Wyoming Powder River Basin, Northern Wyoming Powder River Basin, 

and Gillette coal field, Wyoming.
In-Place Coal Quality 3

R. 837W. R. 85 W. R. 83 W. R. 81 W. R. 79 W. R. 77 W. R. 75 W. R. 73 W. R. 71 W. R. 69 W. R. 67 W. R. 65 W.

MILES

KILOMETERS

0

0

5 10 20

5 10 20

Lake De Smet

Buffalo

Sheridan

Buffalo-Lake De Smet area (Mapel, 1959)

Weston

Big 

Horn Washakie

Campbell

Sheridan

Crook

Johnson

Converse

Natrona

Niobrara

T. 42 N.

T. 40 N.

T. 34 N.

T. 36 N.

T. 38 N.

T. 32 N.

T. 44 N.

T. 46 N.

T. 48 N.

T. 50 N.

T. 52 N.

T. 54 N.

T. 56 N.

T. 58 N.

o

43 00’

o

43 30’

o

44 00’

o

44 30’

o

45 00’

o 107 00’ o 106 00’ o 105 00’ 

BUFFALO

COAL FIELD

BARBER

COAL FIELD

POWDER RIVER

COAL FIELD

SUSSEX 

COAL FIELD

GLENROCK

COAL FIELD

DRY CHEYENNE

COAL FIELD

PUMPKIN

 BUTTES

COAL FIELD

DAVE JOHNSTON MINE

 (abandoned)

BELCO LEASE AREA

STEVENS NORTH

STEVENS SOUTH

LOST SPRING

COAL FIELD

EXPLANATION

County Line

Gillette Coal Field

Buffalo-Lake De Smet area Dave Johnston mine (abandoned)

Stevens North (approximate location)

Stevens South (approximate location)

Southwestern Powder River Basin boundary Belco Lease Area (approximate location)

Northern Wyoming Powder River Basin boundary

INDEX MAP

WYOMING

Figure 2. Location of coal fields in the Southwestern Powder River Basin (modified from Glass, 1976).
4 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

Assessing the coal quality variations is usually straightforward for regions like the Gillette coal field, where mining 

has been extensive and coal analytical data are abundant. 

However, when dealing with relatively few coal analyses, as 

is the case of the SWPRB, it is especially important to try to 

assess both the accuracy and the representativeness of each 

available dataset. During the review of published coal quality 

data for the SWPRB, two recurring problems were apparent. These problems were either anomalously low moisture 

values, representativeness issues, or both. The anomalous 

moisture problems are due to unexplained moisture loss during sampling, shipment, or processing the samples or from 

the sampling of partially dried coal beds. The issue of representativeness is related to partial coal bed samples, whole bed 

core samples with significant core loss, or deliberate exclusion 

of high-ash partings. Because coal quality differs between 

the Fort Union and Wasatch Formations, they are discussed 

separately.

Fort Union Formation Coal Quality

The available in-place coal quality data for the Fort 

Union Formation coal beds in the SWPRB assessment area are 

summarized in table 1. Essentially all of the shallow coal bed 

data in the SWPRB are from the Dry Cheyenne and Glenrock 

coal fields in the south and the Sussex coal field in the west 

(fig. 2). An initial review of the data from table 1 reveals a 

relatively wide range in coal quality, especially for moisture 

contents (18.8 to 28.8 percent). The higher gross calorific 

values (7,581 to 9,160 Btu/lb) are directly dependent on what 

appears to be an unusually wide range in as-received moisture values for a single coal field. Typically, variations in the 

degree of coalification within individual coal fields or adjacent 

fields are not significant. Luppens (1988) demonstrated that 

a linear, inverse relationship exists between moisture and ash 

values in low-rank coals (lignite and subbituminous C ranks) 

for a single coal field. Any moisture values with similar ash 

contents that fell more than 1.5 to 2 percent from a bestfit trendline through an ash versus moisture data crossplot 

were reanalyzed. A similar linear, inverse relationship exists 

between gross calorific and ash values in low-rank coals for 

a single coal field (Hoeft and others, 1983). Because of these 

linear relationships, coal analyses with similar ash values will 

also have similar moisture and gross calorific values.

As moisture or ash contents, or both, decrease, gross 

calorific values increase and vice versa. As a general rule 

of thumb for low-rank coals, a 1-percent change in ash or 

moisture value is equivalent to a change in the gross calorific value of approximately100 to 120 Btu/lb (J.A. Luppens, 

unpub. data, 2011). For example, a 5-percent decrease in 

moisture would result in an increase in the gross calorific 

value of about 500 Btu/lb. A 5-percent decrease in moisture (+500 Btu/lb) coupled with an increased ash content of 

2 percent (−200 Btu/lb) would result in a net increase in the 

gross calorific value of about 300 Btu/lb. Knowledge of these 

coal quality data relationships is particularly useful in identifying potentially anomalous data (Hoeft and others, 1983).

Three sets of published coal quality data from the Dry 

Cheyenne and Glenrock coal fields (fig. 2) were found. The 

analyses from the Stevens North and South areas (table 1) 

were performed on coal cores (Western Fuels Association, 

Inc., 1983). The sample types from the Roland and Smith coal 

beds from the Dave Johnston Mine (table 1) were either face 

channel or coal core samples (Glass, 1975).

The analytical data from the Dry Cheyenne and Glenrock coal fields should be very similar because of the relative 

proximity of the samples (fig. 2). Therefore, samples with 

similar ash contents should also have similar moisture and 

gross calorific values. The published average moisture content 

of 26.6 percent for the produced coal from the Dave Johnston 

mine (Keystone Coal Industry Manual, 1997), owned by 

Pacific Power and Light Company (PPL), is the most reliable 

estimate of the moisture content (at 12.1 percent ash) in the 

Glenrock and Dry Cheyenne coal fields (table 1). However, 

5 of the 10 moisture values from those coal fields, which range 

from 19.5 to 23.5 percent (table 1), appear to be anomalously 

low.

In 1974, an extensive face channeling program was initiated by the Geological Survey of Wyoming to collect coal 

samples to characterize the coal quality in 4 of the 10 major 

coal-bearing regions in the state (Glass, 1975). The extensive 

suite of coal analyses performed on these samples included 

proximate and ultimate analyses, plus tests for gross calorific 

values, sulfur forms, major and minor oxides, ash fusion 

temperatures, Hardgrove grindability, and trace elements. 

As part of that sampling program, Glass (1975) collected 

two face channel samples (74-36 and 74-37, table 1) totaling 

37 and 38 feet in thickness, respectively, from the Smith coal 

bed (originally correlated as the School coal bed) at the Dave 

Johnston mine. The “typical” core analyses for the Smith bed 

supplied by PPL also were presented by Glass (1975) to compare the results of the face channeling results to the expected 

quality at the mine.

