Chapter PG

GILLETTE COALFIELD, POWDER RIVER BASIN:

GEOLOGY, COAL QUALITY, AND COAL RESOURCES

By M.S. Ellis,1 R.M. Flores,1 A.M. Ochs,2 G.D. Stricker,1 G.L. Gunther,2

G.S. Rossi,1 L.R. Bader,

1

 J.H. Schuenemeyer,

1

 and H.C. Power

3

in U.S. Geological Survey Professional Paper 1625-A

1

U.S. Geological Survey

2

Consultant, U.S. Geological Survey, Denver, Colorado

3

University of Delaware, Newark, Delaware
PG-ii

Contents

Structural and Stratigraphic Setting....................................................................PG-1

Depositional Setting............................................................................................PG-3

Wyodak-Anderson Coal Zone—Gillette Coalfield ...........................................PG-3

Gillette Coalfield Structural Cross Section A-A’..........................................PG-3

Gillette Coalfield Structural Cross Section B-B’...........................................PG-5

Gillette Coalfield Structural Cross Section C-C’...........................................PG-6

Gillette Coalfield Structural Cross Section D-D’..........................................PG-7

Gillette Coalfield Structural Cross Section E-E’...........................................PG-8

Gillette Coalfield Structural Cross Section F-F’............................................PG-8

Gillette Coalfield Structural Cross Section G-G’..........................................PG-9

Gillette Coalfield Structural Cross Section H-H’........................................PG-10

Gillette Coalfield Structural Cross Section I-I’ ...........................................PG-10

Coal Quality .....................................................................................................PG-11

Coal Quality—Subareas .............................................................................PG-11

Gillette Coalfield, Wyodak-Anderson Coal Quality ...................................PG-12

Coal Resources—Wyodak-Anderson Coal Zone, Gillette Coalfield ...............PG-15

Resource Estimates—An Overview ...........................................................PG-15

Wyodak-Anderson Coal Resources.............................................................PG-16

Confidence Limits for Resource Calculations ............................................PG-18

References.........................................................................................................PG-20

Figures

PG-1. Geologic map of the Gillette coalfield, Powder River Basin.

PG-2. Index map showing locations of Gillette coalfield cross sections and

mines.
PG-iii

Figures—continued

PG-3. Map showing clinker in the Gillette coalfield, Powder River Basin.

PG-4. Fluvial channel sandstone above the Wyodak–Anderson coal zone.

PG-5. Vertically stacked, multierosional, fluvial sandstone above the WyodakAnderson coal zone.

PG-6. Stacked fluvial channel sandstone containing crossbeds.

PG-7. Crevasse-splay sandstone, siltstone, and mudstone above the

Wyodak–Anderson coal zone.

PG-8. Crevasse channel sandstone lateral to crevasse-splay sandstone, siltstone,

and mudstone above the Wyodak–Anderson coal zone.

PG-9. A thick fluvial channel sandstone with deep erosional base above the

Wyodak–Anderson coal zone.

PG-10. Lateral accretion units in the fluvial channel sandstone.

PG-11. Depositional model of anastomosed streams for the Wyodak–Anderson

coal zone in the Gillette coalfield.

PG-12. A peat deposit near the margin of a raised mire in Central Kalimantan,

Borneo, Indonesia.

PG-13. A river channel at the edge of a raised mire in Sumatra, Indonesia.

PG-14. Gillette coalfield structural cross section A-A’.

PG-15. Modern analog of a peat forming, raised swamp in the Atchafalaya River

Basin, Louisiana.

PG-16. Modern analog of a peat-forming swamp in the Atchafalaya River Basin,

Louisiana.

PG-17. Lateral migration of a fluvial channel caused by avulsion and

abandonment of the Santee River in South Carolina.

PG-18. Gillette coalfield structural cross section B-B’.

PG-19. Gillette coalfield structural cross section C-C’.
PG-iv

Figures—continued

PG-20. Gillette coalfield structural cross section D-D’.

PG-21. Gillette coalfield structural cross section E-E’.

PG-22. Gillette coalfield structural cross section F-F’.

PG-23. Gillette coalfield structural cross section G-G’.

PG-24. Gillette coalfield structural cross section H-H’.

PG-25. Gillette coalfield structural cross section I-I’.

PG-26. Locations of coal quality data points.

PG-27. Ash yield in the Wyodak-Anderson coal zone.

PG-28. Sulfur content in the Wyodak-Anderson coal zone.

PG-29. Moist-mineral-matter free Btu/lb in the Wyodak-Anderson coal zone.

PG-30. Pounds of sulfur dioxide per million Btu in the Wyodak-Anderson coal

zone.

PG-31. Antimony concentration in the Wyodak-Anderson coal zone.

PG-32. Arsenic concentration in the Wyodak-Anderson coal zone.

PG-33. Beryllium concentration in the Wyodak-Anderson coal zone.

PG-34. Cadmium concentration in the Wyodak-Anderson coal zone.

PG-35. Chromium concentration in the Wyodak-Anderson coal zone.

PG-36. Cobalt concentration in the Wyodak-Anderson coal zone.

PG-37. Lead concentration in the Wyodak-Anderson coal zone.

