Preliminary Report on

Coal Resources of the

Wyodak-Anderson Coal Zone,

Powder River Basin,

Wyoming and Montana

M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR

By Margaret S. Ellis, Gregory L. Gunther, Romeo M. Flores,

Allan M. Ochs, Gary D. Stricker, Steven B. Roberts,

Thomas T. Taber, Lisa R. Bader, and John H. Schuenemeyer

U.S. GEOLOGICAL SURVEY OPEN-FILE REPORT 98-0789-A

This report has not been reviewed for conformity with U.S. Geological Survey

editorial standards and stratigraphic nomenclature. Any use of trade names is

for descriptive purposes only and does not imply endorsement by the

U.S. Government.

Online Edition
2

Table of Contents

ABSTRACT

INTRODUCTION

ACKNOWLEDGMENTS

POWDER RIVER BASIN GEOLOGY

DEPOSITIONAL ENVIRONMENTS OF WYODAK-ANDERSON COALS

WYODAK-ANDERSON COAL QUALITY

PREVIOUS STUDIES OF COAL RESOURCES IN THE POWDER RIVER BASIN

NATIONAL COAL RESOURCE ASSESSMENT UNITS

METHODOLOGY

Overview

Resource Reporting Categories

Defining the Assessment Unit and Correlating Stratigraphy

Compiling and Correlating the Data

Preparing Data for Computer Modeling

Creating Layered Spatial Data (ARC/INFO coverages)

Gridding and Modeling Net Coal thickness and Overburden

Creating the ARC/INFO Union Coverage

Calculating Volumetrics

Creating Resource Tables

Estimating Confidence Intervals

CONCLUSION

SELECTED REFERENCES

FIGURES

Figure 1. Index map showing National Coal Resource Assessment Regions

Figure 2. Index map showing the Northern Rocky Mountain and Great Plains Region and the

location of the Powder River Basin

Figure 3. Coal production from 1960 to 1997 from all coal in Wyoming, Montana and North

Dakota.

Figure 4. Late Paleocene paleoenvironments

Figure 5. Cross-section showing the lateral variation of Wyodak-Anderson coal beds.

Figure 6. Relative age correlation of coal assessment units in the Northern Rocky Mountains and

Great Plains Region

Figure 7. Graphic showing the interpretation of StratiFact data used for computer modeling

Figure 8. Map showing spatial data from coverages of the Montana/Wyoming state boundary,

counties, the Powder River Basin boundary, tribal lands, clinker, and mine and lease

areas

Figure 9. Index map showing locations of 7.5-minute quadrangle maps in the study area

Figure 10. Map showing reliability categories for public data points

Figure 11. Map showing Federal coal ownership categories

Figure 12. Net coal isopach map

Figure 13. Shaded relief map generated from the Digital Elevation Model (DEM)

Figure 14. Overburden isopach map showing polygons for reporting categories
3

TABLES

Table 1. Wyodak-Anderson coal quality and geochemistry in the Powder River Basin

Table 2. Stratigraphic column showing nomenclature used for the correlation of the WyodakAnderson coal zone

Table 3. StratiFact data output

Table 4. Internal matrix showing the interpretation of data using the parting/split program

Table 5. Example of data output from the parting/split program for one data point

Table 6. Numeric key to 7.5-minute quadrangle maps in the study area

Table 7. Alphabetic key to 7.5-minute quadrangle maps in the study area

Table 8. Example of information output from the EarthVision volumetrics report

Table 9. Wyodak-Anderson coal resources reported by state and county

Table 10. Wyodak-Anderson coal resources reported by Federal coal and surface ownership

categories

Table 11. Wyodak-Anderson coal resources reported by state and 7.5-minute quadrangle map

area

Table 12. Wyodak-Anderson coal resources reported by state, county, overburden, net coal

thickness, and reliability categories

Table 13. Data used for computation of confidence intervals within reliability categories for the

Wyodak-Anderson coal zone.

Table 14. Estimates of uncertainty for the Wyodak-Anderson coal with and without measurement

error.
4

ABSTRACT

The National Coal Resource Assessment (NCRA) project by the U.S. Geological Survey is

designed to assess US coal with the greatest potential for development in the next 20 to 30 years. Coal

in the Wyodak-Anderson (WA) coal zone in the Powder River Basin of Wyoming and Montana is

plentiful, clean, and compliant with EPA emissions standards. This coal is considered to be very

desirable for development for use in electric power generation.

The purpose of this NCRA study was to compile all available data relating to the WyodakAnderson coal, correlate the beds that make up the WA coal zone, create digital files pertaining to the

study area and the WA coal, and produce a variety of reports on various aspects of the assessed coal

unit. This report contains preliminary calculations of coal resources for the WA coal zone and is one of

many products of the NCRA study.

Coal resource calculations in this report were produced using both public and confidential data

from many sources. The data was manipulated using a variety of commercially available software

programs and several custom programs. A general description of the steps involved in producing the

resource calculations is described in this report.

INTRODUCTION

The Powder River Basin is in the central part of the Northern Rocky Mountains and Great

Plains Region, in Wyoming and Montana (figure 1). The Wyodak-Anderson coal zone in the Powder

River Basin is being studied as part of the U.S. Geological Survey National Coal Resource Assessment,

a study of coals with the highest potential for development within the next 20 to 30 years. The

assessment is driven by the need for clean compliant energy sources. More than 30% of the Nation’s

1997 total coal production of 1.09 billion short tons (EIA, 1998) was produced from 14 Tertiary coal

beds and zones in the Northern Rocky Mountains and Great Plains region. Tertiary coals that we are

studying within this region are from the Powder River, Williston, Hanna and Carbon Basins, and the

Greater Green River Basin (figure 2).

In the northern Rocky Mountain and Great Plains Region, coal production has been from mines

primarily in Wyoming, Montana, and North Dakota (figure 3). The highest coal production in the

Region is from the Wyoming part of the Powder River Basin, from the Wyodak-Anderson coal zone.

According to figures from the Energy Information Administration (EIA, 1998) the total coal

production for 1997 from the Northern Rocky Mountains and Great Plains Region (all coal produced in

Wyoming, Montana and North Dakota) was 350 million short tons. Of the total 280 million short tons

from Wyoming, 41 million short tons from Montana, and 29 million short tons from North Dakota.

Total coal production from Wyoming accounted for approximately 26% of total national coal

production for 1997 (EIA, 1998). Based on 1996 consumption rates, the Energy Information Agency

projected that most of the low-sulfur and low ash coals in the Nation will be produced from Fort Union

Formation coals in the Wyodak-Anderson coal zone.

ACKNOWLEDGMENTS

The authors would like to acknowledge the efforts of agencies and individuals for their

contributions to this study. Data were compiled from many different sources. Much of the data was

obtained from the U.S. Geological Survey (USGS) National Coal Resources Data System (NCRDS),

the Office of Surface Mining, the Bureau of Land Management, the Wyoming Geological Survey, and

the Montana Bureau of Mines and Geology. Individual workers who supplied and correlated data for

the Wyodak-Anderson coal zone include Daniel Vogler (Wyoming Geological Survey), Edith Wilde

(Montana Bureau of Mines and Geology), and Peter Warwick, Ronald Johnson, Frances Wahl Pierce,

and Carol Molnia (USGS). Some other agencies and companies supplied information for this study,
1. Powder River Basin

2. Williston Basin

3. Hanna Basin

4. Carbon Basin

5. Greater Green River Basin

6. Bighorn Basin

 7. Wind River Basin

9. Denver Basin

8. North and Middle Park

 Basins 

10. Raton Basin

MT

NE

ND

SD

CO

WY

NM

8

4

3

Paleocene Rocks

Eocene and

Younger Rocks

Areas of High

Mining Potential

0

6

100 Mi

2

5

7

9

10

1

Figure 2. Northern Rocky Mountains and Great Plains Region basemap. 

(part)

(part)

Northern Rocky Mountains

and Great Plains Region

Rocky Mountains

and Colorado

Plateau

Appalachian

Basin

Coal Basins

Gulf Coast

Illinois 

 Basin

PRB

PRB Powder River Basin

Figure 1. Coal Regions in the National Coal Resource Assessment.

Northern Rocky

Mountains and 

Great Plains Region

5
1960 1964 1968 1972 1976 1980 1984 1988 1992 1996

50

0

100

150

200

250

300

350

400

MILLIONS OF SHORT TONS

YEAR

Regional Total

Wyoming

Montana

North Dakota

Coal Production from Wyoming, Montana, and North Dakota

Figure 3. Coal production from 1960 through 1997 from all coal in Wyoming, Montana, and North Dakota. 

(Data from Energy Information Administration (EIA), 1998). 

6
7

but regrettably cannot be acknowledged due to the proprietary status of the data. Additional workers

that entered data into our data set include Gerald Forney, Timothy Gognat, James Hunsicker, Gregory

Rossi, Scott Kinney, and Kathy Yates. In addition to the contributions of Gregory Gunther and Thomas

Taber, digital data were also produced by Gregory Rossi, Scott Kinney, and Paul Hagen.

Custom programs used in various steps of the coal resource methodology were written and

supplied by Colin Treworgy of the Illinois State Geological Survey, and by Dorsey Blake and Gary

Stricker of the U.S. Geological Survey. Finally, we would like to thank Timothy Rohrbacher, Carol

Molnia, and Kathy Varnes of the U.S. Geological Survey for their critical review and helpful

suggestions to improve this manuscript.

POWDER RIVER BASIN GEOLOGY

The Wyodak-Anderson coals occur in the upper part of the Paleocene Fort Union Formation in

the Powder River Basin of Wyoming and Montana. The Fort Union Formation crops out along the

margin of the basin, and is overlain by exposures of the Eocene Wasatch Formation in the central part

of the basin.

The Powder River Basin is an asymmetrical structural basin with an axis that trends northwest

to southeast along the far western part of the basin. Along the western margin of the basin Fort Union

rocks dip an average 20-25° to the east, and along the eastern margin of the basin the rocks have an

average dip of 2-5° to the west. The Powder River Basin covers more than 12,000 square miles and the

Fort Union Formation is more than 6,000 ft thick in the deepest part (axis) of the basin.

The basin is bounded to the west by the Bighorn Uplift, to the southwest and south by the

Casper Arch, Laramie Mountains, and Hartville Uplift, and to the east by the Black Hills Uplift. The

Miles City Arch and the Cedar Creek Anticline to the north essentially separate the Powder River Basin

from the Williston Basin.

DEPOSITIONAL ENVIRONMENTS OF WYODAK-ANDERSON COALS

The Powder River Basin evolved from an open to a partially enclosed structural basin from

early to late Paleocene time (Flores, 1986). The basin drained to the northeast through interconnected

flow-through fluvial pathways. Early through late Paleocene coal in the Powder River Basin is in the

Fort Union Formation and developed in raised mires. Major coal accumulation took place during the

late Paleocene. In the late Paleocene, paleoenvironments included converging and diverging drainage

patterns, and mire development. The drainage patterns were changed by the influence of meandering,

anastomosed and braided streams, and associated floodplain environments. There were extensive

accumulations of peat in the late Paleocene (figure 4). These accumulations occurred in peat swamps

adjacent to fluvial drainages, and formed the Wyodak-Anderson coals.