Sampling the vertical highwalls of thick beds in the PRB 

posed significant logistical and safety challenges from trying 

to cut a uniform channel through the thick coal beds and collecting the coal by use of multiple 30-foot-long extension ladders. Typically, highwall faces in PRB mines will be exposed 

for weeks to a month or more before the coal is actually 

recovered. As soon as the face is exposed, weathering effects 

begin, initially as moisture loss. This moisture loss results in 

blocky fracturing due to coal shrinkage, which can extend into 

the exposed coal faces for up to several feet. Prolonged exposure can result in a decrease in the gross calorific values due to 

oxidation. Normally, the width and depth of the face channels 

were gauged to yield a minimum of 6 pounds of coal per vertical foot of channel (Glass, 1975). A 3.5-inch by 3.5-inch channel will yield just over 6 pounds per channel foot. Therefore, 

the relatively shallow depth of the face channels would most 

certainly mean that sampling was limited to the zone subjected 

to partial drying.
Table 1. Available as-received coal quality data from the Fort Union formation in the SWPRB assessment area. Average values are shown in bold ty

Area and coal bed name No. of 

analyses

Moisture 

(percent)

Sulfur 

(percent)

Ash 

(percent)

Gross calorific 

value (Btu/lb)

Pounds of SO2 per 

million Btu

G

va

Dry Cheyenne coal field–Stevens North area3

Roland (Baker) coal bed 5 28.7 0.51 7.7 7,831 1.30 Smith coal bed 2 26.1 0.47 7.5 8,247 1.14 

Glenrock coal field–Stevens South area3

Roland (Baker) coal bed 1 22.2 0.42 14.7 7,860 1.07 Smith coal bed 1 23.5 0.38 4.4 9,010 0.84 

Glenrock coal field–Dave Johnston Mine

Roland (Baker) coal bed4 5 27.4 0.45 7.9 7,950 1.13 Smith coal bed face channel sample 74-364 1 20.7 0.5 15.7 7,850Smith coal bed face channel sample 74-374 1 19.5 0.7 8.8 8,870“Typical” Smith coal bed core sample 

analyses from Dave Johnston Mine4

1 26.4 0.5 9.7 7,830

Smith coal bed5,7 (average of 3 analyses above 

from Glass, 1975)

3 22.2 0.6 11.4 8,180 1.47 

Dave Johnston Mine (produced averages)7 – 26.6 0.45 12.1 7,581 1.19 

Sussex coal field4,5

Reported value1,4 1 23.5 0.49 5.2 9,160 1.07 Basin 21,6 1 28.8 0.56 6.7 7,930 1.41 Basin 31,6 1 23.6 0.68 7.8 8,450 1.61 Basin 41,6 1 18.8 0.57 7.8 9,160 1.24 Basin 51,6 1 23.5 0.49 5.2 9,050 1.08 Basin 61,6 1 28.1 0.45 4.6 8,350 1.08 Average of 5 basin analyses 24.6 0.55 6.4 8,588 1.28 Estimated average of 5 basin analyses (adjusted to 30 percent moisture)1 30.0 0.51 6.0 7,969 1.28 

Deep basin coal bed

Smith (Big George) coal bed2,8 – 22.6 0.25 3.5 9,750 0.51 

1Partial bed sample.2Unweighted average of 30 incremental samples from a 200-foot-thick cored portion of the bed below 1,000 feet.3Western Fuels Association, Inc. (1983).4Glass (1975).5Glass and Jones (1992).6Wegemann (1912).7Keystone Coal Industry Manual (1997).8Keystone Coal Industry Manual (2010).9Moist, mineral-matter-free (m,mmf) basis.10Subbituminous C (subC), subbituminous B (subB).
6 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

The coal from both face channel samples was described 

as having “blocky” or “cubical fractures,” which is consistent 

with the blocky fractures caused by shrinkage due to moisture 

loss. Additionally, the condition of sample 74-36 was listed as 

“weathered” (Glass, 1975), which corroborates the assumption that the as-received moisture content of sample 74-36 was 

not representative of the true in-place moisture. Although the 

condition of sample 74-37 was listed as “fresh,” its moisture content was over 1 percent lower than that for sample 

74-36, which confirms that moisture integrity of both channel samples was compromised by significant moisture loss. 

In fact, Glass (1975, p. 164) states that “the routine analysis 

of Sample No. 74-37 compares favorably with the company 

[PPL] analyses except in as-received moisture and heat values 

[gross calorific values]. Like Sample No. 74-36, the moisture 

is lower than the company analyses [19.5 versus 26.4 percent] 

while the heat content is over 1,000 Btu/lb higher [8,870 versus 7,830 Btu/lb]” (table 1).

Using the simple rule of thumb of 100 to 120 Btu/lb 

change per for each 1-percent change in moisture or ash 

contents, the approximately 7-percent difference in moisture 

would equate to a difference in the gross calorific values of 

about 700 to 840 Btu/lb. There was also about a 1-percent difference in ash (table 1), which would account for an additional 

100 to 120 Btu/lb. The combined estimated difference in moisture and ash contents in terms of impacts to the gross calorific 

value is 800 to 960 Btu/lb, which closely approximates the 

“over 1,000 Btu/lb difference” noted by Glass (1975).

Glass and Jones (1992) characterized the coal quality of 

the Smith bed in the Glenrock coal field using only the two 

face channel analyses, 74-36 and 74-37, and the core hole 

analyses provided by PPL (table 1; Glass, 1975), even though 

it was acknowledged that the face channel samples were 

described as weathered and had anomalously low moisture 

contents. Additionally, the coal quality summary was simply 

an average of those three samples, with data ranges, without 

any critical information concerning the sample type, condition 

of the samples, limitations of the data, or source of the data. 

Only the author’s personal knowledge of the 1974 channel 

sampling program allowed tracing of the average values in 

Glass and Jones (1992) back to Glass (1975). The use of data 

that are not representative of the coal to characterize coal 

quality of a given area is disappointing from a professional 

standpoint because of the ripple effect. The three-sample summary, with little documentation from Glass and Jones (1992), 

was cited in subsequent publications (Keystone Coal Industry 

Manual, 1997, 2005), thus perpetuating the impression that the 

coal quality in the Glenrock coal field is better than it actually 

is. The inclusion of samples with anomalously low moisture 

increased the average gross calorific value.

The moisture values for the two cores from the Stevens 

South area of the Glenrock coal field (table 1) also look 

anomalously low. Because this area lies between the Stevens 

North area and the Dave Johnston Mine, the moisture contents 

should be similar. It is assumed that unaccounted-for moisture loss during the coring or core handling occurred, but no 

documentation was available. Additionally, the ash content of 

4.4 percent for the Smith bed analysis (table 1) in the Stevens 

South area also appears to be especially low, especially when 

compared to other Wyoming PRB coal quality. The Gillette 

coal field, which produces coal with some of the lowest ash 

content in the Nation, has as-received ash contents from the 

south to the north areas that range from 5.2 to 6.4 percent, 

respectively. Ranges for other as-received coal quality parameters from the south to the north areas in the Gillette coal field 

were moisture, 26.7 to 30.0 percent; and gross calorific values, 

8,200 to 8,810 Btu/lb.

Given the documented problems with the use of samples with anomalously low moisture contents from Glass 

(1975) and Glass and Jones (1992), more realistic ranges in 

coal quality from these two coal fields are estimated to be 

moisture, 26 to 29 percent; ash, 7.5 to 12 percent; sulfur, 

0.4 to 0.6 percent; and gross calorific values, 7,500 to 8,000 

Btu/lb. The range in moisture is more consistent with that 

for the Gillette coal field. The gross calorific values are also 

consistent with those for the Gillette area, given that the 

average ash content in the Glenrock and Dry Cheyenne coal 

fields is significantly higher than in the Gillette coal field. It is 

interesting to note that one of the reasons for closing the Dave 

Johnston Mine in 2000 was deteriorating coal quality due to 

higher ash (PacifiCorp, 2008).