PG-38. Manganese concentration in the Wyodak-Anderson coal zone.

PG-39. Mercury concentration in the Wyodak-Anderson coal zone.

PG-40. Nickel concentration in the Wyodak-Anderson coal zone.

PG-41. Selenium concentration in the Wyodak-Anderson coal zone.

PG-42. Uranium concentration in the Wyodak-Anderson coal zone.

PG-43. Location of 7.5-minute quadrangle maps in the Wyodak-Anderson study

limit of the Gillette coalfield.
PG-v

Figures—continued

PG-44. Federal coal and Federal surface ownership in the Gillette coalfield.

PG-45. Wyodak-Anderson net coal isopach map for the Gillette coalfield.

PG-46. Wyodak-Anderson overburden isopach map for the Gillette coalfield.

PG-47. Wyodak-Anderson mine and lease areas, clinker, and resource area in the

Gillette coalfield.

Tables

PG-1. Summary data for Wyodak-Anderson coal zone in the Gillette coalfield,

Wyoming

PG-2. Wyodak-Anderson coal resources in the Gillette coalfield, reported by

county, overburden thickness, net coal thickness, and reliability categories

PG-3. Wyodak-Anderson coal resources in the Gillette coalfield, Wyoming,

reported by county and by Federal coal and surface ownership

PG-4. Wyodak-Anderson coal resources in the Gillette coalfield, Wyoming

reported by 7.5-minute quadrangle map area in each county

PG-5. Data used for computation of confidence intervals within reliability

categories for the Wyodak-Anderson coal zone in the Gillette coalfield

PG-6. Estimates of uncertainty (calculated with measurement error) for WyodakAnderson coal resources in the Gillette coalfield, Powder River Basin,

Wyoming
PG-1

STRUCTURAL AND STRATIGRAPHIC SETTING

The Gillette coalfield (fig. PG-1) contains the Fort Union Formation (Paleocene)

that includes, from bottom to top, the Tullock, Lebo, and Tongue River Members.

The Tongue River Member is mainly mapped in the northern part of the coalfield

(see fig. PG-1) Mapel, 1973; Law, 1974, 1975, 1978; Kent and Berlage, 1980;

Love and others, 1990).

The Eocene Wasatch Formation, which was interpreted by Dobbin and Barnett

(1927) as unconformably overlying the Fort Union Formation, is in the western

part of the coalfield. An erosion surface is at the bottom of the thick channel

sandstone beds immediately above the Wyodak-Anderson coal zone. The contact

between these formations is not delineated in the subsurface because they have

basically the same lithology and are difficult to correlate in the absence of the

channel sandstone. The surface contact between the Wasatch and Fort Union

Formations is placed above the Roland coal bed above the channel sandstone.

These rocks dip 2-3 degrees westward.

The Gillette coalfield boundary (see fig. PG-1) approximately follows the area

defined by the U.S. Geological Survey (for example, Keefer and Schmidt, 1973;

Denson and Keefer, 1974) to include early mining activity, as revised by Jones

(1990; 1991).

In this study, the Gillette coalfield (see fig. PG-1) is delineated by the surface

contact line of the Wyodak-Anderson coal and associated coal/clinker burn line on

the east (Heffern and Coates, 1997; Boyd and ver Ploeg, 1997), the closest

coordinate (T. 54 N. and T. 53 N.) to the northernmost mine on the north (fig. PG-
PG-2

2), the closest coordinate (T. 39 N. and T. 38 N.) to the southernmost mine on the

south, and the closest coordinate (R. 74 W. and R. 73 W.) to the mines on the west.

The coal beds of the Wyodak-Anderson coal zone of the Tongue River Member of

the Fort Union Formation are mined at the surface in the Gillette coalfield. These

coal beds are variously identified by the mine operators as the Smith, Anderson,

Wyodak, and Canyon.

Clinker exists along the easternmost outcrops of the Wyodak-Anderson coal zone

(see fig. PG-3). Clinker is an orange-red rock produced from the burning of coal

and related rocks due to spontaneous combustion. Approximately a 250-squaremile area of clinker covers the eastern boundary of the Gillette coalfield from east

of Gillette, Wright, and Rochelle Hills (Heffern and Coates, 1997). The clinker in

the Rochelle Hills east of Wright was dated as being from 700,000 to 18,000 years

before present from downdip to updip toward the burn line using fission-track ages

of zircon grains (Coates and Naeser, 1984; Heffern and Coates, 1997).

The coal beds of the Wyodak-Anderson coal zone exist as merged or split, or as an

individual bed. The coal beds of the Wyodak-Anderson coal zone merge to form a

single bed as much as 145 ft thick. They pinch out into mudstone, siltstone, and

sandstone.

The coal beds of the Wyodak-Anderson coal zone are commonly broken up by

pinch outs along a north-south direction and commonly split westward or

basinward. Coal pinch outs produced pod-like to lenticular-shaped beds.
PG-3

DEPOSITIONAL SETTING

The coal beds are interbedded with carbonaceous shale, mudstone, siltstone,

limestone, and sandstone. The depositional environments of these rocks types

were interpreted by Budai and Cummings (1987).