The Wyodak-Anderson coal zone contains net coal (total thickness of all coals greater than 2.5

ft thick) that is more than 200 ft thick. The entire zone is more than 600 ft thick (measured from the top

of the uppermost coal to the base of the lowermost coal). It consists of as many as six coal beds. The

coal beds average 25 ft in thickness, and are separated by clastic sedimentary rocks ranging from a few

feet to 150 ft in thickness. The Wyodak-Anderson coal beds merge into a single coal bed as much as

202 ft thick in the west-central part of the basin and as much as 120 ft thick in the eastern part of the

basin.

The vertical and lateral stratigraphic variations of the coals in the Powder River are caused by

autocyclic depositional processes represented by fluvial channel sandstones and associated flood

plain deposits formed adjacent to peat swamps. The coal beds merge, split and pinch out over short

distances. Most of the coals formed as thick, discontinuous pod-like coal beds that were split continually by converging and diverging fluvial channels. The west-east trending cross-section in figure
ID

POWDER RIVER 

BASIN AREA

Figure 4. Late Paleocene paleoenvironments. The lateral extent of the upper Fort Union Formation

and age equivalent coals are indicated by areas of peat accumulation in raised and low-lying mires,

shown in dark green. 

Mires

Uplifted mountains

Foothills, low plateaus,

or moderate uplands

River drainage

Volcanoes

Sea and lakes

Cordilleran Thrust Belt

MT ND

SD

NE

WY

CO

8
9

5 shows the merging, splitting, and pinching-out of many of the Wyodak-Anderson coal beds. The

figure also shows increased thickness of detrital deposits to the west and the cutting of the coal beds by

fluvial channel sands. This cross-section is located 13 miles north of Gillette, Wyoming. 

Depositional setting controls the thickness and lateral continuity of the Wyodak-Anderson

coals. The depositional environments of the Fort Union Formation rocks (Flores, 1986) were mainly

fluvial systems consisting of braided, meandering, and anastomosing streams in the basin center and

alluvial fans along the western basin margin. Coals accumulated in peat mires or swamps formed as

topogenous (low-lying) and raised (domed) bogs in fluvial floodplains, abandoned fluvial channels, and

interchannel environments. Thick coals accumulated in raised bogs, well above drainage level, that

were sustained by high rainfall in tropical to subtropical (mean annual temperature of 20° C; annual

precipitation rate of more than 90.55 in.) climate conditions (Nichols and others, 1989).

WYODAK-ANDERSON COAL QUALITY

Powder River Basin coals are generally considered to be clean and compliant. Arithmetic mean

values of the Wyodak-Anderson coal analyses indicate that the coal contains low percentages of ash and

total sulfur and is compliant with EPA standards for pounds of SO2 per million Btu/lb. According to

Wood and others (1983) the standard for low ash yield is from 0-8 weight percent, and low total sulfur

content is from 0-1 weight percent (on an as-received basis). The current Environmental Protection

Agency (EPA) emissions standard, for Phase I compliance is SO2 less than or equal to 2.5 pounds per

million Btu. After the year 2000, the EPA standard for Phase II requires that SO2 emissions from

electrical generation facilities not exceed 1.2 pounds per million Btu.

The Wyodak-Anderson coal beds in the Powder River Basin generally contain less sulfur and

ash than coals produced from other regions in the conterminous United States. As part of our resource

study, we studied coal quality in the Wyodak-Anderson coal zone using weighted averages, based on the

thickness of the coal samples used for the analyses, for each of the coal quality parameters at each data

point location. The means of these weighted averages are total sulfur yield of 0.47 weight percent and

ash content of 6.4 weight percent (n=300) (table 1). The range of total sulfur in Wyodak-Anderson

coal beds is from 0.06 to 2.4 weight percent and the range of ash is from 2.9 to 25 weight percent.

Sulfur is reported as the forms of organic, pyritic, and sulfate. Sulfur content varies from 0.06 to 1.2

weight percent for organic sulfur, from 0.01 to 0.77 weight percent for pyritic sulfur, and from 0.01 to

0.39 weight percent for sulfate. The pyritic and sulfate forms are contained in minerals (e.g., pyrite,

marcasite, and anhydrite) that occur in cleats or as discrete particles in the coal. The organic form of

sulfur is part of the organic structure of the coal macerals.

Pounds of SO2per million Btu for the Wyodak-Anderson coals range from 0.14 to 7.9 with a

mean of 1.2. By comparison, according to the 1998 CoalDat database (Resource Data International,

Inc., 1998), average coal quality of Wyodak-Anderson coal supplied to electrical power plants in 1997

was 5.2 percent ash, 0.32 percent total sulfur, 0.79 pounds of SO2 per million Btu, and 8,700 Btu/lb.






z

z

|

| 























y

y

y

z

z

z

{

{

{

|

|

|

























y

y

y

z

z

z

{

{

{

|

|

|

Coal

Quaternary Rocks

WA = Wyodak-Anderson

 Coal Zone

Sandstone

Mudstone

zSedimentary Rock

8.26 Miles

0

100 ft

West East

WA

WA

{

}

Figure 5. Cross section showing the lateral variation of Wyodak-Anderson coal beds.

10
11

Table 1. Wyodak-Anderson coal quality and geochemistry in the Powder River Basin. Statistics were

run on our coal quality/geochemistry data set using weighted average values (weighted on bed thickness

where several sample analyses were available at a given location). Analyses are on an as-received basis

(*= weight percent, lbSO2/mmBtu = pounds SO2 per million Btu, and MmmfBtu = moist, mineralmatter free Btu/lb).

POWDER RIVER BASIN, WYODAK-ANDERSON COAL ZONE

Parameter Number of

Samples (n)

Mean Minimum Maximum Standard

Deviation

Gross Calorific Value

(Btu/lb)

277 8220 3736 9950 817

Moisture* 300 28 15 42 3.97

Ash* 279 6.4 2.9 25 3.37

Total Sulfur* 279 0.47 0.06 2.4 0.35

LbSO2/mmBtu 277 1.2 0.14 7.9 1.09

MmmfBtu 277 8819 4576 10563 745

Volatile Matter* 105 30.1 24 36 2.03

Fixed Carbon* 105 36 20.3 45 3.77

Organic Sulfur* 71 0.25 0.060 1.2 0.16

Pyritic Sulfur* 67 0.10 0.010 0.77 0.13

Sulfate* 58 0.040 0.010 0.39 0.07

PREVIOUS STUDIES OF COAL RESOURCES IN THE POWDER RIVER BASIN

There have been a number of publications that included coal assessment and coal resource

calculations for coals in the Powder River Basin, Wyoming and Montana. Several of the publications

consisted of general geologic studies of the basin area that included either coal resource tonnages in

tables, or showed the distribution of coal resources by state, basin, or strippable coal status. These

publications include Averitt (1975), Berryhill and others (1950), Combo and others (1949, 1978), and

Smith and others (1972).

A number of publications report coal resources for all coals in the Powder River Basin. Gary

Glass (1997) sites several sources for coal resource calculations. He reports that the remaining

identified resources in the Powder River Basin are 110 billion short tons (modified from Berryhill, and

others, 1950) with identified resources that include coal that is >2.5 ft in thickness and under less than

3000 ft of overburden. To further refine this number, he reports a reserve base of 45.5 billion short

tons of coal in the Powder River Basin, Wyoming. Of this total, 22.6 billion were deemed to be

recoverable through underground mining, and 22.9 billion short tons were considered surface minable.

In that report the reserve base for all coal in Wyoming is reported at 55.9 billion short tons. Richard

Jones and Gary Glass (1991) reported demonstrated reserves of strippable coal in the Powder River

Basin in Wyoming as 23.7 billion short tons. In a more detailed study, Smith and others (1966) (Glass,

1996) reported 17.5 billion short tons of strippable Wyodak coal in the Wyoming portion of the Powder

River Basin.
12

Additional detailed studies of coal resources in the area of the Powder River Basin include a

series of 243 7.5-minute quadrangle map studies of coal resources for non-leased Federal coal. These

reports were produced as part of a CRO/CDP (Coal Resource Occurrence/Coal Development Potential)

project by the U.S. Geological Survey. This series of maps, authored by many different workers, is

referenced and synopsized in Trent (1986) and in Roberts (1998). Other more specific studies include

those by Ayers (1986), Barnum (1975), Culbertson and others (1978), Glenn (1979), Mapel (1959),

and Molnia and others (1997). These publications are valuable sources of information on coal

resources in the Powder River Basin. They report coal resource calculations for areas that range from

a single 7.5-minute quadrangle to portions of 24 7.5-minute quadrangles.

In this study, we have drawn information from both published and unpublished sources to

provide information on the coal resources for all of the coal beds in they Wyodak-Anderson coal zone

throughout the Powder River Basin study area. The study provides an overview of this assessment unit

and a consistently correlated data set for the basin area that is available for future investigations.

NATIONAL COAL RESOURCE ASSESSMENT UNITS

In each of the Tertiary basins studied for the National Coal Resource Assessment (NCRA)

there are from 1 to 3 study areas containing coals that were studied as assessment units. Assessment

units consist either as distinct individual beds, or as related coal beds that have been combined and

studied as coal zones. The assessment units for the Northern Rocky Mountains and Great Plains

Region are shown in figure 6. This figure shows the relative age of the coals (the time of peat

accumulation) with age correlation based on palynological biozone analyses by Doug Nichols of the

U.S. Geological Survey (Stricker and others, 1998 and Flores and others, 1996).

The major assessment unit for our study of the coals in the Powder River Basin is the Late

Paleocene Wyodak-Anderson coal and stratigraphically equivalent coal beds. This unit is referred to as

the Wyodak-Anderson coal zone. Due to the vertical and lateral stratigraphic variations of coal beds

within the Wyodak-Anderson coals it was determined that the assessment of individual beds was

impractical. Coal beds included in the zone are (from north to south) the Anderson, Dietz, Canyon,

Smith, Swartz, Werner, Wyodak, Sussex, School, and Badger. The coals being assessed consisted of

from 1 to 6 beds at each data point location. The data set that we compiled consists of approximately

6,200 data points.

In addition to the regional study of Wyodak-Anderson coals in the Powder River Basin, local

studies of the Wyodak-Anderson coals in the Sheridan and Gillette coal fields in Wyoming are also

being conducted by the authors. National Coal Resource Assessment studies in the Montana part of

the Powder River Basin are being conducted by Steven B. Roberts. These studies include the AndersonCanyon coal in the Decker coal field, which is included in the Wyodak-Anderson coal zone; the

Rosebud-Robinson coal in the Colstrip coal field; and the Knobloch coal in the Ashland coal field.