The author encountered a similar situation while evaluating purchase of a coal property in the Gillette coal field 

near Gillette, Wyo. (J.A. Luppens, unpub data, 1998). The 

seller provided coal quality data from cores drilled in the 

1960s, with moisture values ranging from 18 to 34 percent 

and averaging about 28 percent. However, moisture values 

from adjacent mines ranged from 30 to 32 percent. A coring 

program was initiated to resample over 20 original core sites 

with suspect analyses. The result of the recoring program 

was a revised average moisture content of about 31.5 percent, 

consistent with that from the neighboring mines. Subsequent 

interviews with laboratory personnel who conducted the original analyses confirmed suspicions regarding the conditions of 

the core samples as received at the laboratory. Many samples 

had been stored for weeks before shipment to the laboratory in 

unlined cardboard boxes without plastic bags or other means 

of preventing moisture loss. Problems caused by unaccountable moisture loss are more common than often realized, 

especially with coal samples collected years ago when neither 

materials to prevent moisture loss nor expedited shipping 

options were available.

The problems with the published coal quality data from 

the Sussex coal field (fig. 2) stem not only from moisture 

issues similar to those for the Dry Cheyenne and Glenrock 

coal fields but also from representativeness issues. Additionally, only select data were reported in more recent publications, and citation errors were introduced.

Table 2 illustrates the progression of reported coal quality data over the past nearly 100 years since the original data 

were reported. The original assessment of the Sussex coal 

field reported a total of six coal sample analyses (Wegemann, 
In-Place Coal Quality 7

1912). Smith and others (1972) reported three of the original 

six analyses. More recently, only a single coal analysis for 

the Sussex coal field was reported by Glass and Jones (1992; 

tables 1 and 2), and it does not exactly match any of the analyses from the original data (table 2). Evidently, the moisture, 

ash, volatile, fixed carbon, and sulfur contents from sample 

10827 were combined with the gross calorific value from 

sample 10804. It is unknown whether this reporting was due 

to intentional compositing or was a result of a typographical 

error. Nevertheless, it demonstrates the value in reviewing the 

original reports rather than relying on subsequent interpretations. The quotation of the single erroneous analysis from 

Glass and Jones (1992) by the Keystone Coal Industry Manual 

(2005) demonstrates how misleading and incorrect citations 

can easily be perpetuated.

Based on this single reported proximate analysis (Glass 

and Jones, 1992), the coal quality of the Sussex field appears 

to be relatively high for the PRB. However, after one reviews 

the original report on the Sussex coal field (Wegemann, 

1912), a different perspective is gained. A total of five analyses from various “basins” within the Sussex coal field with 

at least as-received moisture, ash, sulfur, and gross calorific 

values was given in the original report (table 1). The analytical results show a fairly wide range in moisture (and consequently gross calorific values), whereas ash contents are all 

fairly low. The lower moisture analyses (less than 28 percent 

moisture) likely reflect unaccountable moisture loss. This 

conclusion is supported by the nature of the samples. All of 

the analyses were performed on samples collected at small, 

shallow, wagon mines along the coal bed outcrops. The author 

even cautioned “this form [as-received] is not well suited for 

comparison, because the amount of moisture as it comes from 

the mine is largely a matter of accident, and consequently 

analyses of the same coal in this form may vary widely” 

(Wegemann, 1912, p. 468).

The averages of the five individual basin analyses from 

the Sussex coal field are also presented in table 1. Although 

the average gross calorific value of 8,588 Btu/lb is significantly lower than the 9,160 Btu/lb reported by Glass and Jones 

(1992), the average moisture values still appear to be anomalously low. Assuming that all samples encountered at least 

some unaccounted-for moisture loss is reasonable, given the 

nature of the samples as well as the logistics and time required 

to ship the samples from the field to the laboratory in 1910; an 

average moisture content of 30 percent would be more realistic. The 30 percent moisture estimate is only about 1 percent 

higher than the moisture content for the Basin 2 sample. With 

an as-received moisture level of 30 percent, the 9,160 Btu/lb 

reported by Glass and Jones (1992) would be reduced to 

8,380 Btu/lb. More realistic estimated average values of the 

basin samples for all parameters adjusted to an as-received 

moisture level of 30 percent (ASTM International, 2008c) 

are also shown in table 1.

The issue of anomalously low moisture data was not 

the major problem encountered in assessing the coal quality 

of the Sussex coal field. It is important to recognize that the 

analytical data for the Sussex coal field presented in table 1 

should not be considered representative of average values 

for the whole field. All of the samples are partial coal bed 

samples. For example, the Basin 6 sample (Laboratory No. 

11048) came from a 5-foot-thick bench of lower-ash coal in 

the middle of an approximately 13-foot-thick coal bed which 

has several high-ash layers (up to 1 foot thick) in the upper 

and lower benches of the coal bed. Similarly, the other four 

samples are from lower-ash coal bed benches, the higher-ash 

benches being avoided at each sampling location. In fact, the 

presence of higher-ash coal layers and mineral partings within 

the coal measures described by Wegemann (1912) are the rule 

rather than the exception. Statements such as “too broken by 

shale partings to be minable” are common. The presence of 

partings and higher-ash layers is consistent with the coal bed 

geology in the southwestern part of the PRB. On the western 

flank, the thick coal beds in the eastern and deep central parts 

of the basin thin, split, and pinch out (Flores and others, 1999). 

Unlike problems associated with anomalously low moisture 

contents, which can be straightforwardly adjusted to more 

realistic levels, the analytical results on coal samples that are 

not representative of the entire coal bed are especially difficult 

to assess.

The high frequency and discontinuous nature of these 

high-ash layers mean that the typical quality of coals in 

the Sussex coal field is expected to be even lower than the 

average values adjusted to 30 percent moisture shown in 

table 1 (higher ash contents and lower gross calorific values). It is impossible to reasonably predict the impacts of 

inclusion of higher-ash benches on the average coal quality. 

The same is true for sulfur content predictions. However, it 

is not unreasonable to expect that the average ash content 

of 6.0 percent (at the estimated 30 percent moisture basis) 

would increase to at least 10 percent given the abundance of 

high-ash layers in the Sussex coal field. At 10 percent ash, 

the calorific value would decrease to about 7,550 Btu/lb. 

This calorific value adjustment assumes a decrease of about 

100 Btu/lb per 1-percent ash increase. An estimated coal 

quality of 30 percent moisture, 10 percent ash, and a calorific 

value of 7,550 Btu/lb is markedly dissimilar from the reported 

analysis by Glass and Jones (1992; tables 1 and 2).

Excluding the samples with anomalously low moisture 

contents (table 1), the near-surface Fort Union Formation coal 

beds have an apparent rank of subbituminous C (subC; ASTM 

International, 2008a). However, the apparent rank increases 

with depth as evidenced by the deep Smith (Big George) bed 

analysis in table 1. The apparent rank reaches subbituminous 

A (subA) in the deepest part of the basin (fig. 3; Stricker and 

others, 2007).
8 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

Table 2. A comparison of reported coal quality data in the Sussex coal field, Wyoming, from successive publications. Analyses not 

reported are indicated by NR.