The Wyodak-Anderson coal zone is split by floodplain mudstone, siltstone, silty

sandstone, and associated fluvial channel sandstone as seen in the highwall at the

Antelope coal mine (fig. PG-4). The coal zone is overlain by vertically stacked

and multi-erosional sandstone deposited in fluvial channels, as seen in the highwall

at the Belle Ayr coal mine (figs. PG-5 and PG-6). This fluvial channel sandstone

grades laterally into floodplain and crevasse-splay mudstone, siltstone, and silty

sandstone as exhibited in the highwall of the Cordero Rojo coal mine (figs. PG-7

and PG-8). The fluvial channel sandstone is thick and crossbedded and has a deep

erosional base as seen in the highwall of the Buckskin coal mine (figs. PG-9 and

PG-10)

The vertical and lateral variations, thickness, distribution, and shape of the

Wyodak-Anderson coal beds reflect their deposition in raised mires formed in

meandering and anastomosed fluvial systems (figs. PG-11, PG-12, and PG-13;

Warwick and Stanton, 1986, 1988).

WYODAK ANDERSON COAL ZONE—GILLETTE COALFIELD

GILLETTE COALFIELD STRUCTURAL CROSS SECTION A-A’

• The north-south oriented structural cross section A-A’ (fig. PG-14) is 80 mi

long. The Wyodak-Anderson coal zone shows merging, splitting, and re-
PG-4

splitting of as many as two to eight coal beds. The cross section was drawn

using data from 185 drill holes, which are not labeled on the cross section.

• Coal beds are interbedded with, and split by, mudstone (shown in green), which

was deposited in floodplains and floodplain lakes, and sandstone (shown in

yellow), which was deposited in fluvial channels.

• The coal beds commonly pinch out into these fluvial mudstone and sandstone

deposits.

• Coal beds are lenticular in the southern part of the cross section and in the

middle and southern subareas of the Gillette coalfield.

• Coal beds in the northern part of the cross section, in the northern subarea of the

Gillette coalfield, display pod and lenticular geometry.

• The lenticular coal beds in the southern part of the cross section split and

merge, forming a zigzag pattern.

• The peat deposits (precursor of coal) accumulated in interchannel raised mires.

The areal size and shape of the peat in these mires was influenced by the types

and patterns of surrounding streams.

• Merging and diverging fluvial channels of anastomosed streams yielded small,

isolated, interchannel mires that produced pod-shaped peat deposits (figs. PG15 and PG-16).
PG-5

• Lateral migration and avulsion, or shift of fluvial channels of meandering

streams (fig. PG-17) created widespread mires on interchannel and abandoned

fluvial deposits that produced lenticular peat deposits.

• A combination of raised mires, confinement of the fluvial channels by peat

deposits, local and regional subsidence, and rain-fed mires controlled the

accumulation of thick coal beds and kept the ash content low.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone consist of intervals more than 300 ft

thick. The intervals include interbedded fluvial-channel sandstone, and

overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION B-B’

• The west-east oriented structural cross section B-B’ (fig. PG-18) is more than 8

mi long. The Wyodak-Anderson coal zone, an interval as much as 350 ft thick,

contains merging and splitting coal beds that dip to the west. The cross section

was drawn using data from 20 drill holes, which are not labeled on the cross

section.

• The coal beds merge toward the east to form a bed more than 100 ft thick.

• Toward the west, coal beds are interbedded with, and split by, mudstone (shown

in green), which was deposited in floodplains and floodplain lake (fig. PG-16),

and by sandstone (shown in yellow), which was deposited in fluvial channels

and crevasse splays.
PG-6

• The peat deposits accumulated in interchannel raised mires, which encroached

and occupied abandoned fluvial-channel and floodplain deposits. The peat

deposits formed in these mires produced low-ash and low-sulfur coal.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone are as much as 400 ft thick and more

than 400 ft thick, respectively. They are composed of interbedded fluvialchannel sandstone, and overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION C-C’

• The west-east oriented structural cross section C-C’ (fig. PG-19) is 12 mi long.

The Wyodak-Anderson coal zone, as much as 400 ft thick, contains merging

and splitting coal beds that dip westward. The cross section was drawn using

data from 20 drill holes, which are not labeled on the cross section.

• The coal beds merge eastward to form a bed about 200 ft thick.

• Toward the west, coal beds are interbedded with, and split by, mudstone (shown

in green), which was deposited in floodplains and floodplain lakes, and by

sandstone (shown in yellow), which was deposited in fluvial channels and

crevasse splays.

• The coal beds merge, split, and pinch out, to form a zigzag pattern. This pattern

yields a discontinuous distribution of coal.
PG-7

• The peat deposits accumulated in interchannel raised mires, which are adjacent

to abandoned fluvial channel and floodplain deposits. Raised mires formed

coal with low-ash and low-sulfur contents.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone are as much as 300 ft thick and more

than 500 ft thick, respectively. The rocks consist of interbedded fluvial-channel

sandstone, and overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION D-D’

• The west-east oriented structural cross section D-D’ (fig. PG-20) is 15 mi long.

The Wyodak-Anderson coal zone, as much as 450 ft thick, contains coal beds

that merge, split, pinch out, and dip to the west. The cross section was drawn

using data from 17 drill holes, which are not labeled on the cross section.