METHODOLOGY

Overview

The general procedure for our study consisted of a number of crucial steps. The first step was

to define the coal unit to be assessed and determine the lateral extent of the coal. Next we established

coal resource reporting categories so that we knew what data were essential to be included in our data

sets. We then compiled and correlated the lithologic data for the rock and the coal beds in our coal

zone. The correlated data were edited and downloaded from our relational database. These data were

modified to produce an ASCII file for use in creating coal thickness and overburden grids, and isopach

maps. Maps of coal thickness and overburden were then combined with other layers of
Wyodak-Anderson (PRB)

(200 ft)*

Beulah-Zap (64 ft)* (WB)

Hanna Nos. 77-80 (HB)

(55 ft)*

Hagel (53 ft)* (WB)

late

(Zones

P5-P6)

middle

(Zones

P3-P4)

early

(Zones

P1-P2)

Composite section of

coal producing units

Harmon (39 ft)*

Hansen (24 ft)*

Knobloch (60 ft)*

Rosebud (30 ft)*

Ferris Nos. 23, 25, 

31, 50 & 65 (34 ft)*

Deadman Seams 2-5 (GRB)

(40 ft)*

(HB)

} (PRB)

} (WB)

}

Paleocene

Fort Union Age Coals

PRB = Powder River Basin

WB = Williston Basin

HB = Hanna Basin

GRB = Green River Basin

* = maximum net 

 thickness of coal

Coal assessment units Relative age of coal beds and zones based 

on palynological biozones 

 

KEY

Johnson-107 (110 ft)* (HB)

Figure 6. Relative age correlation of coal assessment units in the Northern Rocky Mountains and

Great Plains Region.

13
14

spatial data containing information needed for creating coal resource reports. The digital file (polygon

union coverage) containing all of the necessary information was used in conjunction with the coal

thickness grid information to calculate the tonnage of coal resources in each polygon. The information

from the volumetrics report was combined with the attributes from the union coverage and resource

tables were created for different reporting parameters.

Resource Reporting Categories

Standardized reporting criteria were mandated for the National Coal Resource Assessment.

Mandatory and optional reporting categories have been established in U.S. Geological Survey Circular

891 (Wood and others, 1983). The following reporting categories are mandatory for reporting coal

resources for surface minable lignite and subbituminous coal:

1. Overburden thickness of 0-100 ft, 100-200 ft, 200-500 ft, and >500 ft;

2. Net coal thickness of 2.5-5 ft, 5-10 ft, 10-20 ft, 20-40 ft, and >40 ft;

3. Reliability categories (areas based on distance from a data point) of measured (0-1/4 mi), indicated

(1/4-3/4 mi), inferred (3/4-3 mi), and hypothetical (>3 mi).

Additional coal resource reporting categories used in this study are state, county, 7.5-minute quadrangle

map, and Federal coal and surface ownership.

Defining the Assessment Unit and Correlating Stratigraphy

The Wyodak-Anderson assessment unit is a coal zone. Beds included in the zone (from north to

south) are the Anderson, Dietz, Canyon, Smith, Schwartz, Werner, Wyodak, Sussex, School, and

Badger (table 2) The coal zone extends from the top of the uppermost coal bed to the base of the

lowermost coal bed. The total thickness of the coal zone includes the rock within the interval. The net

coal thickness in the zone is the sum of all of the coal beds greater than or equal to 2.5 ft thick.

For consistency of stratigraphic correlations, a stratigraphic column was prepared establishing

nomenclature for the coal zone and for the rocks above and below the zone. The standardized

stratigraphic column was used for editing and labeling spreadsheet data, and for establishing the

organization of stratigraphic units in our relational database. Information from the standardized

stratigraphic column was entered in the relational database and used to define primary zones and subzones.
15

Table 2. Stratigraphic column showing nomenclature used for the correlation of the Wyodak-Anderson

coal zone. The relative age and approximate stratigraphic relationships of the coal beds are shown.

The boxes for the coal beds are not proportional.

Age Formation StratiFact

Nomenclature

Northwest

Northeast

and northcentralEast South

Coal Bed Names

Eocene Wasatch

Formation

Above

WyodakWasatch-Fort

Union?

Anderson coal

zone

Smith (WY) Badger

Swartz

Late upper Wyodak- Anderson Anderson School

Paleocene Fort Union Anderson (Dietz 1) WyodakFormation coal zone Dietz 2 Anderson

(Tfu) Dietz 3

Canyon Canyon

(Monarch) Werner

(WY)

Tfu

undifferentiated

Below

WyodakAnderson coal

zone

Compiling and Correlating the Data

 The study began with the compilation of all stratigraphic and geochemical data for the

assessment unit. Sources of the data included federal and state agencies (e.g., U.S. Geological Survey,

Bureau of Land Management, Office of Surface Mining, State geological surveys, and State

Departments of Environmental Quality), industry (mining companies), and private (individual

geologists) sources. Much of the industry and private data are proprietary and will not be released to

the public. Some of the data acquired existed on computer files in spreadsheet format, however, much of

the data had to be hand entered from paper copy onto spreadsheets using commercially available

computer software.

Information on the spreadsheets was checked, edited, and supplemented to conform to

standardized column names and data entries. As the data sets from various sources were completed,

the files were loaded into our relational database (StratiFact (GRG Corp., 1996)). In the relational

database, standardized stratigraphic nomenclature was used to define the top and base of the coal zone,

and the units above and below the zone at each data point throughout the study area. Standardized

lithologic nomenclature was also established and applied to the rock and coal units in the data set. The

final Powder River Basin data set contains 6,216 data points, of these 3,013 points are from oil and gas

exploratory drilling, 3,178 are from coal exploratory drilling, and 25 points are measured sections. The

Wyodak-Anderson data set contains 5,169 data point locations that contain Wyodak-Anderson coal,

897 of which contain confidential information.

When the data set was in its final form we downloaded selected information from StratiFact in

ASCII format. The data dump consisted of one line of data for each coal unit within the coal zone.
16

There were therefore, multiple entries at each data point location. Data included; point ID, sequence

number, drill hole depth, surface elevation, depth to the top of the uppermost coal, depth to the base of

the lowermost coal, elevation of the top of the coal, elevation of the base of the coal, coal thickness,

decimal latitude, and decimal longitude. The downloaded information also included two rows of

information that synopsize the information for just the coal in the zone (ZY) and for all of coal zone

(ZZ). An example of the data included in the ASCII file for one data point location is shown in table 3.

Table 3. StratiFact data output. The table shows sample StratiFact output for one location, WT108,

from the Powder River Basin, Wyoming. The entry for the ZY sequence number represents the coal

data only, and the entry for the ZZ sequence number represents the data for the entire coal zone.

DBS

Number

Point

ID

Sequence

 Number

Drill

Hole

Depth

Surface

Elevation

(ft)

Top

Depth

(ft)

Bottom

 Depth

(ft)

Top

Elevation

(ft)

Bottom

Elevation

(ft)

Coal

Thickness

(ft)

Latitude

(decimal

degrees)

Longitude

(decimal

degrees)

036I WT108 01 430.0 4146.0 28.0 32.0 4118.0 4114.0 4.0 44.8257 105.5934

036I WT108 02 430.0 4146.0 36.0 58.0 4110.0 4088.0 22.0 44.8257 105.5934

036I WT108 03 430.0 4146.0 219.0 221.0 3927.0 3925.0 2.0 44.8257 105.5934

036I WT108 04 430.0 4146.0 224.0 234.0 3922.0 3912.0 10.0 44.8257 105.5934

036I WT108 05 430.0 4146.0 304.0 322.0 3842.0 3824.0 18.0 44.8257 105.5934

036I WT108 06 430.0 4146.0 400.0 403.0 3746.0 3743.0 3.0 44.8257 105.5934

036I WT108 ZY 430.0 4146.0 28.0 403.0 4118.0 3743.0 59.0 44.8257 105.5934

036I WT108 ZZ 430.0 4146.0 28.0 403.0 4118.0 3743.0 375.0 44.8257 105.5934

Preparing Data for Computer Modeling

The StratiFact data dump is used as input for a parting/split computer program, which modifies

the data so that it consists of one line of data for each data point location. The program determines

whether a rock unit between two coal units is a parting in a coal bed or if the rock unit splits two

separate coal beds, as defined by Wood and others (1983, p. 36). This information is important when

summing the net coal thickness of the coal. Coal units are combined to form a single coal bed if the

thickness of rock between them is thinner than the coal on either side. If the rock is thinner it is

considered to be a parting and if it is thicker it is considered to be a split. After the program calculates

the thickness of coal in each coal bed it must determine if the coal beds are greater than minimum

thickness requirement for calculating coal resources, which is 2.5 ft for lignite and subbituminous coal

(Wood and others, 1983, p. 34). Data is interpreted by the program as shown in table 4, which

represents an intermediate step in the program processing.
17

Table 4. Internal matrix showing the interpretation of our data using the parting/split program. It

calculates rock thickness between coals and determines whether the rock unit is a parting in the coal or if

it splits two coal beds.

Point

 ID

Drill

Hole

Depth

Surface

Elevation

(ft)

Top

Depth

(ft)

Base

Depth

(ft)

Top

Elevation

(ft)

Base

Elevation

(ft)

Unit

Thickness

(ft)

Latitude

(decimal

degrees)

Longitude

(decimal

degrees)

Lithology

Type

Parting

or Split

WT108 430.0 4146.0 28.0 32.0 4118.0 4114.0 4.0 44.8257 105.5934 Coal

WT108 430.0 4146.0 32.0 36.0 4114.0 4110.0 4.0 44.8257 105.5934 Rock Parting

WT108 430.0 4146.0 36.0 58.0 4110.0 4088.0 22.0 44.8257 105.5934 Coal

WT108 430.0 4146.0 58.0 219.0 4088.0 3927.0 161.0 44.8257 105.5934 Rock Split

WT108 430.0 4146.0 219.0 221.0 3927.0 3925.0 2.0 44.8257 105.5934 Coal

WT108 430.0 4146.0 221.0 224.0 3925.0 3922.0 3.0 44.8257 105.5934 Rock Split

WT108 430.0 4146.0 224.0 234.0 3922.0 3912.0 10.0 44.8257 105.5934 Coal

WT108 430.0 4146.0 234.0 304.0 3912.0 3842.0 70.0 44.8257 105.5934 Rock Split

WT108 430.0 4146.0 304.0 322.0 3842.0 3824.0 18.0 44.8257 105.5934 Coal

WT108 430.0 4146.0 322.0 400.0 3824.0 3746.0 78.0 44.8257 105.5934 Rock Split

WT108 430.0 4146.0 400.0 403.0 3746.0 3743.0 3.0 44.8257 105.5934 Coal

To determine the net coal thickness for the coal at a single location the parting/split program sums

coal bed thickness of only the coal beds that are greater than minimum coal thickness requirements. The

program outputs a single line of data for each location with net coal thickness and location parameters.

Also included in the data output is the number of coal beds that were utilized in calculating the net coal

thickness. Figure 7 and table 5 show how the program interpreted the original data, and the output for

data point WT108.