Basin – coal bed Laboratory 

number Analysis Wegemann (1912)

Smith and 

others (1972)

Glass and 

Jones (1992)

Keystone 

Coal Industry 

Manual (2005)

Basin 2 – ? 10683 Moisture (%) 28.8

Ash (%) 6.7

Volatile matter (%) NR

Fixed carbon (%) NR

Sulfur (%) 0.56

Calorific value (Btu/lb) 7,930

Basin 3 – ? 10728 Moisture (%) 23.6

Ash (%) 7.8

Volatile matter (%) NR

Fixed carbon (%) NR

Sulfur (%) 0.68

Calorific value (Btu/lb) 8,450

Basin 3 – ? 10694 Moisture (%) NR

Ash (%) 14.6

Volatile matter (%) NR

Fixed carbon (%) NR

Sulfur (%) 0.56

Calorific value (Btu/lb) 7,980

Basin 4 – Upper bed 10804 Moisture (%) 18.8 18.8

Ash (%) 7.6 7.6

Volatile matter (%) 35.7 35.7

Fixed carbon (%) 37.9 37.9

Sulfur (%) 0.57 0.57

Calorific value (Btu/lb) 9,160 9,160

Basin 4 – Lower bed 10827 Moisture (%) 23.5 23.5 23.5 23.5

Ash (%) 5.17 5.17 5.17 5.2

Volatile matter (%) 35.6 35.6 35.6 35.6

Fixed carbon (%) 35.7 35.7 35.7 35.7

Sulfur (%) 0.49 0.49 0.49 0.49

Calorific value (Btu/lb) 9,050 9,050 9,160 9,160

Basin 6 – Upper bed 11048 Moisture (%) 28.1 28.1

Ash (%) 4.62 4.62

Volatile matter (%) 31.6 31.6

Fixed carbon (%) 35.7 35.7

Sulfur (%) 0.45 0.45

Calorific value (Btu/lb) 8,350 8,350

Additional information Assessment of moisture validity YES NO NO NO

Data to assess representativeness YES NO NO NO
In-Place Coal Quality 9

 

 (subC)

(subB)

(subA)

Figure 3. Variation in moist, mineral-matter-free (m, mmf) British thermal units (Btu) in the Wyodak-Anderson 

coal zone in the Powder River Basin, Wyoming (Stricker and others, 2007), showing a trend towards increasing 

apparent rank of the coal (ASTM International, 2008a) from subbituminous C (subC) at shallow depths to 

subbituminous B (subB), and finally subbituminous A (subA) in the deepest part of the basin. 
10 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

Estimation of the Quality of Deep Fort Union 

Formation Resources

The SWPRB contains significant volumes of thick Fort 

Union Formation coal beds that are too deep to mine by means 

of conventional surface mining technology. As an integral part 

of the assessment of these deep coal resources, an estimation of the coal quality by depth is an important parameter in 

assessing the viability of resources potentially recoverable 

by underground mining methods. Coal quality data for the 

deeper coal resources are especially limited to a relatively few 

coal cores sampled for coal-bed natural gas assessment work. 

Furthermore, the results from the analyses of core samples following gas desorption testing require critical evaluation from 

the standpoint of both representativeness and sample degradation during the desorption process.

The coal quality dataset from Stricker and others (2007) 

is a compilation of analyses from 963 core samples from 

37 core holes that were placed in sealed canisters for gas 

desorption measurements. Each sample was 2 feet long and 

4.0 inches in diameter. The coal, after gas desorption in the 

canisters, was split into subsamples for an extensive analytical suite including proximate analyses, ultimate analyses, and 

gross calorific value determinations. The analytical results 

from the 37 core holes representing 52 beds were summarized 

by Stricker and others (2007, table 2, p. 11–14). Those data 

were then imported into a spreadsheet (table 3) to determine 

whether a relationship between sample depth and gross 

calorific values could be established by creating crossplots 

of depth versus gross calorific values.

The process of analyzing coal quality data from desorbed 

canister samples can be challenging. The desorption process 

can take several months or longer. During that time, coal may 

be partially oxidized, resulting in lower gross calorific values. 

Furthermore, samples tend to dry out during gas desorption 

(Mavor and Pratt, 1996). Moisture losses result in anomalously higher as-received gross calorific values. Perhaps one of 

the greatest challenges is deciding whether or not the canister 

samples are representative of the entire coal bed. For example, 

for Bore Hole No. 26 (table 3), a total of fifty-one 2-foot-long 

canister samples from the Smith (Big George) bed were 

desorbed, but proximate and gross calorific value analyses 

were performed on only three of those canister samples.

Representativeness refers not only to parameters like ash 

but also to the maceral assemblages of each canister sample. 

Although North American coals tend to be rich in vitrinite 

(Mathews and others, 2007), there are typically petrographic 

facies or layers within a coal bed where exinite or inertinite 

may be enriched relative to vitrinite (Flores and others, 1989). 

Exinite has a markedly higher calorific value and volatile 

matter content than vitrinite due to its high hydrogen content. 

Conversely, the relatively low calorific value of inertinite is 

due to its lower hydrogen content (Stach and others, 1982). 

Thus, a canister from a facies enriched in exinite might have 

an anomalously higher calorific value compared to a whole 

bed sample from the same sampling location, and the opposite 

would be true for a facies enriched in inertinite.

Given all the challenges and limitations of interpreting 

coal quality data from canister samples, it is not surprising that the first attempt to establish a trend of depth versus 

as-received gross calorific values from table 3 was unsatisfactory (fig. 4). The plot exhibits fairly poor agreement between 

the two parameters. The least squares trendline had an R2 of 

only 0.255. Furthermore, the trendline projection of about 

7,000 Btu/lb at shallow depths does not reflect typical values 

for the PRB at similar depths (table 1).

A review of the data from table 3 suggests that the 

previously discussed problems of moisture loss, sample 

oxidation, anomalously high and low calorific values, 

and representativeness were all encountered to at least 

some degree. Equilibrium moisture values will typically 

be lower than the respective inherent (bed) moisture values for a sample that has been collected at its inherent 

moisture level (Luppens, 1982; Luppens and Hoeft, 1991). 

In fact, ASTM D1412 (“Standard Test Method for Equilibrium Moisture of Coal at 96 to 97 Percent Relative Humidity and 30°C”) states that “based on experience with low 

rank coals, equilibrium moisture values that exceed inherent 

moisture values are due to one or both of two situations, 

either the sample was not collected with its full complement 

of inherent moisture (partially dried) or an analytical problem 

occurred, or both” (ASTM International, 2008b, p. 290). For 

Wyoming subbituminous coals, equilibrium moisture values 

ranged from 0.5 to 1.3 percent lower than inherent moisture 

levels (Luppens and Hoeft, 1991). Therefore, equilibrium 

moisture values that are higher than total moisture values in 

table 3 indicate moisture was lost during gas desorption for 

some samples. Additionally, the 2,557-Btu/lb range between 

the maximum and minimum moisture- and ash-free (DAF) 

calorific values for coal beds within the same area is quite 

large. This is probably due in large part to the impact of atypical maceral assemblages on the relatively few canister intervals with calorific analyses and perhaps the effects of sample 

oxidation, which lowers DAF calorific values.

An additional tool for verifying coal quality is data interrelationships. The verification process is based on the fact that 

data interrelationships are almost axiomatic in coal analyses. 