• The coal beds merge into a bed more than 100 ft thick and pinch out eastward

into fluvial-channel sandstone (shown in yellow).

• The coal beds are interbedded with, and split by, mudstone (shown in green),

which was deposited on floodplains and in floodplain lakes, and by sandstone

(yellow), which was deposited in fluvial channels.

• The peat deposits accumulated in interchannel raised mires flanked by deposits

of a fluvial-channel complex.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone are as much as 450 ft thick and about
PG-8

400 ft thick, respectively. The rocks are composed of interbedded fluvialchannel sandstone, and overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION E-E’

• The west-east oriented structural cross section E-E’ (fig. PG-21) is 13 mi long.

The Wyodak-Anderson coal zone, more than 200 ft thick, contains a merged

coal bed that dips westward. The cross section was drawn using data from 15

drill holes, which are not labeled on the cross section.

• The merged coal bed is about 80 ft thick.

• The coal bed is interbedded with, and split by, mudstone (shown in green) that

was deposited on floodplains and in floodplain lakes.

• The peat deposits accumulated in interchannel raised mires.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone are more than 750 ft thick, and are

composed of interbedded fluvial-channel sandstone, and overbank, floodplain,

and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION F-F’

• The west-east oriented structural cross section F-F’ (fig. PG-22) is more than 15

mi long. The Wyodak-Anderson coal zone, more than 125 ft thick, contains

merging and splitting coal beds that dip westward. The cross section was

drawn using data from 15 drill holes, which are not labeled on the cross section.
PG-9

• The coal beds merge westward into a single bed that is about 130 ft thick.

• To the east the coal beds are interbedded with, and split by, mudstone (shown in

green) that was deposited on floodplains and in floodplain lakes.

• The peat deposits accumulated in interchannel raised mires.

• The undifferentiated Fort Union rocks above and Fort Union rocks below the

Wyodak-Anderson coal zone are more than 400 ft thick and 1,100 ft thick,

respectively. The intervals are composed of interbedded fluvial-channel

sandstone, and overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION G-G’

• The west-east oriented structural cross section G-G’ (fig. PG-23) is 13 mi long.

The Wyodak-Anderson coal zone, more than 100 ft thick, contains coal beds

that merge, split, pinch out, and dip westward. The cross section was drawn

using data from 20 drill holes, which are not labeled on the cross section.

• The coal beds merge to form a bed about 90 ft thick.

• The coal bed is interbedded with, and split by, mudstone (shown in green) that

was deposited on floodplains and in floodplain lakes.

• The peat deposits accumulated in interchannel raised mires.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone are more than 600 ft thick and 500 ft
PG-10

thick, respectively. The intervals are composed of interbedded fluvial-channel

sandstone, and overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION H-H’

• The west-east oriented structural cross section H-H’ (fig. PG-24) is 17 mi long.

The Wyodak-Anderson coal zone, more than 100 ft thick, contains coal beds

that merge, split, pinch out, and dip westward. The cross section was drawn

using data from 16 drill holes, which are not labeled on the cross section.

• The coal beds merge eastward into a single bed that is about 80 ft thick.

• The coal bed is interbedded with, and split by, mudstone (shown in green) that

was deposited on floodplains and in floodplain lakes.

• The peat deposits accumulated in interchannel raised mires.

• The rocks above and below the Wyodak-Anderson coal zone are more than 600

ft thick and 350 ft thick, respectively. They are composed of interbedded

fluvial-channel sandstone, and overbank, floodplain, and lacustrine mudstone.

GILLETTE COALFIELD STRUCTURAL CROSS SECTION I-I’

• The west-east oriented structural cross section I-I’ (fig. PG-25) is 8 mi long.

The Wyodak-Anderson coal zone contains as many as four coal beds in an

interval more than 300 ft thick that dip westward. The cross section was drawn

using data from 17 drill holes, which are not labeled on the cross section.
PG-11

• The coal beds are as much as 35 ft thick.

• The coal beds are interbedded with, and split by, mudstone (shown in green),

which was deposited in floodplain lakes, floodplains, and crevasse splays, and

sandstone (shown in yellow), which was deposited in fluvial channels.

• The peat deposits accumulated in interchannel raised mires and low-lying

swamps.

• The undifferentiated Fort Union and Wasatch rocks above and Fort Union rocks

below the Wyodak-Anderson coal zone are more than 200 ft thick and 400 ft

thick, respectively. The rocks consist of interbedded fluvial-channel sandstone,

and overbank, floodplain, and lacustrine mudstone.

COAL QUALITY

COAL QUALITY—SUBAREAS

• The Gillette coalfield may be divided from north to south into three subareas

(see figure PG-2) based on the quality of the Wyodak-Anderson coal.

• The northern subarea from north to southeast of the city of Gillette contains

coal with a total average heat value of produced coal of 8,326 Btu/lb (Resource

Data International, Inc., 1998).

• The central subarea south of the city of Gillette and to the east of the town of

Wright contains coal with a total heat value of produced coal of 8,445 Btu/lb

(Resource Data International, Inc., 1998).
PG-12

• The southern subarea east-southeast of the town of Wright contains coal with a

total heat value of produced coal of 8,764 Btu/lb (Resource Data International,

Inc., 1998).