A negative sign is assigned to the coal thickness if the drill hole was prematurely terminated. The

drill hole was inferred to be terminated if the base of the final unit is equal to the total depth of the drill

hole, indicating that the data may not be complete for the entire coal zone. A negative sign on the net coal

thickness allows for special handling within the isopach gridding option in EarthVision (Dynamic

Graphics, Inc., 1997). The example shown in table 5 is not a terminated drill hole.

Table 5. Example of data output file from parting/split program for data point WT108.

Point

ID

Drill

Hole

Depth

(ft)

Surface

Elevation

(ft)

Top

Coal

Depth

(ft)

Bottom

Coal

Depth

(ft)

Top Coal

Elevation

(ft)

Bottom

Coal

Elevation

(ft)

Original

Coal

Thickness

(ft)

Latitude

(decimal

degrees)

Longitud

e

(decimal

degrees)

Calculated

Net Coal

Thickness

(ft)

Number

of Coal

Beds

WT108 430.0 4146.0 28.0 403.0 4118.0 3743.0 59.0 44.8257 105.5934 57.0 5
Tertiary Wasatch

/Fort Union?

Rock (R) (p=parting, s=split)

Tertiary Fort Union 

Formation

Tertiary Fort Union 

Formation undifferentiated

Coal (C)

4 Coal thickness in feet

WT108

4 C

3 C

78 Rs

18 C

70 Rs

10 C

3 Rs

2 C

161 Rs

22 C

4 Rp

Number of coal beds = 5 

 (2 ft bed not included)

Net coal thickness = 57 ft

(includes beds > or equal to 2.5 ft 

 in thickness)

Zone thickness = 375 ft

Overburden thickness = 28 ft

28 R

WyodakAnderson

coal zone

KEY

Figure 7. Graphic showing the interpretation of Stratifact data to be used for computer modeling.

STRATIGRAPHIC

CORRRELATIONS

LITHOLOGIC 

COLUMN

FINAL REPRESENTATION 

OF DATA 

18
19

Creating Layered Spatial Data (ARC/INFO coverages)

To calculate coal resources using computerized methods, a number of computer software

programs were used. For our study we created and managed layered spatial information using

ARC/INFO (ESRI, 1998), ArcView (ESRI, 1998), and EarthVision (Dynamic Graphics, Inc., 1997)

software. All of the layers of information were created as ARC/INFO point or polygon coverages and

were limited to a lateral extent just outside of the Powder River Basin area.

For the study of the Wyodak-Anderson coal zone the coverages are in Lambert Conformal Conic

projection, Clark 1866, with first standard parallel of 33 degrees, second standard parallel of 45 degrees,

and Central Meridian of 106 degrees (the approximate center of the Powder River Basin). We used an

origin for the projection of 0 degrees and false northing and false easting of 0. The projection parameters

used were selected considering the scale at which the information would be displayed and the lateral extent

of the digital information. The layers of data used for the resource study include: State boundaries,

counties, the Powder River Basin boundary defined by the contact of Tertiary and Cretaceous age rocks

(Ross and others, 1955, and Love and Christiansen, 1985), tribal lands for the Northern Cheyenne and

Crow Indian Reservations (Biewick and others, 1998), areas of mapped Wyodak-Anderson clinker (Boyd

and Van Ploeg, 1997, Heffern and others, 1993, Heffern, 1998, and Kanizay, 1978), and WyodakAnderson mine and lease boundaries, shown in figure 8; the Wyodak-Anderson study limit defined by the

lateral extent of the Wyodak-Anderson coal and 7.5-minute quadrangles maps in the study area, shown in

figure 9 and indexed in tables 6 and 7; point locations and reliability categories, (public data only) shown

in figure 10; Federal coal and surface ownership (Biewick and others, 1997) shown in figure 11; net coal

thickness, shown in figure 12; digital elevation model (DEM), represented by a shaded relief map shown

in figure 13; and overburden isopach map, shown in figure 14. Additional information on the spatial data

(coverages) used for the Wyodak-Anderson study can be found in Ellis and others (in press).
Tribal 

Lands Tribal 

Lands

Tribal 

Lands

Mi

Km

Figure 8. Graphic showing information from coverages of the Montana/Wyoming state boundary,

counties, the Powder River Basin boundary, tribal lands, clinker, and mine and lease boundaries.

The mines shown are only those that contain Wyodak-Anderson coal.

KEY

Powder River Basin

Boundary

County Lines

State Line

Wyodak-Anderson

Mines and Lease Areas

Wyodak-Anderson

Clinker

MT

WY

MT

WY

Johnson

Sheridan

Big Horn

Powder River

Campbell

Natrona Converse

Washakie

Rosebud Custer

Carter

Niobrara

Weston

Fallon

Treasure

Crook

Johnson

Sheridan

Big Horn

Powder River

Campbell

Natrona Converse

Washakie

Rosebud Custer

Carter

Niobrara

Weston

Fallon

Treasure

Crook

0 20 40 60

0 40 80

107 106 105 107 106 105

46

45

44

43

46

45

44

43

20
21

Table 6. Numeric key to 7.5-minute quadrangle maps in the study area. Locations are shown in figure 9

and coal resources are reported by 7.5-minute quadrangle in table 11.

Number 7.5-minute Quadrangle Map Number 7.5-minute Quadrangle Map

1 COOK CREEK BUTTE 57 HOMESTEAD DRAW

2 CLUBFOOT CREEK 58 ROCKY BUTTE

3 BIRNEY DAY SCHOOL 59 HULTZ DRAW

4 GREEN CREEK 60 SHERIDAN

5 KING MOUNTAIN 61 WYARNO

6 THREEMILE BUTTES 62 JONES DRAW

7 SONNETTE 63 S R SPRINGS

8 SPRING CREEK RANCH 64 SHULER DRAW

9 KIRBY 65 GARDNER GULCH

10 TAINTOR DESERT 66 FAWN DRAW

11 BIRNEY SW 67 CABIN CREEK SE

12 BIRNEY 68 KLINE DRAW

13 BROWNS MOUNTAIN 69 RESERVOIR CREEK

14 POKER JIM BUTTE 70 HOMESTEAD DRAW SW

15 FORT HOWES 71 WHITE TAIL BUTTE

16 GOODSPEED BUTTE 72 ROCKY BUTTE SW

17 PHILLIPS BUTTE 73 BEAVER CREEK HILLS

18 HODSDON FLATS 74 BIG HORN

19 BAR V RANCH NE 75 BUFFALO RUN CREEK

20 HALF MOON HILL 76 VERONA

21 TONGUE RIVER DAM 77 ULM

22 SPRING GULCH 78 CLEARMONT

23 LACEY GULCH 79 LEITER

24 STROUD CREEK 80 ARVADA

25 HAMILTON DRAW 81 ARVADA NE

26 OTTER 82 LAREY DRAW

27 REANUS CONE 83 SPOTTED HORSE

28 SAYLE 84 RECLUSE

29 BLOOM CREEK 85 PITCH DRAW

30 LITTLE BEAR CREEK 86 OLIVER DRAW

31 BAR V RANCH 87 STORY

32 PEARL SCHOOL 88 BANNER

33 DECKER 89 HORSE HILL

34 HOLMES RANCH 90 UCROSS

35 PINE BUTTE SCHOOL 91 JULIO DRAW

36 FORKS RANCH 92 ARPAN BUTTE

37 QUIETUS 93 JEWELL DRAW

38 BEAR CREEK SCHOOL 94 LARIAT

39 SAYLE HALL 95 CROTON

40 BRADSHAW CREEK 96 TRUMAN DRAW

41 MOORHEAD 97 WILDCAT

42 THREE BAR RANCH 98 CALF CREEK

43 BAY HORSE 99 WESTON SW

44 RANCHESTER 100 STONE MOUNTAIN

45 MONARCH 101 LAKE DE SMET WEST

46 ACME 102 LAKE DE SMET EAST

47 BAR N DRAW 103 BUFFALO NE

48 CEDAR CANYON 104 FREDRICK DRAW

49 O T O RANCH 105 FLOATE DRAW

50 ROUNDUP DRAW 106 MITCHELL DRAW

51 BOX ELDER DRAW 107 LIVINGSTON DRAW

52 CABIN CREEK NW 108 ECHETA

53 CABIN CREEK NE 109 TWENTYMILE BUTTE

54 BLACK DRAW 110 ORIVA NW

55 DEAD HORSE LAKE 111 RAWHIDE SCHOOL

56 CORRAL CREEK 112 MOYER SPRINGS
22

Table 6. Continued.

Number 7.5-minute Quadrangle Map Number 7.5-minute Quadrangle Map

113 NORTH RIDGE 171 GREASEWOOD RESERVOIR

114 BUFFALO 172 ROCKY BUTTE GULCH

115 BUFFALO SE 173 RENO JUNCTION

116 PINE GULCH 174 HILIGHT

117 BEAR DRAW 175 OPEN A RANCH

118 SOMERVILLE FLATS WEST 176 KAYCEE NE

119 SOMERVILLE FLATS EAST 177 FIGURE 8 RESERVOIR

120 CARR DRAW 178 SUSSEX

121 JEFFERS DRAW 179 HOUSE CREEK

122 ORIVA 180 DRY FORK RANCH

123 GILLETTE WEST 181 ROLLING PIN RANCH

124 GILLETTE EAST 182 SOUTH BUTTE

125 FORTIN DRAW 183 BAKER SPRING

126 KLONDIKE RANCH 184 RATTLESNAKE DRAW

127 T A RANCH 185 LITTLE THUNDER RESERVOIR

128 TA RANCH NE 186 RENO RESERVOIR

129 CRAZY WOMAN RANCH 187 PINEY CANYON NW

130 PLOESSERS DRAW 188 LINCH

131 JUNIPER DRAW 189 TAYLOR RANCH

132 LASKIE DRAW 190 ARTESIAN DRAW

133 MORGAN DRAW 191 PINE TREE

134 SCOTT DAM 192 TURNERCREST

135 FOUR BAR J RANCH 193 RENO FLATS

136 APPEL BUTTE 194 TECKLA SW

137 THE GAP 195 TECKLA

138 COYOTE DRAW 196 PINEY CANYON SW

139 COON TRACK CREEK 197 SAWMILL CANYON

140 PURDY RESERVOIR 198 ROSS

141 TRABING 199 ROSS FLAT

142 BROWN RANCH 200 MACKEN DRAW

143 BOON 201 COAL DRAW NORTH

144 BOWMAN FLAT 202 BETTY RESERVOIR

145 NEGRO BUTTE 203 DUGOUT CREEK NORTH

146 BOGIE DRAW 204 COAL BANK DRAW

147 DOUBLE TANKS 205 GILLAM DRAW EAST

148 PLEASANTDALE 206 MARSH DRAW

149 SCAPER RESERVOIR 207 THOMPSON DRAW

150 THE GAP SW 208 BEAR CREEK

151 SADDLE HORSE BUTTE 209 COAL DRAW SOUTH

152 ANTELOPE DRAW 210 ALTA CREEK

153 ELAINE DRAW 211 DUGOUT CREEK SOUTH

154 PROVENCE RANCH 212 SEVEN L CREEK EAST

155 HOE RANCH 213 FLY DRAW

156 THE NIPPLE 214 SOUTH FORK RESERVOIR

157 FATS DRAW 215 SUICIDE HILL

158 WAGS PINNACLE 216 RED HILL

159 PEPSSON DRAW 217 PATSY DRAW

160 THREEMILE CREEK RESERVOIR 218 BEAUCHAMP RESERVOIR

161 EAGLE ROCK 219 GUMBO HILL

162 NEIL BUTTE 220 COAL HILL

163 ROUGH CREEK 221 HOLDUP HOLLOW

164 DRY CREEK RESERVOIR 222 WHIPPLE HOLLOW

165 FOURMILE RESERVOIR 223 BOBBY DRAW

166 SOLDIER CREEK 224 GLENROCK NW

167 FORT RENO 225 HYLTON RANCH

168 FORT RENO SE 226 LEUENBERGER RANCH

169 NORTH BUTTE 227 GILBERT LAKE

170 SAVAGETON
WyodakAnderson 

Study Limit

POWDER RIVER BASIN

Figure 9. Index map showing 7.5-minute quadrangle maps in the study area.