As the analytical result for one parameter changes, the esults 

for some other parameter usually changes too. The pair of 

test results may vary inversely or in direct proportion to each 

other. Correlations based on these data interrelationships can 

be used to identify possible outliers or estimate missing data 

(Hoeft and others, 1983). For example, moisture and ash have 

an inverse linear relationship (Luppens, 1982). As the ash 

content increases, moisture content decreases. Thus, for two 

analyses with similar ash contents from the same coal bed and 

depth, the moisture contents should be similar as well. 
Table 3. Properties and composition of coal beds cored in the Powder River Basin, Wyoming (modified from Stricker and others, 2007,table 2). Figure depth versus as-received calorific values for these data.—Continued

Core 

hole

Mean 

depth, in 

feet

Coal zone Coal bed name as used by gas operators Number of samples

Moisture

Ash yield 

(%)

Asreceived 

calorific 

value 

(Btu/lb)

Dry ash 

yield

(Btu/lb)

Total (%) Equilibrium (%)

9 143.4 Wyodak-Anderson Upper Wyodak 5 19.27 22.18 12.22 9,280 15.14 

10 167.0 Wyodak-Anderson Upper Wyodak 2 26.46 27.45 9.31 8,910 12.66 

9 181.5 Wyodak-Anderson Middle/Lower Wyodak 26 22.69 25.57 4.52 8,790 5.85 

10 209.2 Wyodak-Anderson Middle/Lower Wyodak 27 27.53 26.78 4.59 8,750 6.33 13 212.0 Wyodak-Anderson Upper Wyodak 8 27.25 ---- 8.88 8,790 12.21 20 248.9 Wyodak-Anderson Middle/Lower Wyodak 34 27.81 24.77 4.50 8,990 6.23 13 252.4 Wyodak-Anderson Middle/Lower Wyodak 27 28.86 ---- 4.56 8,750 6.41 14 289.0 Wyodak-Anderson Middle/Lower Wyodak 30 28.88 25.18 3.46 8,890 4.87 15 293.1 Wyodak-Anderson Middle/Lower Wyodak 32 27.93 25.61 3.69 8,740 5.12 17 293.1 Wyodak-Anderson Middle/Lower Wyodak 39 26.54 25.49 3.82 8,770 5.20 16 319.4 Wyodak-Anderson Middle/Lower Wyodak 28 27.13 26.13 4.22 8,600 5.79 28 329.3 Wyodak-Anderson Anderson 8 27.46 27.23 4.22 8,820 5.82 34 345.0 Wyodak-Anderson Anderson 17 24.98 25.35 4.68 9,010 6.24 23 346.0 Wyodak-Anderson School 9 25.49 26.25 12.20 8,020 16.37 18 360.2 Wyodak-Anderson Middle/Lower Wyodak 40 25.56 ---- 3.75 9,090 5.04 

8 385.0 Wyodak-Anderson Smith 15 26.13 ---- 5.87 9,080 7.95 

34 519.5 Wyodak-Anderson Canyon 13 26.31 24.26 3.71 9,090 5.03 30 640.0 Wyodak-Anderson Canyon 7 25.31 25.53 3.74 9,120 5.01 

6 648.7 Wyodak-Anderson Canyon 7 32.01 ---- 5.20 7,630 7.65 6 952.0 ---- Cook 8 27.77 ---- 4.54 8,670 6.29 

33 960.2 Wyodak-Anderson Big George 10 24.77 23.83 4.22 9,180 5.61 

8 990.0 Wyodak-Anderson Canyon 4 25.82 ---- 4.19 9,340 5.65 6 1,074.0 ---- Wall 4 27.40 ---- 5.67 8,760 7.81 5 1,076.0 Wyodak-Anderson Big George 13 26.42 ---- 3.95 8,920 5.37 2 1,138.9 Wyodak-Anderson Big George 50 19.78 ---- 3.38 10,300 4.21 4 1,142.0 ---- Wall 16 25.55 ---- 5.78 9,440 7.76 

37 1,166.1 ---- Pawnee 7 21.60 20.91 5.58 9,740 7.12 

3 1,192.0 Wyodak-Anderson Anderson 2 27.76 26.40 6.30 7,790 8.72 

35 1,230.9 Wyodak-Anderson Big George 20 25.55 24.77 3.65 9,150 4.90 25 1,243.0 ---- Pawnee 5 25.19 24.34 8.14 8,120 10.88 
Table 3. Properties and composition of coal beds cored in the Powder River Basin, Wyoming (modified from Stricker and others, 2007,table 2). Figure depth versus as-received calorific values for these data.—Continued Core hole Mean depth, in feet Coal zone Coal bed name as used by gas operators Number of samples Moisture Ash yield (%) As- received calorific value (Btu/lb) Dry ash yield (Btu/lb) 

Total (%) Equilibrium (%) 36 1,266.4 ---- Pawnee 4 22.01 34.07 6.09 10,030 7.81 

5 1,276.5 Wyodak-Anderson Wyodak 24 24.61 ---- 4.50 9,440 5.97 1 1,287.2 Wyodak-Anderson Big George 60 19.22 18.50 3.67 10,440 4.54 

19 1,308.0 Wyodak-Anderson Big George 6 23.49 23.69 3.36 9,420 4.39 26 1,365.2 Wyodak-Anderson Big George 51 19.23 17.67 3.86 10,330 4.78 

11 1,366.4 Wyodak-Anderson Big George 12 24.33 23.48 4.51 9,510 5.96 

12 1,401.0 Wyodak-Anderson Big George 32 21.90 19.89 4.58 9,300 5.86 31 1,440.3 Wyodak-Anderson Big George 32 17.94 18.15 5.97 9,870 7.28 27 1,537.6 ---- Pawnee 11 23.43 20.41 4.55 9,200 5.94 32 1,541.8 Wyodak-Anderson Big George 17 17.95 16.51 3.41 10,440 4.16 27 1,709.8 ---- Cache 16 23.10 22.24 4.77 9,100 6.20 

11 1,769.0 Wyodak-Anderson Werner 13 24.60 22.81 2.69 9,570 3.57 

29 2,197.5 ---- Roberts 7 22.64 16.85 9.75 9,820 12.60 Average 24.78 23.62 5.22 9,140 6.94 Maximum value (MAX) 32.01 34.07 12.22 10,440 16.37 Minimum value (MIN) 17.94 16.51 2.69 7,630 3.57 MAX-MIN range 14.07 17.56 9.53 2,810 12.81 
0

 200 400 600 800

1,000 1,200

1,4001,6001,8002,000

6,500 7,000 7,500 8,000 8,500 9,000 9,500 10,000 10,500 11,000 11,500

Depth, in feet

As-received gross calorific value, in British thermal units per pound

Figure 4. Estimated variability of as-received gross calorific values versus depth from table 3 data for the Wyodak coal beds in the Powder Rive
14 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

For Core Holes 30 and 6 (table 3), the ash contents and depths 

are very similar, but the moisture contents are significantly 

different (25.31 versus 32.01, respectively). Furthermore, the 

relatively high moisture content for Core Hole 6 resulted in 

anomalously lower dry and DAF calorific values. Therefore, 

Core Hole 6 was considered suspect. Core hole samples with 

equilibrium moisture values that were more than 1 percent 

higher than the total moisture values also were discarded. 

The remaining data in table 3 were similarly edited, and coal 

bed analyses that appeared to be anomalous (mostly because 

of suspect moisture and calorific values) also were discarded. 

Also, to reduce variations due to differences in bed characteristics, only coal beds in the Wyodak-Anderson coal zone 

(Smith through the Werner beds) were used (fig. 5).

Of the 43 coal bed analyses in table 3, only 25 were 

retained as a result of the editing process (table 4). All the 

ranges in the maximum and minimum values in table 4 are 

much more realistic. For example, the range in DAF calorific 

values was reduced from 2,557 Btu/lb (table 3) to 938 Btu/lb 

(table 4). More importantly, the plot of depth versus calorific 

values from the edited data (fig. 6) visually demonstrates a 

more reliable relationship than the plot made from the unedited data (fig. 4). Additionally, the R2

 of 0.744 from figure 6 

represents a significant improvement over that in figure 4. 