• The higher value of Btu/lb in coal of the southern subarea may have been

caused by deeper burial of the peat deposits that formed the Fort Union coal.

GILLETTE COALFIELD, WYODAK-ANDERSON COAL QUALITY

Coal from the Wyodak-Anderson coal zone is produced from 19 mines in the

Gillette coalfield, Wyoming. This coal is utilized for out-of-state electric power

generation in power plants in 26 states. Wyodak-Anderson coal is sometimes

blended with higher sulfur coals to produce a compliant fuel.

Coal in the Wyodak-Anderson coal zone in the Gillette coalfield in the Powder

River Basin, Wyoming, is considered to be ‘clean coal’. This coal zone (see

chapter PS in this CD-ROM) for a description of the coal zone) contains lowcontaminant, compliant, subbituminous apparent rank coal. Analyses of the coal

have the following arithmetic mean values (on an as-received basis):

moisture—27.47%, ash—7.45%, total sulfur—0.48%, heat value—8,220

(Btu/lb), lb SO2 per million Btu—1.25, and moist, mineral-matter-free

Btu—8,910. Arithmetic mean values (on a whole coal, parts per million, and

remnant moisture basis) of elements of environments concern for the WyodakAnderson coal zone are: antimony—0.72, arsenic—2.3, beryllium—0.35,

cadmium—0.39, chromium—7.0, cobalt—2.3, lead—2.9, manganese—22,

mercury—0.17, nickel—5.0, selenium—1.4, and uranium—1.5.
PG-13

Summary data for the 18 variables mentioned in the previous paragraph were

calculated for the Wyodak-Anderson coal zone using all available data. However,

some of the coal quality data in this basin is proprietary. This type of data, along

with public data, is used for the summary table (table PG-1), but is not shown on

location maps or graphic displays. The locations of public data points used in the

summary are shown on figure PG-26.

A common problem in statistical summaries of trace-element data arises when

element values are below the limits of detection. This results in a censored

distribution. To compute unbiased estimates of censored data for the elements in

this table, we adopted the protocol of reducing all ‘less than’ values by 50 percent

to generate a real value for these data. Summary statistics of range (minimum and

maximum value) and arithmetic means were generated using the modified data.

Moisture values are reported on an as-received basis (ASTM, 1996, designation

D388-6). Because no equilibrium moisture values are available for this report,

apparent ranks can not reliably be determined.

Between 1974 and 1994, the U.S. Geological Survey analyzed samples of coal for

contents of major-, minor-, and trace-elements. Prior to performing the analyses

most of the coal samples were dried at room temperature and humidity for up to 80

hours. However, some samples may have been dried only long enough to allow

grinding (to less than 100 mesh). Moisture content in the above mentioned

samples is unknown, although moisture contents were probably similar to that

which would remain after air-dry loss determination (ASTM Standards, 1991,

D3302). Since the actual moisture content of the samples analyzed between 1974

and 1994 is unknown, the elemental analysis of the samples cannot be converted to

any other moisture basis. In addition, these analyses can only provide an

approximation of load factors (such as, grams of mercury per trillion Btu).
PG-14

For graphic display of public analytical data, data points were color keyed to show

the values of the 17 variables, moisture and calorific value (Btu/lb) are not shown.

When more than one analysis was available per location, the analytical values were

weight averaged on thickness. For ash and total sulfur (figs. PG-27 and PG-28),

the values are keyed to low, medium, and high, following guidelines established in

U.S. Geological Survey Circular 891 (Wood and others, 1983). For calorific value

as represented by moist, mineral-matter-free Btu (apparent rank) (fig. PG-29),

apparent rank designations established in ASTM (1994) were utilized. For pounds

of SO2 per million Btu (fig. PG-30), values are keyed according to the

Environmental Protection Agency’s Phase I, Phase II, and non-compliant limits for

sulfur emission from coal fired power plants.

No guidelines have been established for elements of environmental concern (also

referred to as ‘hazardous air pollutants’ or ‘HAPs’). Analytical values for these

elements (figs. PG-31 through PG-42) are coded based on the following

parameters: (1) each element of environmental concern was ranked from the

lowest to the highest value for all data in the northern Rocky Mountains and Great

Plains region; and (2) quartiles were established for each element. Low represents

those values that are less than the .25 quartile (also known as the lower quartile or

the 25th percentile). Medium represents those values that are within the .25 to .75

quartile (or between the 25th to 75th percentile). High represents those values that

are in the upper .25 quartile (or greater than the 75th percentile).
PG-15

COAL RESOURCES—WYODAK-ANDERSON COAL ZONE

GILLETTE COALFIELD

RESOURCE ESTIMATES—AN OVERVIEW

• Coal resources are calculated using the specific gravity of the coal calculated

from apparent coal rank, which is the weight of coal per unit volume, net coal

thickness, and areal extent of the coal.

• Resource tables for the Wyodak-Anderson coal zone in the Gillette coalfield

include coal and overburden thickness categories from Wood and others (1983),

which are based on apparent coal rank. Additional categories have been added

to provide more detail in this area. Resources are also reported by county,

Federal coal and surface ownership categories, and 7.5-minute quadrangle.