23
24

Table 7. Alphabetic key to 7.5-minute quadrangle maps in the study area. Locations are shown in figure

9 and coal resources are reported by 7.5-minute quadrangle in table 11.

7.5-minute Quadrangle Map Number 7.5-minute Quadrangle Map Number

ACME 46 DRY CREEK RESERVOIR 164

ALTA CREEK 210 DRY FORK RANCH 180

ANTELOPE DRAW 152 DUGOUT CREEK NORTH 203

APPEL BUTTE 136 DUGOUT CREEK SOUTH 211

ARPAN BUTTE 92 EAGLE ROCK 161

ARTESIAN DRAW 190 ECHETA 108

ARVADA 80 ELAINE DRAW 153

ARVADA NE 81 FATS DRAW 157

BAKER SPRING 183 FAWN DRAW 66

BANNER 88 FIGURE 8 RESERVOIR 177

BAR N DRAW 47 FLOATE DRAW 105

BAR V RANCH 31 FLY DRAW 213

BAR V RANCH NE 19 FORKS RANCH 36

BAY HORSE 43 FORT HOWES 15

BEAR CREEK 208 FORT RENO 167

BEAR CREEK SCHOOL 38 FORT RENO SE 168

BEAR DRAW 117 FORTIN DRAW 125

BEAUCHAMP RESERVOIR 218 FOUR BAR J RANCH 135

BEAVER CREEK HILLS 73 FOURMILE RESERVOIR 165

BETTY RESERVOIR 202 FREDRICK DRAW 104

BIG HORN 74 GARDNER GULCH 65

BIRNEY 12 GILBERT LAKE 227

BIRNEY DAY SCHOOL 3 GILLAM DRAW EAST 205

BIRNEY SW 11 GILLETTE EAST 124

BLACK DRAW 54 GILLETTE WEST 123

BLOOM CREEK 29 GLENROCK NW 224

BOBBY DRAW 223 GOODSPEED BUTTE 16

BOGIE DRAW 146 GREASEWOOD RESERVOIR 171

BOON 143 GREEN CREEK 4

BOWMAN FLAT 144 GUMBO HILL 219

BOX ELDER DRAW 51 HALF MOON HILL 20

BRADSHAW CREEK 40 HAMILTON DRAW 25

BROWN RANCH 142 HILIGHT 174

BROWNS MOUNTAIN 13 HODSDON FLATS 18

BUFFALO 114 HOE RANCH 155

BUFFALO NE 103 HOLDUP HOLLOW 221

BUFFALO RUN CREEK 75 HOLMES RANCH 34

BUFFALO SE 115 HOMESTEAD DRAW 57

CABIN CREEK NE 53 HOMESTEAD DRAW SW 70

CABIN CREEK NW 52 HORSE HILL 89

CABIN CREEK SE 67 HOUSE CREEK 179

CALF CREEK 98 HULTZ DRAW 59

CARR DRAW 120 HYLTON RANCH 225

CEDAR CANYON 48 JEFFERS DRAW 121

CLEARMONT 78 JEWELL DRAW 93

CLUBFOOT CREEK 2 JONES DRAW 62

COAL BANK DRAW 204 JULIO DRAW 91

COAL DRAW NORTH 201 JUNIPER DRAW 131

COAL DRAW SOUTH 209 KAYCEE NE 176

COAL HILL 220 KING MOUNTAIN 5

COOK CREEK BUTTE 1 KIRBY 9

COON TRACK CREEK 139 KLINE DRAW 68

CORRAL CREEK 56 KLONDIKE RANCH 126

COYOTE DRAW 138 LACEY GULCH 23

CRAZY WOMAN RANCH 129 LAKE DE SMET EAST 102

CROTON 95 LAKE DE SMET WEST 101

DEAD HORSE LAKE 55 LAREY DRAW 82

DECKER 33 LARIAT 94

DOUBLE TANKS 147 LASKIE DRAW 132
25

Table 7. Continued.

7.5-minute Quadrangle Map Number 7.5-minute Quadrangle Map Number

LEITER 79 ROUGH CREEK 163

LEUENBERGER RANCH 226 ROUNDUP DRAW 50

LINCH 188 S R SPRINGS 63

LITTLE BEAR CREEK 30 SADDLE HORSE BUTTE 151

LITTLE THUNDER RESERVOIR 185 SAVAGETON 170

LIVINGSTON DRAW 107 SAWMILL CANYON 197

MACKEN DRAW 200 SAYLE 28

MARSH DRAW 206 SAYLE HALL 39

MITCHELL DRAW 106 SCAPER RESERVOIR 149

MONARCH 45 SCOTT DAM 134

MOORHEAD 41 SEVEN L CREEK EAST 212

MORGAN DRAW 133 SHERIDAN 60

MOYER SPRINGS 112 SHULER DRAW 64

NEGRO BUTTE 145 SOLDIER CREEK 166

NEIL BUTTE 162 SOMERVILLE FLATS EAST 119

NORTH BUTTE 169 SOMERVILLE FLATS WEST 118

NORTH RIDGE 113 SONNETTE 7

O T O RANCH 49 SOUTH BUTTE 182

OLIVER DRAW 86 SOUTH FORK RESERVOIR 214

OPEN A RANCH 175 SPOTTED HORSE 83

ORIVA 122 SPRING CREEK RANCH 8

ORIVA NW 110 SPRING GULCH 22

OTTER 26 STONE MOUNTAIN 100

PATSY DRAW 217 STORY 87

PEARL SCHOOL 32 STROUD CREEK 24

PEPSSON DRAW 159 SUICIDE HILL 215

PHILLIPS BUTTE 17 SUSSEX 178

PINE BUTTE SCHOOL 35 T A RANCH 127

PINE GULCH 116 TA RANCH NE 128

PINE TREE 191 TAINTOR DESERT 10

PINEY CANYON NW 187 TAYLOR RANCH 189

PINEY CANYON SW 196 TECKLA 195

PITCH DRAW 85 TECKLA SW 194

PLEASANTDALE 148 THE GAP 137

PLOESSERS DRAW 130 THE GAP SW 150

POKER JIM BUTTE 14 THE NIPPLE 156

PROVENCE RANCH 154 THOMPSON DRAW 207

PURDY RESERVOIR 140 THREE BAR RANCH 42

QUIETUS 37 THREEMILE BUTTES 6

RANCHESTER 44 THREEMILE CREEK RESERVOIR 160

RATTLESNAKE DRAW 184 TONGUE RIVER DAM 21

RAWHIDE SCHOOL 111 TRABING 141

REANUS CONE 27 TRUMAN DRAW 96

RECLUSE 84 TURNERCREST 192

RED HILL 216 TWENTYMILE BUTTE 109

RENO FLATS 193 UCROSS 90

RENO JUNCTION 173 ULM 77

RENO RESERVOIR 186 VERONA 76

RESERVOIR CREEK 69 WAGS PINNACLE 158

ROCKY BUTTE 58 WESTON SW 99

ROCKY BUTTE GULCH 172 WHIPPLE HOLLOW 222

ROCKY BUTTE SW 72 WHITE TAIL BUTTE 71

ROLLING PIN RANCH 181 WILDCAT 97

ROSS 198 WYARNO 61

ROSS FLAT 199
Wyodak-Anderson

Study Limit

26

(based on distance from 

data point)

Measured (< 1/4 mi)

Indicated (1/4-3/4 mi)

Inferred (3/4-3 mi)

Hypothetical (> 3 mi)

Mi

Km

Figure 10. Map showing reliability categories. Only the circles drawn around non-confidential (public) 

data points are shown on this map.

Powder River Basin 

Boundary

0 20 40

0 20 40 60 80

Reliability Categories

107 106 105

44

45

43

MT

WY
27
28

Gridding and Modeling Net Coal Thickness and Overburden

The gridding and modeling of net coal and overburden thickness data were performed using

EarthVision (Dynamic Graphics, Inc., 1997) software. Data files were brought into EarthVision (EV) in

fixed ASCII format, with locations given in decimal latitude and longitude. The data fields and projection

were defined (as geographic, with a central meridian of Longitude 106 degrees west). The file was then

projected to Lambert Conformal conic, Clarke 1886, with first standard parallel of 33 degrees, second

standard parallel of 45 degrees, and central meridian of 106 degrees (the approximate center of the basin).

The origin of the projection is 0 degrees with 0 false northing and 0 false easting.

To accurately grid the net coal thickness, the data set was augmented to include interpretive

points. This was necessary because the isopach map contours were extrapolated throughout the study

area using the existing data set and the distribution of data did not include information on where the coal

pinched-out (depositional “want” areas) or was eroded away (erosional want areas). These additional data

were taken from published and unpublished mapping. In extensive areas where the coal pinched-out along

the Wyodak-Anderson boundary, portions of the polygon file (the locations of the nodes) were assigned

coal thickness values of 0 and added to the data set. Other data were hand digitized and added to the data

set.

The grid for net coal thickness was created using the EV isopach gridding option. This option

considers terminated data (negative sign in front of the net coal thickness) as "greater than" values when

assigning the net coal thickness values to the grid nodes. The Wyodak-Anderson data set included data

from 714 drill holes that terminated before the Wyodak-Anderson coal zone was completely penetrated

(therefore, the data may not represent all of the coal in the Wyodak-Anderson coal zone). Grids were

created using 4 multiple data points (the program looks at the 4 closest points to determine the grid

values), with a coal thickness z range from .1 to 285 ft (actual data ranged from –87 to 284 ft), and

extrapolated to cover the full extent of the study area.

Several different grid spacings were tested to determine which grids were most accurate and

appropriate for our purposes. There were several criteria for choosing the correct grids to use for graphic

output, creating the net coal isopach coverage, and calculating coal resources. It was important to choose

a grid that used as many of the data points in the data set as possible and that had a relatively small

absolute Z grid error (shown in the grid report). The grid must also have fine enough grid spacing to

produce an acceptable level of detail for graphics and the calculation of volumetrics.