A trendline based on the moist, mineral-matter-free calorific 

values from table 4 was also plotted on figure 6 to illustrate the 

relationship between apparent coal rank and depth. It must be 

stressed that, given the limitations of the desorbed-gas-canister 

analyses combined with the discarding of analyses through an 

editing process based solely on author experience with coal 

quality relationships in the PRB, the results should be refined 

as more data become available.

Two sources of independent data verification shown on 

figure 6 corroborate the relationship between calorific values 

and depth. Testa and Pratt (2003) also examined the relationship between calorific values and depth. The trendline of their 

depth versus moist, mineral-matter-free calorific values was 

added to figure 6. The two trendlines exhibit marked concurrence. The slopes are nearly parallel, and calorific values 

generally agree within 100 Btu/lb. The second source of verification is the published deep basin analyses of the Smith bed 

(Big George) from table 1. The calorific value of 9,750 Btu/lb 

represents the “unweighted average of 30 incremental samples 

from a 200-ft thick cored portion of the bed below 1000 feet” 

(Keystone Coal Industry Manual, 2010, p. 617). Further investigation revealed that the exact depth interval for this Smith 

bed was 1,580 to 1,780 feet (N.R. Jones, unpub. data, 2010). 

This 200-foot sample interval with an average calorific value 

is plotted on figure 6 as a box, which the trendline for the 

calorific values versus depth trendline nearly bisects. Although 

the depth versus calorific values plot in figure 6 is based on 

relatively few selected data points, the multiple sources of 

independent data verification support the use of that trendline 

as a reasonable technique for estimating the expected coal 

quality of the thick coals to a depth of 2,000 feet.

Wasatch Formation Coal Quality

As with the Fort Union Formation, only limited coal 

quality data for the Wasatch Formation are available. No significant, large-scale mining has occurred in the Wasatch coal 

beds in the PRB despite the fact that the Healy bed in the Lake 

De Smet coal field (fig. 2) is the thickest known coal bed in 

the contiguous United States (Keystone Coal Industry Manual, 

2010). A summary of coal quality data for the Wasatch Formation is presented in table 5.

In general, Wasatch coal beds have higher ash and sulfur 

contents and lower calorific values than the Fort Union Formation coals currently being produced in the PRB (Luppens 

and others, 2008). Partings are common and fairly persistent 

in many Wasatch coal beds (Mapel and others, 1953; Mapel, 

1959; Keystone Coal Industry Manual, 2010), which probably 

accounts, in part, for the lower quality. The apparent rank of 

Wasatch coal beds based on published data ranges from lignite 

A to subbituminous C (table 5). As with the Fort Union Formation coal quality for the SWPRB, limitations with existing 

published Wasatch coal quality information stem from both 

the accuracy and the representativeness of the data.

The issue of accuracy is, again, basically one of the 

reliability of the moisture results. The total moisture contents 

forthe Lake De Smet coal cores in table 5 ranges from 

23.6 to 30.5 percent. This range in moisture values for cores 

with similar ash contents from the same area seems large, 

especially because the face channel samples from the mine 

in the Lake De Smet area indicate moisture contents exceeding 30 percent (table 5). The USGS-2A core had a moisture 

content of 30.5 percent, which is comparable to the two mine 

face channel samples. It was recognized that moisture in the 

Bureau of Reclamation (BOR) core (table 5) was at least 

5 percent below that of the other cores, which was attributed 

to moisture loss during handling and shipping (Mapel and 

others, 1953). Yet, despite this core having an acknowledged 

anomalously low moisture content, it was used as reported in 

the calculation of the average coal quality parameters, biasing 

the average calculated coal quality.

The fluids used in the coring process can add excess 

surface moisture to the cores (Luppens and others, 1992). 

Therefore, it would be expected that the average total moisture 

of the cores should be at or at least slightly higher than—

rather than significantly less than—the total moisture values 

of the mine face channel samples. It is suspected that moisture 

was lost during shipping and that subsequent partial drying 

occurred through handling and processing of the core. All the 

cores were either coated with paraffin wax or wrapped in wax 

paper to preserve the moisture in the core samples; however, 

both practices are imperfect barriers to moisture loss. Any 

additional core breakage during shipping could have compromised the wax coating. Furthermore, the cores were sliced in 

half by means of a saw with a dry blade (Mapel and others, 

1953) prior to core logging and analysis, which also could 

have contributed to additional, unaccounted-for moisture loss.
Formation 

name

Kent and others 

(1980)

Pierce and others 

(1990)

USGS Fort Union 

Coal Assessment 

Team (1999) and 

Glass (1997)

Ellis and others 

(2002)

Gillette coal field 

(Luppens and 

others, 2008)

Northern Wyoming 

Powder River Basin 

(Scott and others, 

2010)

Bed names

Wasatch

Upper Healy 

Healy 

Ucross 

Felix Upper Felix Felix 

Felix Lower Felix 

Fort Union

(Tongue

River

Member)

Roland/ Wyodak Roland (Upper Rider) R

R

Smith Badger Roland Rider Roland Roland of Baker, 1929 R

Roland of Taff, 1929

Wyodak-Anderson

coal zone=Smith 

(Swartz), Badger, 

School, Sussex, 

Big George, Wyodak, 

Anderson, Dietz, 

Canyon (Monarch),

Werner

Smith Smith 

Upper Upper Anderson Rider, Anderson 

Wyodak Wyodak or Wyodak Anderson Lower Anderson

Upper Wyodak Dietz 1, Dietz 2, 

(Anderson/ Dietz Dietz 3, Dietz 4 

Canyon Canyon) Canyon Canyon Canyon 

Lower Canyon 

Lower Ferry

Werner Lower Wyodak Werner Werner 

Wyodak

Otter 

Gates/ Upper Gates/

Kennedy Kennedy Kennedy Gates Gates 

Pawnee Pawnee 

Odell

Roberts 

Figure 5. Coal bed and coal zone names used in different publications in the Powder River Basin, including current SWPRB coal assessment ar

U.S. Geological Survey, unpub. data, 2010).
Table 4. Edited version of table 3 in which core holes with anomalous data were deleted. Of the total of the 43 coal bed analyses in table 3, only 25 wthe editing process. Figure 6 shows the plot of the depth versus as-received calorific values for these data. A trendline for the calculated moist, minervalue is also shown on figure 6 to illustrate the relationship with apparent rank versus depth. Core hole Mean depth, in feet Coal zone Coal bed name as used by gas operators Number of samples Moisture Ash yield (%) As- received calorific value (Btu/lb) Dry ash yield (Btu/lb) 

Total (%) Equilibrium (%) 10 167.0 Wyodak-Anderson Upper Wyodak 2 26.46 27.45 9.18 8,790 12.49 

10 209.2 Wyodak-Anderson Middle/Lower Wyodak 27 27.53 26.78 4.59 8,750 6.33 13 212.0 Wyodak-Anderson Upper Wyodak 8 27.25 ---- 8.88 8,790 12.21 20 248.9 Wyodak-Anderson Middle/Lower Wyodak 34 27.81 24.77 4.50 8,990 6.23 13 252.4 Wyodak-Anderson Middle/Lower Wyodak 27 28.86 ---- 4.56 8,750 6.41 14 289.0 Wyodak-Anderson Middle/Lower Wyodak 30 28.88 25.18 3.46 8,890 4.87 15 293.1 Wyodak-Anderson Middle/Lower Wyodak 32 27.93 25.61 3.69 8,740 5.12 17 293.1 Wyodak-Anderson Middle/Lower Wyodak 39 26.54 25.49 3.82 8,770 5.20 16 319.4 Wyodak-Anderson Middle/Lower Wyodak 28 27.13 26.13 4.22 8,600 5.79 28 329.3 Wyodak-Anderson Anderson 8 27.46 27.23 4.22 8,820 5.82 34 345.0 Wyodak-Anderson Anderson 17 24.98 25.35 4.68 9,010 6.24 18 360.2 Wyodak-Anderson Middle/Lower Wyodak 40 25.56 ---- 3.75 9,090 5.04 