• Following USGS published guidelines (Wood and others, 1983); coal resource

estimates are divided into measured, indicated, and inferred categories

according to relative abundance and reliability of data.

• Where data are widely spaced, a hypothetical resource may be extrapolated.

• Measured resources are tonnage estimates of coal in the coal zone within a

radius of 0.25 mi of a control point where the net thickness of coal is measured.

• Indicated resources are tonnage estimates of coal that is within a radius of 0.25-

0.75 mi of a control point where the net thickness of the coal is measured.

• Inferred resources are tonnage estimates of coal that is within a radius of 0.75-3

mi of a control point where the net thickness of the coal is measured.
PG-16

• Hypothetical resources are tonnage estimates of coal that is beyond a radius of 3

mi of a control point where the net thickness of coal is measured.

• These resource categories assume a high to low degree of geologic certainty. A

statistical method, which measures levels of uncertainty (confidence limits) for

the Wyodak-Anderson resource estimates in the Gillette coalfield, is also

included in this study.

• Resource estimates are reported in millions of short tons with two significant

figures.

WYODAK-ANDERSON COAL RESOURCES

The lateral extent (study limit) of the Wyodak-Anderson coal zone in the Gillette

coalfield is based on the Wyodak-Anderson coal outcrop and Wyodak-Anderson

clinker to the east from published maps by Kent and Berlage (1980), and Heffern

and others (1993; and unpublished data, undated). The eastern boundary was

generalized in some areas to include small areas containing Wyodak-Anderson

coal that were outside of the main boundary. The contact of Cretaceous and

Tertiary rocks (Love and Christiansen, 1985) was used in some areas where

mapped coal outcrop was not available. The northern, southern, and western

boundaries of the study area were delineated by the closest Township and Range

lines (T54-53N, T39-38 N, R 74-73 W respectively) outside of the active Gillette

coalfield mine areas. The total study area is about 1,026,000 acres (415,208

hectares) in size.
PG-17

Wyodak-Anderson coal resources in the Gillette coalfield, Powder River Basin,

Wyoming were calculated using several software packages and custom programs.

Details of the methodology used are given in Ellis and others (1999, in press).

To calculate the Wyodak-Anderson coal resources in the Gillette coalfield data was

compiled in a StratiFact* (GRG Corporation, 1996) relational database. The coal

beds in the Wyodak-Anderson coal zone, including the Smith, Anderson, Canyon,

Werner, Upper Werner, Swartz, and Sussex beds (see the explanation of coal bed

nomenclature in chapter PS of this CD-ROM), were correlated in the database. A

custom program was used to calculate net coal thickness at each data point (drill

hole or measured section) location.

The net coal thickness and overburden were gridded, and isopach maps were

produced using EarthVision* (Dynamic Graphics, Inc., 1997) software. The grids

were made using an isopach grid option (special handling of 0 values and

terminated data) with a grid spacing of 200 x 200-meters.

The spatial parameters for querying the coal resources (for example, 7.5-minute

quadrangle map area (fig. PG-43; U.S. Geological Survey National Mapping

Division unpublished data, undated), Federal coal and surface ownership (fig. PG44; Biewick and others, 1998), reliability, coal thickness (fig. PG-45), and

overburden categories (fig. PG-46)) were created on individual layers as

ARC/INFO* (ESRI, 1998a) polygon coverages. The coverages were unioned to

make one polygon coverage with many attributes for each polygon. The polygons

in the union coverage were edited in ARC/INFO* and ArcView* (ESRI, 1998b).

Coal resources were calculated using the EarthVision* (EV) volumetrics tool,

which calculates tonnages in each polygon in the union coverage using the net coal
PG-18

thickness grid, the area of each polygon, and a conversion factor of 1,770 short

tons per acre-ft for subbituminous rank coal (Wood and others, 1983). Data from

the EV volumetrics report and the union coverage polygon attribute table were

combined in Excel* (Microsoft, 1997) spreadsheet software. Data for polygons

containing Wyodak-Anderson clinker (fig. PG-47) (Heffern and others, 1993; Boyd

and others, 1997; Heffern unpublished data, undated), Wyodak-Anderson mine or

lease areas (fig. PG-47) (Bureau of Land Management, 1996) or areas of net coal

less than 2.5 ft in thickness were deleted from the data set. Lease areas may include

public and/or state leases in addition to Federal leases. Resource tables were

created using data from the remaining polygons (tables PG-2 through PG-4). The

final resource area (fig. PG-47) (area that met all coal resource criteria) was about

892,000 acres (360,980 hectares) in area.

*Commercial Software package.

CONFIDENCE LIMITS FOR RESOURCE CALCULATIONS

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

a point value estimate. In this study we computed 90-percent confidence intervals

on the volume (total resource in millions of short tons) of coal in the WyodakAnderson coal zone in the measured, indicated, inferred, and hypothetical

categories.