The EV method of calculating volumetrics uses grid nodes and subgrid nodes from EarthVision for

coal thickness information in conjunction with the area of the polygons from the ARC/INFO (ESRI, 1998)

union coverage. Therefore, the grid we used for calculating coal resources had to be fine enough to have

at least one grid node value within each polygon. Because we were considering the necessity of having at

least one grid node within each polygon, we chose grid spacing that was indicated by the distance between

nodes measured in meters. The default EV grid spacing was 3,520 X 3,606 meters, which was very

coarse and created a grid that only used 2,800 out of 5,346 points and had an average absolute z error of

about 4%, which is quite high. We tested several other grid spacings and determined that the best grid

spacing to use for the union coverage and for the calculation of coal resources was 300 X 300 meters.

This grid spacing used 5,261 out of 5,346 points and had an absolute z error of .5%.

The coal thickness isopach map was created in EarthVision and is shown in figure 12. The isopach

map for use in the ARC/INFO union coverage was produced in the EV visualization module and saved as

an ASCII file. The ASCII file was modified using a custom program, ismarc, and generated into a

polygon coverage in ARC/INFO using an Arc Macro Language (AML) that was supplied to us from the

Illinois State Geological Survey. This AML clips the isopach map to the study area and assigns thickness

values to each polygon that is the average of the value of the isopach lines on either side of the polygon.

The overburden for the Wyodak-Anderson coal zone is the thickness of rock above the uppermost

coal in the coal zone, and therefore represents minimum overburden. The overburden isopach grid was

created by subtracting the grid for the top of the coal zone (grid spacing of 500 X 500
2.5

5

10

20

30

40 

50

75

100

150

200

250

0

Figure 12. Net coal isopach map.

Wyodak-Anderson

Net Coal Thickness 

(ft)

County Lines

29

Powder 

River

Basin

Miles

Kilometers

0 10 20 30

0 20 40

WyodakAnderson

Study Limit

Rosebud

Johnson

Powder River

Natrona

Campbell

Converse

Sheridan

Big 

Horn

107 

45 

44 

43 

106 

o

o o

o

o

Montana

Wyoming
30

meters) from the grid for the digital elevation model (DEM) which represents a detailed topographic

surface (grid spacing of 500 X 500 meters). The shaded relief map in figure 13 indicates the topography

generated from the digital elevation model. The overburden isopach map (figure 14) was created in

EarthVision and was generated into a polygon coverage in ARC/INFO using the same procedure and

programs as described for the generation of the net coal thickness polygon coverage.

Creating the ARC/INFO Union Coverage

Spatial information for use in the study of the coal resources for the Wyodak-Anderson coal zone

were created and manipulated as ARC/INFO polygon and point coverages. The coverages were created

by importing or digitizing data, projecting all of the coverages to a standard projection, editing and

cleaning the coverages, generalizing the data where necessary, assigning attributes to the polygons or

points, and clipping the coverages to the appropriate study extent.

The coverages were combined (unioned) to create an ARC/INFO union coverage with many

different attributes for each polygon. Because some of the coverages had similar boundaries that came

from different sources, extra polygons (or slivers) had to be edited out of the union coverage. This was

accomplished by eliminating polygons that had an area of less than 20,000 square meters and manually

editing (dissolving) additional problematic polygons.

Calculating Volumetrics

Volumetrics were calculated using a combination of EarthVision and ARC/INFO products. The

union coverage was imported into EarthVision, and labels were assigned to the polygons based on polygon

ID. The coverage was saved as a polygon file and the projection of the file was defined. In EarthVision

utilities, volumetrics were calculated with the layered volumetrics option using the ARC/INFO union

coverage polygon, the unclipped 300 X 300-meter net coal thickness grid, and a conversion factor of 1770

short tons per acre-ft for subbituminous coal. The results of the calculation of short tons of coal in each

polygon are given in the EarthVision volumetrics report. The volumetrics report was modified using a

custom program, evrpt, which striped-off the file header and added a calculated field containing coal

thickness (calculated from EarthVision positive area and the short tons in each polygon.) An example of

the file output is shown in table 8.
(Elevation in ft

above Mean 

Sea Level)

KEY

Figure 13. A shaded relief map generated from the Digital Elevation Model (DEM).

31
Overburden Thickness 

(ft)

0

100

200

300

400

500

1000

2000

Mi

Km

0 20 40

0 20 40 60 County 

Lines

Powder 

River 

Basin

WyodakAnderson

 Study Limit

Figure 14. Overburden isopach map showing polygons for reporting categories.

107 106

45

44

43

32 

Rosebud

Johnson

Powder River

Natrona

Campbell

Converse

Sheridan

Big 

Horn MT

WY
33

Table 8. Example of information output from the EarthVision volumetrics report after it has been

modified by the evrpt program.

Polygon

 ID

Short Tons EV Positive Area

(square meters)

EV Coal

Thickness (ft)

2 998,838 197,200.2 11.6

3 308,697 58,976.1 12

4 444,720 87,232.6 11.7

5 619,674 119,618.2 11.8

6 313,039 62,791 11.4

7 145,275 28,581.8 11.6

8 358,032 70,026.8 11.7

9 318,819 64,151.8 11.4

10 1,969,375 401,293 11.2

11 1,025,328 205,536.2 11.4

12 270,140 55,757.8 11.1

13 132,878 26,525.4 11.5

14 4,840,237 928,079.3 11.9

15 443,408 87,635.1 11.6

16 934,074 201,994.8 10.6

17 720,379 132,345.6 12.4

18 718,352 150,964.6 10.9

Creating Resource Tables

Information from the modified EarthVision Volumetrics report and the ARC/INFO union

coverage polygon attribute tables were combined to create coal resource tables. This merging of

information was based on the polygon ID. This merging of data can be accomplished in ArcView (ESRI,

1998) or in spreadsheet or relational database software. For this study, we imported the polygon attribute

table and the volumetrics report in ASCII format into Excel spreadsheet software. The files were merged,

additional fields were calculated (e.g., millions of short tons and acres), and text fields were added (so

that the categories in our resource tables would print in the proper order). Polygons containing lease and

mine areas, clinker, and net coal less than 2.5 ft thick were then deleted. The polygons in WyodakAnderson (WA) mine and lease areas and in areas containing WA clinker were removed because we did

not have enough detailed information to determine how much WA coal remained in those areas. Coal less

than 2.5 ft in thickness was deleted because it is not included in standard resource reporting categories.

Coal resources tables were produced using pivot tables in Excel. The following tables (tables 9

through 12) show coal resources for the Wyodak-Anderson coal zone reported on various parameters. All

coal tonnages are reported in millions of short tons and with two significant figures.
34

Table 9. Wyodak-Anderson coal resources reported by state and county. Resources are shown in millions of

short tons (MST) and with two significant figures. Zeros (0) indicate that no coal resources were calculated

within those categories. The table does not include coal resources in mine or lease areas, areas containing

Wyodak-Anderson clinker, or areas containing coal less than 2.5 ft thick. County outlines are shown on

figure 8.

State County Total (MST)

Montana BIG HORN 29,000

POWDER RIVER 11,000

ROSEBUD 2,800

Montana Total 42,000

Wyoming CAMPBELL 280,000

CONVERSE 15,000

JOHNSON 160,000

SHERIDAN 52,000

Wyoming Total 510,000

Grand Total (MST) 550,000
35

Table 10. Wyodak-Anderson coal resources reported by state and by Federal coal and Federal surface

ownership categories. Resources are shown in millions of short tons (MST) and with 2 significant figures.

The table does not include coal resources in mine or lease areas, areas containing Wyodak-Anderson

clinker, or areas containing coal less than 2.5 ft thick. Federal ownership categories are shown on figure

11.

State Federal Ownership Total (MST)

Montana No Federal coal ownership or Federal surface ownership 3,900

No Federal coal ownership, but Federal surface ownership 2.9

Federal coal ownership, but no Federal surface ownership 32,000

Federal coal ownership and Federal surface ownership 6,700

Total Federal coal in Montana 39,000

Montana Total 42,000

Wyoming No Federal coal or Federal surface ownership 64,000

No Federal coal ownership, but Federal surface ownership 1,900

Federal coal ownership, but no Federal surface ownership 360,000

Federal coal and Federal surface ownership 79,000

Total Federal coal in Wyoming 440,000

Wyoming Total 510,000

Grand Total (MST) 550,000
36

Table 11. Wyodak-Anderson coal resources reported by state and 7.5-minute quadrangle map area.

Resources are shown in millions of short tons (MST) and with 2 significant figures. The table does not

include coal resources in mine or lease areas, areas containing Wyodak-Anderson clinker, or areas

containing coal less than 2.5 ft thick. The index map showing locations of the 7.5-minute quadrangles is

on figure 9 and the numeric and alphabetized keys are shown on tables 6 and 7.

State 7.5-minute Quadrangle Map Numeric Key Total (MST)

Montana ACME 46 160

BAR N DRAW 47 150

BAR V RANCH 31 320

BAR V RANCH NE 19 740

BAY HORSE 43 2.1

BEAR CREEK SCHOOL 38 2,700

BIRNEY 12 74

BIRNEY DAY SCHOOL 3 2.2

BIRNEY SW 11 130

BLACK DRAW 54 51

BLOOM CREEK 29 65

BOX ELDER DRAW 51 89

BRADSHAW CREEK 40 1,100

BROWNS MOUNTAIN 13 60

CABIN CREEK NE 53 150

CABIN CREEK NW 52 150

CEDAR CANYON 48 130

CLUBFOOT CREEK 2 2.4

COOK CREEK BUTTE 1 81

DECKER 33 1,900

FORKS RANCH 36 2,700

FORT HOWES 15 160

GOODSPEED BUTTE 16 43

GREEN CREEK 4 56

HALF MOON HILL 20 1,600

HAMILTON DRAW 25 2,400

HODSDON FLATS 18 210

HOLMES RANCH 34 2,700

KING MOUNTAIN 5 22

KIRBY 9 1,700

LACEY GULCH 23 1,100

MONARCH 45 1.2

MOORHEAD 41 43

O T O RANCH 49 140

OTTER 26 630

PEARL SCHOOL 32 4,600

PHILLIPS BUTTE 17 600

PINE BUTTE SCHOOL 35 3,400

Table 11. Continued on next page.
37

Table 11. Continued.