8 385.0 Wyodak-Anderson Smith 15 26.13 ---- 5.87 9,080 7.95 

34 519.5 Wyodak-Anderson Canyon 13 26.31 24.26 3.71 9,090 5.03 30 640.0 Wyodak-Anderson Canyon 7 25.31 25.53 3.74 9,120 5.01 33 960.2 Wyodak-Anderson Big George 10 24.77 23.83 4.22 9,180 5.61 

8 990.0 Wyodak-Anderson Canyon 4 25.82 ---- 4.19 9,340 5.65 5 1,076.0 Wyodak-Anderson Big George 13 26.42 ---- 3.95 8,920 5.37 

35 1,230.9 Wyodak-Anderson Big George 20 25.55 24.77 3.65 9,150 4.90 

5 1,276.5 Wyodak-Anderson Wyodak 24 24.61 ---- 4.50 9,440 5.97 

19 1,308.0 Wyodak-Anderson Big George 6 23.49 23.69 3.36 9,420 4.39 

11 1,366.4 Wyodak-Anderson Big George 12 24.33 23.48 4.51 9,510 5.96 

12 1,401.0 Wyodak-Anderson Big George 32 21.90 19.89 4.58 9,300 5.86 31 1,440.3 Wyodak-Anderson Big George 32 17.94 18.15 5.97 9,870 7.28 

11 1,769.0 Wyodak-Anderson Werner 13 24.60 22.81 2.69 9,570 3.57 

Average 25.74 24.47 4.58 9,079 6.17 Maximum value (MAX) 28.88 27.45 9.18 9,870 12.49 Minimum value (MIN) 17.94 18.15 2.69 8,600 3.57 MAX-MIN range 10.94 9.30 6.49 1,270 8.92 
De(R2 

De(T

De(fro

200(D

Da

2,0001,800

1,6001,400

1,2001,000

800

600

400

200

0

8,000 8,500 9,000 9,500 10,000 10,500

Depth, in feet

Gross calorific value, in British thermal units per pound

subC

subBsubB

subA

11,000

Figure 6. Estimated variability of as-received (AR) and moist, mineral-matter-free (m,mmf) gross calorific values versus depth from table 4 for th

in the Powder River Basin, Wyoming. The m,mmf gross calorific values calculated from the table 4 data are used to determine apparent rank, and subbituminous C (subC), less than 9,500 Btu/lbm,mmf; subbituminous B (subB), 9,500–10,500 Btu/lbm,mmf; and subbituminous A (subA), greater than 10
Table 5. Available as-received coal quality data from the Wasatch Formation in the Lake De Smet area and other Watsatch coal beds. Average valu

Area and coal bed name No. of analyses Coal bed interval Total coal bed interval cored Total feet shale, pyrite excluded Net total coal feet analyzed6 Percent of excluded shale to total recovered core Percent of core interval analyzed Moisture (%) Sulfur (%) Ash (%) Calorific value (Btu/lb) Poun

of SO2 

milli

Btu 

From To Lake De Smet area Healy coal bed1,2 15 28.5 0.60 7.6 7,880 1.52 

USGS-1 core1,3 1 59.0 141.7 82.7 7.1 47.1 13.1 57.0 29.1 0.60 5.1 7,970 1.5USGS-2A core1,3 66.0 119.0 53.0 1.8 40.8 4.2 77.0 30.5 0.50 6.1 7,845 1.27 USGS-3 core1,3 101.0 159.7 58.7 1.1 37.2 2.9 63.4 28.3 0.60 7.7 7,890 1.52 USGS-6 core1,3 172.0 292.0 120.0 33.8 68.1 33.2 56.8 29.3 0.60 9.2 7,515 1.60 Bureau of Recla- mation core1,3 95.0 224.9 129.9 20.2 101.6 16.6 78.2 23.6 1.00 9.7 8,270 2.42 Average of all 5 core holes3 5 12.8 14.0 66.5 27.0 0.70 8.1 7,940 1.76 Estimated average of 5 core holes (at 30.5% moisture) 5 30.5 0.67 7.7 7,559 1.76 Healy bed (face channel mine sample) 1 30.7 0.40 5.1 7,900 1.0Bed 120 feet be- low Healy bed (face channel mine sample) 1 31.1 1.40 8.8 7,350 3.8

Other Wasatch coal beds Felix coal bed2 42 28.0 0.89 7.8 8,050 2.2

Murray/Ucross beds2 15 30.2 1.50 4.3 8,000 3.7

1Partial bed sample. 2Keystone Coal Manual (2010). 3Mapel and others (1953). (Individual cores from Healy beds.) 4Moist, mineral-matter-free (m,mmf) basis.5Subbituminous C (subC), subbituminous B (subB), lignite A (ligA).6Total coal recovered minus shale partings deliberately excluded from analytical samples.
In-Place Coal Quality 19

To derive a more realistic estimate of the coal quality 

for the Lake De Smet coal field, the weighted average of the 

five cores in table 5 was normalized to 30.5 percent moisture. The 30.5-percent moisture estimate was based largely 

on the moisture levels of the mine face channel samples and 

USGS-2A core (table 5). The moisture adjustment resulted in a 

decrease of more than 300 Btu/lb in the average calorific value 

to 7,559 Btu/lb and lowered the estimated apparent rank to the 

lignite A/subbituminous C boundary (table 5). Because the 

Wasatch Formation is younger than the underlying Fort Union 

Formation, the slightly lower apparent rank of the Wasatch 

coal beds relative to the Fort Union coal beds in the nearby 

Sussex coal field (fig. 1, table 1) is consistent with their relative burial depths.

Unlike the moisture issue, the representativeness of the 

data is much more difficult to evaluate because of a combination of incomplete core recovery for all core holes and the 

intentional exclusion of all high-ash intervals. The percentage 

of the coal bed recovered ranged from about 57 to 78 percent 

(table 5). This range of core loss is significant (Luppens and 

others, 1992) and certainly raises questions about the representativeness of the core samples. Frequently, higher-ash 

zones with interbedded coal and non-coal lamina sustain 

lower recovery rates during the coring process. The dissimilar lamina create more bedding-plane weaknesses, leading 

to more fractured core and greater potential for core loss. If 

this situation contributed to increased core loss, the recovered 

samples would be biased towards lower-ash contents. However, without corroborating data such as geophysical logs, it is 

impossible to evaluate the recovered cores from a representativeness perspective.

The most serious problem from a representativeness 

standpoint was the systematic removal of all high-ash zones 

and partings greater than 3/8 inch in thickness from all core 

samples (Mapel and others, 1953). U.S. Geological Survey 

guidelines in place at the time of sampling mandated that 

their samplers strictly follow the exclusionary procedure 

for partings and pyrite of certain thicknesses (Golightly and 

Simon, 1989). The elimination of the high-ash layers follows 

the practices in the ASTM D388 standard on the classification 

of coals by rank (ASTM International, 2008b). This standard 

specifies excluding mineral partings more than 1 centimeter (3/8 inch) for samples used for the determination of coal 

rank. The excluded intervals were not analyzed separately. 