The three main potential sources of error that might bias the confidence intervals

are preferential sampling, measurement errors, and model fitting. The probabilistic

interpretation of a confidence interval is based upon a random sample, which does

not apply in this situation, because there is preferential sampling in those areas

deemed to be minable. Measurement error can be caused by an error in recording
PG-19

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 in the

Gillette coalfield were calculated by J.H. Schuenemeyer and H.C. Power,

University of Delaware. The data set that they used contained net coal

measurements from 2,009 locations. This data set only included locations that

contained Wyodak-Anderson coal (no 0 values) and data that 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 coal thickness trends

using a nonparametric regression algorithm called loess (with span=0.5), using

residual thickness to compute a semivariogram, and fitting the semivariogram to a

spherical model. Parameter estimates were sill=393.98 ft2, nugget=299.14 ft2, and

range=1.69 miles. Standard deviations of coal thickness were obtained from the

semivariogram model. Differences in point densities were compensated for by

calculating sample size, called a pseudo n, within each reliability category and

calculating the variability of volume for each of the reliability categories.

Volumes of Wyodak-Anderson coal were then calculated at a 90-percent

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

results of the confidence interval calculations are shown in tables PG-4 and PG-5.

A detailed description of the methodology used is given in Schuenemeyer and

Power (in press) and in Ellis and others (1999, in press).
PG-20

REFERENCES

American Society for Testing and Materials, 1994a, Annual book of ASTM

Standards, Section 5, Petroleum products, lubricants and fossil fuels, vol.

05.05 Gaseous fuels; coal and coke; section D388-92a; Standard

Classification of Coal by Rank: American Society for Testing and

Materials, Philadelphia, Pennsylvania, p. 168-171.

_____1994b, Annual book of ASTM Standards, Section 5, Petroleum products,

lubricants and fossil fuels, vol. 05.05 Gaseous fuels; coal and coke; section

D3180-89; Standard Practice for Calculating Coal and Coke Analysis from

As-Determined to Different Bases: American Society for Testing and

Materials, Philadelphia, Pennsylvania, p. 318-320.

_____1994c, Annual book of ASTM Standards, Section 5, Petroleum products,

lubricants and fossil fuels, vol. 05.05 Gaseous fuels; coal and coke; section

D3302-91; Standard Test Method for Total Moisture in Coal: American

Society for Testing and Materials, Philadelphia, Pennsylvania, p. 330-336.

Biewick, L.R.H., Urbanowski, S.R., Cain, Sheila, and Neasloney, Larry, 1998,

Land status and Federal mineral ownership in the Powder River Basin,

Wyoming and Montana: a digital data set for geographic information

systems: U.S. Geological Survey Open-File Report 98-108, coverage scale

1:100,000.

Boyd, C.S., and ver Ploeg, A.J., 1997, Geologic map of the Gillette 30’ x 60’

quadrangle, Campbell, Crook, and Weston Counties, northeastern Wyoming:

Wyoming State Geological Survey Map Series 49, scale 1:100,000.

Bragg, L.J., Oman, J.K., Tewalt, S.J., Oman, C.L., Rega, N.H., Washington, P.M.,

and Finkleman, R.B, 1994, U.S. Geological Survey Coal Quality (Coalqual)

Database: Version 1.3: U.S. Geological Survey Open-File Report 94-205,

CD-ROM.
PG-21

Bryson, R.P., and Bass, N.W., 1973, Geology of the Moorhead coalfield, Powder

River, Big Horn, and Rosebud Counties, Montana: U.S. Geological Survey

Bulletin 1338, 116 p.

Budai, C.M., and Cummings, M.L., 1987, A depositional model of the Antelope

coalfield, Powder River Basin, Wyoming: Journal of Sedimentary

Petrology, v. 57, p. 30-38.

Bureau of Land Management, 1996, Powder River Basin Coal Production—1995;

N. Braz, C. Gaskill and R. Nelson compilers, U.S. Department of the

Interior, Bureau of Land Management, Casper, Wyoming, file

D\drawings\wo-map.dwg.

Coates, D.A., and Naeser, C.W., 1984, Map showing fission-track ages of clinker

in the Rochelle Hills, southern Campbell and Weston Counties, Wyoming:

U.S. Geological Survey Miscellaneous Investigations Series Map I-1462,

scale 1:50,000.

Denson, N.M., and Keefer, W.R., 1974, Map of the Wyodak-Anderson coal bed in

the Gillette area, Campbell County, Wyoming: U.S. Geological Survey

Miscellaneous Investigations Series Map I-848-D, scale 1:125,000.

Dobbin, C.E., and Barnett, V.H., 1928, The Gillette coalfield, northeastern

Wyoming: U.S. Geological Survey Bulletin 796, p. 1-50.

Dynamic Graphics, Inc., 1997, EarthVision, v.4: Dynamic Graphics, Inc., 1015

Atlantic Ave., Alameda, CA 94501.

Ellis, M.S., Gunther, G.L., Flores, R.M., Ochs, A.M., Stricker, G.D., Roberts,

S.B., Taber, T.T., Bader, L.R., and Schuenemeyer, J.H., 1999, Preliminary

report on coal resources of the Wyodak-Anderson coal zone, Powder River

Basin, Wyoming and Montana: U.S. Geological Survey Open-File Report

98-789A.

Ellis, M.S., Gunther, G.L., Flores, R.M. Stricker, G.D., and Ochs, A.M., in press,

Preliminary report on methodolgy for calculating coal resources of the
PG-22

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

Montana: U.S. Geological Survey Open-File Report 98-789B.