State 7.5-minute Quadrangle Map Numeric Key Total (MST)

Montana POKER JIM BUTTE 14 1,000

QUIETUS 37 3,500

REANUS CONE 27 360

ROUNDUP DRAW 50 110

SAYLE 28 1,100

SAYLE HALL 39 2,600

SONNETTE 7 46

SPRING CREEK RANCH 8 47

SPRING GULCH 22 790

STROUD CREEK 24 1,000

TAINTOR DESERT 10 1,100

THREE BAR RANCH 42 39

THREEMILE BUTTES 6 3.4

TONGUE RIVER DAM 21 250

Montana Total 42,000

Wyoming ACME 46 2,700

ALTA CREEK 210 6.0

ANTELOPE DRAW 152 59

APPEL BUTTE 136 5,000

ARPAN BUTTE 92 2,800

ARTESIAN DRAW 190 4,600

ARVADA 80 2,200

ARVADA NE 81 3,700

BAKER SPRING 183 4,200

BANNER 88 93

BAR N DRAW 47 2,900

BAY HORSE 43 0.25

BEAR CREEK 208 680

BEAR DRAW 117 6,700

BEAUCHAMP RESERVOIR 218 29

BETTY RESERVOIR 202 1,700

BIG HORN 74 520

BLACK DRAW 54 1,200

BOBBY DRAW 223 67

BOGIE DRAW 146 6,900

BOON 143 3,400

BOWMAN FLAT 144 7,000

BOX ELDER DRAW 51 1,200

BROWN RANCH 142 750

BUFFALO 114 1600

BUFFALO NE 103 3,300

BUFFALO RUN CREEK 75 880

BUFFALO SE 115 3,900

CABIN CREEK NE 53 1,400

Table 11. Continued on next page.
38

Table 11. Continued.

State 7.5-minute Quadrangle Map Numeric Key Total (MST)

Wyoming CABIN CREEK NW 52 2,500

CABIN CREEK SE 67 2,400

CALF CREEK 98 3,400

CARR DRAW 120 7,800

CEDAR CANYON 48 2,700

CLEARMONT 78 1,500

COAL BANK DRAW 204 64

COAL DRAW NORTH 201 1,200

COAL DRAW SOUTH 209 230

COAL HILL 220 1,300

COON TRACK CREEK 139 51

CORRAL CREEK 56 1,600

COYOTE DRAW 138 1,400

CRAZY WOMAN RANCH 129 2,700

CROTON 95 6,200

DEAD HORSE LAKE 55 1,300

DOUBLE TANKS 147 7,500

DRY CREEK RESERVOIR 164 570

DRY FORK RANCH 180 7,300

DUGOUT CREEK NORTH 203 840

DUGOUT CREEK SOUTH 211 2.7

EAGLE ROCK 161 4,200

ECHETA 108 4,900

ELAINE DRAW 153 620

FATS DRAW 157 7,300

FAWN DRAW 66 2,600

FIGURE 8 RESERVOIR 177 670

FLOATE DRAW 105 5,900

FLY DRAW 213 250

FORT RENO 167 3,600

FORT RENO SE 168 7,600

FORTIN DRAW 125 390

FOUR BAR J RANCH 135 4,600

FOURMILE RESERVOIR 165 2,000

FREDRICK DRAW 104 4,500

GARDNER GULCH 65 2,000

GILBERT LAKE 227 80

GILLAM DRAW EAST 205 1.1

GILLETTE EAST 124 2,400

GILLETTE WEST 123 4,900

GLENROCK NW 224 96

GREASEWOOD RESERVOIR 171 4,700

GUMBO HILL 219 390

HILIGHT 174 3,500

HOE RANCH 155 4,600

Table 11. Continued on next page.
39

Table 11. Continued.

State 7.5-minute Quadrangle Map Numeric Key Total (MST)

Wyoming HOLDUP HOLLOW 221 770

HOMESTEAD DRAW 57 480

HOMESTEAD DRAW SW 70 2,700

HORSE HILL 89 1,100

HOUSE CREEK 179 4,700

HULTZ DRAW 59 240

HYLTON RANCH 225 590

JEFFERS DRAW 121 4,500

JEWELL DRAW 93 3,900

JONES DRAW 62 1,700

JULIO DRAW 91 2,800

JUNIPER DRAW 131 7,900

KAYCEE NE 176 110

KLINE DRAW 68 3,200

LAKE DE SMET EAST 102 2,200

LAKE DE SMET WEST 101 380

LAREY DRAW 82 3,800

LARIAT 94 5,200

LASKIE DRAW 132 9,500

LEITER 79 1,600

LEUENBERGER RANCH 226 480

LINCH 188 390

LITTLE BEAR CREEK 30 1.2

LITTLE THUNDER RESERVOIR 185 5,700

LIVINGSTON DRAW 107 5,700

MACKEN DRAW 200 670

MARSH DRAW 206 330

MITCHELL DRAW 106 6,000

MONARCH 45 690

MOORHEAD 41 1.8

MORGAN DRAW 133 9,700

MOYER SPRINGS 112 530

NEGRO BUTTE 145 9,200

NEIL BUTTE 162 2,200

NORTH BUTTE 169 8,000

NORTH RIDGE 113 78

O T O RANCH 49 2,200

OLIVER DRAW 86 36

OPEN A RANCH 175 430

ORIVA 122 4,400

ORIVA NW 110 5,100

PATSY DRAW 217 33

PEPSSON DRAW 159 5,500

PINE GULCH 116 5,100

PINE TREE 191 4,200

Table 11. Continued on next page.
40

Table 11. Continued.

State 7.5-minute Quadrangle Map Numeric Key Total (MST)

Wyoming PINEY CANYON NW 187 110

PINEY CANYON SW 196 520

PITCH DRAW 85 1,100

PLEASANTDALE 148 5,300

PLOESSERS DRAW 130 5,700

PROVENCE RANCH 154 2,000

PURDY RESERVOIR 140 35

RANCHESTER 44 180

RATTLESNAKE DRAW 184 4,000

RAWHIDE SCHOOL 111 4,300

RECLUSE 84 3,400

RED HILL 216 630

RENO FLATS 193 4,300

RENO JUNCTION 173 6,000

RENO RESERVOIR 186 3,100

RESERVOIR CREEK 69 2,600

ROCKY BUTTE 58 100

ROCKY BUTTE GULCH 172 6,200

ROCKY BUTTE SW 72 240

ROLLING PIN RANCH 181 7,000

ROSS 198 770

ROSS FLAT 199 1,200

ROUGH CREEK 163 1,500

ROUNDUP DRAW 50 1,500

S R SPRINGS 63 1,300

SADDLE HORSE BUTTE 151 370

SAVAGETON 170 5,600

SAWMILL CANYON 197 33

SCAPER RESERVOIR 149 5,900

SCOTT DAM 134 5,600

SEVEN L CREEK EAST 212 59

SHERIDAN 60 1,100

SHULER DRAW 64 1,500

SOLDIER CREEK 166 1,100

SOMERVILLE FLATS EAST 119 6,800

SOMERVILLE FLATS WEST 118 7,300

SOUTH BUTTE 182 5,300

SOUTH FORK RESERVOIR 214 900

SPOTTED HORSE 83 5,400

STORY 87 7.5

SUICIDE HILL 215 1,100

SUSSEX 178 1,900

T A RANCH 127 310

TA RANCH NE 128 1,400

TAYLOR RANCH 189 2,600

Table 11. Continued on next page.
41

Table 11. Continued.

State 7.5-minute Quadrangle Map Numeric Key Total (MST)

Wyoming TECKLA 195 3,700

TECKLA SW 194 4,800

THE GAP 137 3,800

THE GAP SW 150 3,200

THE NIPPLE 156 7,300

THOMPSON DRAW 207 720

THREE BAR RANCH 42 1.2

THREEMILE CREEK RESERVOIR 160 6,400

TRABING 21 450

TRUMAN DRAW 96 6,200

TURNERCREST 192 2,900

TWENTYMILE BUTTE 109 4,600

UCROSS 90 2,700

ULM 77 710

VERONA 76 630

WAGS PINNACLE 158 5,800

WESTON SW 99 100

WHIPPLE HOLLOW 222 400

WHITE TAIL BUTTE 71 1,500

WILDCAT 97 4,100

WYARNO 61 2,700

WY Total 510,000

Grand Total (MST) 550,000
42

Table 12. Wyodak-Anderson coal resources reported by state, county, and overburden, net coal thickness,

and reliability categories. Resources are shown in millions of short tons (MST) and with two significant

figures. Zeros (0) indicate that no coal resources were calculated within those categories. The table does not

include coal resources in mine or lease areas, areas containing Wyodak-Anderson clinker, or areas containing

coal less than 2.5 ft thick. County outlines are shown on figure 8.