By analyzing both the retained and the excluded materials as 

well as the coal quality for each, whole bed core could have 

been mathematically composited. Whole bed core analyses are 

a fundamental component for all coal resource assessments. 

Despite the exclusionary practice, the detailed documentation 

of the core sampling process (Mapel and others, 1953) makes 

it possible to at least estimate the magnitude of the effect on 

the reported average coal quality (table 5) if the excluded high 

ash and pyrite partings were reconstituted with the analyzed 

core samples. The total thickness of the excluded shale 

partings for the five cores ranged from 1.1 to 33.8 feet and 

averaged 12.8 feet (table 5).

Because the total percentage of excluded shale to total 

coal core analyzed for drill core USGS-1 was very close to 

the average of the five cores (table 5), USGS-1 was studied 

more closely to better understand impacts of this exclusionary 

sampling methodology. To begin, the laboratory descriptions 

of drill core USGS-1 were reviewed. The USGS-1 core hole 

spanned a total of 18 black shale partings totaling 7.1 feet, 

which were removed and discarded prior to analyzing the core 

samples. The thickness of individual partings ranged from 

1/2 inch to 16 3/4 inches. Figure 7 illustrates the distribution 

and magnitude of the excluded partings, as well as the intervals of lost core. As indicated by this figure, it seems unlikely 

that selective mining to exclude these parting would be practical given the range in parting thicknesses and their interbedded relationship with the coal layers throughout the entire bed; 

therefore, from the standpoint of a coal resource assessment, a 

predicted quality for the whole coal bed is needed.

The first step in arriving at the composited quality is 

estimating ash and calorific values for the excluded shale 

fraction. Based on the analyses of numerous coal bed roof and 

floor samples from Texas, Louisiana, Mississippi, and Wyoming (J.A. Luppens, unpub. data, 2010), the as-received ash 

and calorific values chosen were 65 percent and 1,500 Btu/lb, 

respectively. When compositing analyses, it is also important 

to consider the method by which the proportional amounts of 

each sample are calculated. Often, one can make the mistake 

of simply using a volumetric approach (dividing the length of 

each sample by the total length of the interval to be composited). If the densities of each interval are similar, the volumetric method is appropriate; however, in cases where there are 

significant density differences (such as partings composed of 

mineral matter versus coal), the volumetric method is flawed 

(Luppens and others, 1992). The density of mineral matter should be determined for each lithology and then used to 

calculate composite values. To evaluate compositing on a mass 

basis, the average densities of the coal and shale are estimated 

to be 1.3 grams per cubic centimeter (g/cm3

) and 1.9 g/cm3.

Table 6 illustrates the differences between results from 

volume (thickness) weighting versus density weighting to 

calculate coal quality averages for the USGS-1 core hole 

interval samples. The volumetric method overestimates the 

calorific value of the recovered core by nearly 300 Btu/lb 

and underestimates the ash by 2.7 percent. Most important 

are the significant differences between the estimated quality 

from the composited results (table 6) and the original reported 

values in table 5 for the USGS-1 core hole from Mapel and 

others (1953). The combined effects of adjusting the moisture 

level to a more realistic value of 30.5 percent and estimating 

the impact of including the discarded shale partings was an 

increase in ash from 5.1 to over 15 percent and a decrease in 

the calorific value from 7,970 to about 6,700 Btu/lb.

With a significant volume of unrecovered core (fig. 7) 

for USGS-1 (28.5 feet, or 34.4 percent of the total interval cored), the issue of representativeness cannot be fully 

resolved (table 6). A compositing case that assumed that the 

entire unrecovered core was coal is presented in table 6 as a 
20 A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming

 

 

Depth, in feet

60.0 Core top at 59.0 feet

Core bottom at 141.6 feet

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

105.0

110.0

115.0

120.0

125.0

130.0

135.0

140.0

Core USGS-1

 

 

 

Coal intervals analyzed

Shale, rock, pyrite intervals 

excluded from analyses

No core recovery

EXPLANATION

Figure 7. Core log for core hole USGS-1 in the Lake De Smet coal field, Wyoming, illustrating core intervals 

analyzed, shale intervals excluded from the analyses, and lost-recovery intervals (from Mapel and others, 1953).
References Cited 21

Table 6. Estimated impacts of compositing the originally excluded shale, pyrite, and other high-ash parting material with the coal 

analyses for the USGS-1 core hole (Mapel and others, 1953) from the Lake De Smet coal field, Wyoming. Because these coal quality 

parameters are reported on a weight basis, mass weighting is the more appropriate method of mathematically compositing samples 

(Luppens and others, 1992).

Lithologies

Total core 

thickness 

(ft)

Analytical 

parameters Volumetric weighting Mass weighting

Ash 

(%)a

Calorific 

valuea

(Btu/lb)

Volume Ash % × 

vol %

Calorific 

value × 

vol % 

(Btu/lb)

Density 

(g/cm3

)

b

Thickness × 

density

Mass Ash % × 

mass %

Calorific 

value × 

mass % 

(Btu/lb)

Shale excluded 7.1 65.0 1,500 13.1% 8.5 196 1.9 13.5 17.7% 11.5 266

Total coal analyzed 47.1 5.0 7,813 86.9% 4.3 6,790 1.3 62.6 82.3% 4.1 6,429

Total 54.2 100.0% 12.9 6,986 76.1 100.0% 15.6 6,694

a

Ash and calorific values are the averages for USGS-1 core adjusted to 30.5 percent moisture and estimated values for the excluded shale.

b

Density values are estimated. 

best-case scenario in terms of deriving a whole coal bed estimate. However, this best-case estimate still more than doubles 

the average ash content from the original reported value. More 

than likely, the actual quality of the whole coal bed interval is 

somewhere between the volumetric and mass-weighted estimates in table 6. Additionally, the effects on the average sulfur 

content of the composited estimates were impossible to assess 

because of a lack of data; but intuitively, the sulfur content for 

the whole bed should be higher than reported given the deliberate exclusion of the black shale and pyrite-rich partings.

The thorough review of the original report for the 

Lake De Smet coal field resulted in a projected average coal 

quality that was significantly lower than the published data. 

With estimated values of 12 to nearly 16 percent ash, calorific 

values of about 6,700 to 7,000 Btu/lb, and sulfur contents 

perhaps as high as 1.0 percent or higher, the coal quality of 

this coal field is not competitive with any other coal currently 

being produced in the PRB. Despite the thickness of the coal 

bed, no significant, large-scale mining has occurred to date. 

It is suspected that the relatively inferior coal quality might be 

a contributing factor to the lack of development in the area.

Conclusions

From the previous discussion, it is demonstrated why it is 

always preferable to research and evaluate the circumstances 

regarding the sampling and analytical results from the original data sources when evaluating coal quality information, 

especially if only limited data are available. Cited data often 

omit important information, such as moisture integrity and the 

information needed to evaluate the issue of representativeness. 

Furthermore, only selected data may be quoted or data may be 

misquoted, as in the case of Glass and Jones (1992).

Using the available published data at face value would 

have significantly overestimated the coal quality for all 

the coal fields from both the Fort Union and the Wasatch 

Formations in the SWPRB assessment area. However, if sufficient sampling information is provided, it may be possible to 

make adjustments to reported data to derive more reasonable 

estimates of coal quality.

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Luppens—A Critical Review of Published Coal Quality Data From the Southwestern Part of the Powder River Basin, Wyoming—Open-File Report 2011–1148