Overburden Net Reliability Categories (distance from data point) Grand

State County Thickness Coal

Thickness

Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

Montana BIG HORN 0-100 ft 2.5-5 ft 0 0.35 2.3 0 2.7

5-10 ft 0.26 0.20 10 0 10

10-20 ft 21 54 79 0 150

20-40 ft 120 420 240 0 780

>40 ft 650 2,100 1,100 0 3,900

0-100 ft Total 790 2,600 1,500 0 4,900

100-200 ft 2.5-5 ft 0 0.29 0.76 0 1.0

5-10 ft 0 1.9 6.6 0 8.4

10-20 ft 11 41 36 0 89

20-40 ft 92 350 220 0 670

>40 ft 1,000 3,200 1,400 0 5,700

100-200 ft Total 1,100 3,600 1,700 0 6,400

200-500 ft 2.5-5 ft 0.14 0.40 0.076 0 0.61

5-10 ft 0 4.1 0.55 0 4.6

10-20 ft 2.9 28 64 0 95

20-40 ft 27 140 430 0 600

>40 ft 1,500 7,300 5,100 0 14,000

200-500 ft Total 1,500 7,500 5,600 0 15,000

>500 ft 2.5-5 ft 0 0.25 0 0 0.25

5-10 ft 0.75 1.2 0 0 2.0

10-20 ft 0.95 0.15 0 0 1.1

20-40 ft 3.4 1.1 0 0 4.5

>40 ft 190 1,100 1,400 0 2,700

>500 ft Total 200 1,100 1,400 0 2,700

BIG HORN Total 3,600 15,000 10,000 0 29,000

POWDER

RIVER

0-100 ft 2.5-5 ft 0.42 1 19 4.1 25

5-10 ft 2.3 15 110 13 140

10-20 ft 59 260 450 32 800

20-40 ft 220 930 670 93 1,900

>40 ft 200 730 760 40 1,700

0-100 ft Total 490 1,900 2,000 180 4,600

100-200 ft 2.5-5 ft 0 0 3.8 0.52 4.3

5-10 ft 0.55 6.6 39 4.3 51

10-20 ft 2.0 47 140 14 200

20-40 ft 30 180 370 50 630

>40 ft 130 730 730 7.1 1,600

100-200 ft Total 160 970 1,300 75 2,500

Table 12. Continued on next page
43

Table 12. Continued

Overburden Net Reliability Categories (distance from data point) Grand

State County Thickness Coal

Thickness

Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

Montana POWDER 200-500 ft 2.5-5 ft 0 0 0.7 0 0.70

RIVER 5-10 ft 0 2.0 8.9 1.4 12

10-20 ft 0 3.4 27 9.2 40

20-40 ft 9.5 88 180 24 300

>40 ft 200 1,100 2,200 0 3,500

200-500 ft Total 210 1,100 2,400 34 3,800

>500 ft >40 ft 0 0.53 0 0 0.53

>500 ft Total 0 0.53 0 0 0.53

POWDER RIVER Total 860 4,100 5,700 290 11,000

ROSEBUD 0-100 ft 2.5-5 ft 0.38 0.47 6.3 1.5 8.6

5-10 ft 11 31 66 30 140

20-40 ft 29 120 180 0 320

>40 ft 62 330 220 0 610

0-100 ft Total 120 590 610 56 1,400

100-200 ft 2.5-5 ft 0 0.46 4.2 0.094 4.8

5-10 ft 2.4 15 24 8.3 50

10-20 ft 7.2 34 79 16 140

20-40 ft 9.4 89 110 0 210

>40 ft 75 340 340 0 750

100-200 ft Total 94 470 560 25 1,200

200-500 ft 2.5-5 ft 0 0 0.61 0 0.61

5-10 ft 0.097 6.2 9.6 4.0 20

10-20 ft 0 20 61 7.0 89

20-40 ft 3.4 3.9 19 0 26

>40 ft 11 82 27 0 120

200-500 ft Total 15 110 120 11 250

ROSEBUD Total 230 1,200 1,300 92 2,800

Montana Total 4,700 20,000 17,000 380 42,000

Wyoming CAMPBELL 0-100 ft 2.5-5 ft 3.6 9.6 57 12 83

5-10 ft 24 36 83 33 180

10-20 ft 140 140 300 32 620

20-40 ft 380 780 1,100 0 2,300

>40 ft 1,900 4,400 2,900 420 9,600

0-100 ft Total 2,500 5,300 4,500 490 13,000

100-200 ft 2.5-5 ft 1.1 5.4 14 0.58 21

5-10 ft 6.9 32 31 5.6 76

10-20 ft 55 180 140 2.7 380

20-40 ft 290 880 660 3.3 1,800

>40 ft 3,200 6,800 1,800 0 12,000

100-200 ft Total 3,500 7,900 2,600 12 14,000

200-500 ft 2.5-5 ft 3.5 9.5 5.3 0.73 19

5-10 ft 31 49 23 0.96 100

10-20 ft 120 360 200 0 680

20-40 ft 540 1,400 830 0 2,800

>40 ft 9,300 29,000 11,000 20 49,000

200-500 ft Total 10,000 30,000 12,000 22 52,000

Table 12. Continued on next page.
44

Table 12. Continued.

State County Overburden Net Reliability Categories (distance from data point) Grand

Thickness Coal

Thickness

Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

Wyoming CAMPBELL >500 ft 2.5-5 ft 0.25 3.8 9.0 0 13

5-10 ft 2.3 18 59 0 79

10-20 ft 1.3 37 160 0 200

20-40 ft 120 390 1,400 49 1900

>40 ft 13,000 57,000 120,000 6,900 200,000

>500 ft Total 13,000 58,000 130,000 6,900 200,000

CAMPBELL Total 29,000 100,000 140,000 7,500 280,000

CONVERSE 0-100 ft 2.5-5 ft 0.10 0.36 1.8 190 200

5-10 ft 1.6 21 80 620 720

10-20 ft 41 280 570 380 1,300

20-40 ft 92 440 650 22 1,200

>40 ft 46 100 280 0 430

0-100 ft Total 180 850 1,600 1,200 3,800

100-200 ft 2.5-5 ft 0.33 1.1 15 30 46

5-10 ft 1.4 17 89 120 230

10-20 ft 47 230 370 54 700

20-40 ft 76 360 440 88 960

>40 ft 65 270 100 0 440

100-200 ft Total 190 880 1,000 300 2,400

200-500 ft 2.5-5 ft 1.6 7.3 92 92 190

5-10 ft 20 97 290 710 1,100

10-20 ft 70 350 960 530 1,900

20-40 ft 41 260 730 230 1,200

>40 ft 150 480 230 0 860

200-500 ft Total 280 1,200 2,300 1,600 5,300

>500 ft 2.5-5 ft 2.3 14 16 2.4 35

5-10 ft 20 120 430 430 1,000

10-20 ft 22 150 590 1,000 1,800

20-40 ft 1.3 20 350 340 710

>40 ft 0 0 190 0 190

>500 ft Total 45 300 1,600 1,800 3,700

CONVERSE Total 700 3,200 6,500 4,900 15,000

JOHNSON 0-100 ft 2.5-5 ft 0 2.3 3.3 10 16

5-10 ft 0 5.6 13 9.9 28

10-20 ft 5.4 15 52 26 99

20-40 ft 33 62 42 10 150

>40 ft 32 82 150 0 270

0-100 ft Total 71 170 260 56 560

100-200 ft 2.5-5 ft 0 2.3 2.0 10 15

5-10 ft 0 4.0 4.3 12 20

10-20 ft 2.0 5.3 17 8.5 33

20-40 ft 9.2 42 100 7.9 160

>40 ft 30 130 220 0 380

100-200 ft Total 41 180 350 39 610

Table 12. Continued on next page
45

Table 12. Continued

Overburden Net Reliability Categories (distance from data point) Grand

State County Thickness Coal

Thickness

Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

Wyoming JOHNSON 200-500 ft 2.5-5 ft 0 2.9 4.7 36 44

5-10 ft 0.036 7.0 17 85 110

10-20 ft 5.6 25 74 38 140

20-40 ft 23 72 440 23 560

>40 ft 32 310 1,300 16 1,600

200-500 ft Total 61 410 1,800 200 2,500

>500 ft 2.5-5 ft 1.6 6.6 33 88 130

5-10 ft 5.7 49 500 420 970

10-20 ft 20 130 1,500 1,200 2,900

20-40 ft 85 620 5,000 3,200 9,000

>500 ft >40 ft 6,800 32,000 96,000 7,700 140,000

>500 ft Total 6,900 33,000 100,000 13,000 160,000

JOHNSON

Total

7,100 34,000 110,000 13,000 160,000

SHERIDAN 0-100 ft 2.5-5 ft 0.053 8.2 2.2 22 33

5-10 ft 12 40 11 37 100

10-20 ft 36 170 120 40 370

20-40 ft 45 230 420 5.5 700

>40 ft 100 350 740 0 1,200

0-100 ft Total 200 790 1,300 110 2,400

100-200 ft 2.5-5 ft 1.4 4.1 2.0 15 23

5-10 ft 4.6 11 14 51 81

10-20 ft 22 62 90 54 230

20-40 ft 98 410 660 46 1,200

>40 ft 290 620 850 81 1,800

100-200 ft Total 420 1,100 1,600 250 3,400

200-500 ft 2.5-5 ft 1.4 8.1 7.6 16 33

5-10 ft 6.7 23 20 49 98

10-20 ft 34 160 530 180 910

20-40 ft 260 1,100 3,400 370 5,100

>40 ft 840 3,700 6,500 220 11,000

200-500 ft Total 1,100 4,900 11,000 840 17,000

>500 ft 2.5-5 ft 0.51 3.0 4.7 87 95

5-10 ft 4.9 33 73 580 690

10-20 ft 23 140 760 1,600 2,500

20-40 ft 120 790 7,800 5,200 14,000

>40 ft 310 1,800 8,600 1,300 12,000

>500 ft Total 450 2,700 17,000 8,700 29,000

SHERIDAN Total 2,200 9,600 31,000 9,900 52,000

WY Total 39,000 150,000 290,000 35,000 510,000

Grand Total (MST) 44,000 170,000 300,000 36,000 550,000
46

Confidence Interval Estimation Procedure

A confidence interval is a statistic designed to capture uncertainty associated with a point

estimate. In this study we computed 90-percent confidence intervals on the volume of coal in the WyodakAnderson coal zone in the measured, indicated, inferred, and hypothetical categories.

The three main potential sources of error exist that might bias the confidence interval are

preferential sampling, measurement errors, and model fitting. The probabilistic interpretation of a

confidence interval is based upon a random sample, which does not occur in this situation, as there is

preferential sampling in those areas deemed to be minable. Measurement error can be caused by an error

in recording the coal bed thickness or in the definition of coverage areas. Modeling fitting variability and

bias result from the choice of models and fitting procedures.

Confidence limits for coal resources of the Wyodak-Anderson coal zone were calculated by Jack

Schuenemeyer and Helen Power using a data set containing net coal measurements from 4,462 locations

(only data from those locations that contained Wyodak-Anderson coal (no 0 values) and were

representative of the entire coal zone (no terminated holes)). The confidence limits were derived through a

complex series of steps. These steps included modeling coal thickness trends and removing the trends

using a nonparametric regression algorithm loess; using residual thickness to compute a semivariogram

and fitting the semivariogram to an exponential model to determine measurement error; calculating

standard deviations of coal thickness from the variogram model; compensating for differences in point

density by calculating a pseudo n within each reliability category and calculating the variability of volume

for each of the reliability categories; and calculating the volumes of Wyodak-Anderson coal at a 90-

percent confidence interval with measurement error. Some of the parameters used and results of the

confidence limit calculations are shown in tables 13 and 14. A detailed description of the methodology

used is given in Schuenemeyer and Power (1998), and in Ellis and others, 1998.

Table 13. Data used for computation of confidence intervals within reliability categories for the WyodakAnderson coal zone. Rows may not sum due to independent rounding.

Parameter Reliability Category Entire

Measured Indicated Inferred Hypothetical Area

Area (in square meters) 1,488,759,443 5,670,020,546 11,140,313,431 3,612,068,158 21,911,161,578

Percent of area 7 26 51 16 100

Acres 367,880 1,401,092 2,752,831 892,561 5,414,365

SD (Standard deviation

(in ft) from variogram

model)

18.38 22.78 26.67 26.89

Acre feet (Acres x SD) 6,759,910 31,921,428 73,423,804 24,000,492

Volume standard

deviation (MST)

227 1,634 10,675 43,028 55,564

Pseudo n (Minimum

number of points in the

area)

2,928 1,239 152 1
47

Table 14. Estimates of uncertainty for the Wyodak-Anderson coal with measurement error. Resource

calculations are in millions of short tons (MST) with four significant figures. Rows may not sum due to

independent rounding.

Parameter Reliability Category Entire

Measured Indicated Inferred Hypothetical Area

Volume (in millions of short tons (MST)) 43,670 167,700 303,700 35,610 550,700

Lower 90% confidence bound (MST) 43,300 165,000 286,100 0 459,300

Upper 90% confidence bound (MST) 44,050 170,400 321,200 106,400 642,100

CONCLUSION

The coal resources for the Wyodak-Anderson coal zone are approximately 550,000 million short

tons with a lower confidence limit (at the 90-percent confidence interval) of 460,000 million short tons and

an upper confidence limit (at the 90-percent confidence interval) of 640,000 million short tons. These

calculations represent net coal (all coal beds greater or equal to 2.5 ft in thickness added together at each

data point) and does not include coal in mine or lease areas, within areas of mapped clinker, or areas

where coal thickness was extrapolated to be less than 2.5 feet in thickness. All coal was considered to be

subbituminous in apparent rank, with a conversion factor of 1,770 short tons per acre-foot.

This report represents one aspect of the National Coal Resource Assessment for the WyodakAnderson coal zone. The production of comprehensive stratigraphic and coal quality data sets containing

detailed correlations, digital data in the form of layered and attributed spatial data, and other reports on

various aspects of this study, which will be available to the public, add a new dimension to future

investigations.

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