ESRI–Environmental Systems Research Institute, Inc., 1998a, ARC/INFO, v.

7.1.1, 380 New York Street, Redlands, CA 92373, USA.

_____1998b, ArcView, v.3.0a, 380 New York Street, Redlands, CA 92373, USA.

GRG Corporation, 1996, StratiFact, relational database software, v. 4.5: GRG

Corporation, 4175 Harlan Street, Wheatridge, CO 80033-5150, USA.

Heffern, E.L., Coates, D.A., Whiteman, Jason, and Ellis, M.S., 1993, Geologic

map showing distribution of clinker in the Tertiary Fort Union and Wasatch

Formations, Northern Powder River Basin, Montana: U.S. Geological

Survey Coal Investigations Map C-142, scale 1:175,000.

Heffern, E.L., and Coates, D.A., 1997, Clinker—its occurrence, uses, and effects

on coal mining in the Powder River Basin, in Harris, R.E., and Jones, R.W.

eds., Proceedings of the 32nd annual forum on the geology of industrial

minerals: Wyoming State Geological Survey, Public Information Circular

38, p. 151-165.

Law, B.E., 1974, Preliminary geologic map and coal resources of the Fortin Draw

quadrangle, Campbell County, Wyoming: U.S. Geological Survey OpenFile Report 74-36, scale 1:24,000.

_____1975, Preliminary geologic map and coal resources of the Oriva quadrangle,

Campbell County, Wyoming: U.S. Geological Survey Open-File Report 75-

195, scale 1:24,000

_____1978, Geologic map of the Gillette West quadrangle, Campbell County,

Wyoming: U.S. Geological Survey Miscellaneous Field Studies Map MF974, scale 1:24,000

Love, J.D., and Christiansen, A.C., 1985, Geologic map of Wyoming: U.S.

Geological Survey, scale 1:500,000.
PG-23

Jones, R.W., 1990, Coal map of the Powder River Basin: Wyoming Geological

Survey Map Series 33, scale 1:500,000.

_____1991, Coal map of Wyoming: Wyoming Geological Survey Map Series 33,

scale 1:500,000.

Keefer, W.R., and Schmidt, P.W., 1973, Energy Resources map of the Powder

River Basin, Wyoming and Montana: U.S. Geological Survey

Miscellaneous Investigations Series Map I-847-A, scale 1:500,000.

Kent, B.H., and Berlage, L.J., 1980, Geologic map of the Recluse 1° x 1/2°

quadrangle, Campbell and Crook Counties, Wyoming: U.S. Geological

Survey Coal Investigations Map C-81D, scale 1:100,000.

Love, J.D., Christiansen, A.C., McGrew, L.W., and King, J.K., 1990, Geologic

map of the Gillette 1° x 2° quadrangle, northeastern Wyoming and western

South Dakota: Geological Survey of Wyoming Map Series 25-G, scale

1:250,000.

Mapel, W.J., 1973, Preliminary geologic map of the Rawhide School quadrangle,

Campbell County, Wyoming: U.S. Geological Survey Open-File Report 73-

177, scale 1:24,000.

Microsoft, 1997, Excel spreadsheet software, v. Office 97: Microsoft Corporation,

1 Microsoft Way, Redman, WA 98052.

Resource Data International, Inc., 1998, COALdat coal database: Resource Data

International, Inc., 1320 Pearl St., Suite 300, Boulder, Colorado 80302.

Schuenemeyer, J.H. and Power H.C., in press, Uncertainty estimation for resource

assessment—An application to coal: Mathematical Geology.

U.S. Environmental Protection Agency, 1996, Standards of Performance for New

Stationary Sources, 40CFR, Part 60.43, Standards for Sulfur Dioxide:

Environmental Protection Agency, 27 p.

Warwick, P.D., and Stanton, R.W., 1986, Depositional controls on the geometry of

the Wyodak-Anderson coal bed, northeastern Wyoming, in Carter, L.M.H.,
PG-24

ed., The Second Annual McKelvey Forum on Mineral and Energy

Resources: U.S. Geological Survey Circular 974, p. 71-72.

_____1988, Depositional models for two Tertiary coal-bearing sequences in the

Powder River Basin, USA: Journal of the Geological Society of London, v.

145, p. 613-620.

Wood, G.H. Jr., Kehn, T.M., Carter, M.D., and Culbertson, W.C., 1983, Coal

resource classification system of the U.S. Geological Survey: U.S.

Geological Survey Circular 891, 65 p.

Gillette

Coalfield Gillette

Wright

44°

43°30'

Standby, operating, and

proposed coal mines

Buckskin

Rawhide

Dry Fork

Eagle Butte

Fort Union

Clovis Point

Wyodak

Rocky Butte

Caballo

Belle Ayr

Caballo Rojo

Cordero

Coal Creek

Jacob's Ranch

Black Thunder

Rochelle

North Antelope

Antelope

BLM or state lease

Northern

Sub-Area

Middle

Sub-Area

Southern

Sub-Area

A'

C C'

D

D'

E

E'

F

F'

G G'

H

H'

I

I'

(part)

hnson Campbell

Converse

Weston

Crook

SR 387

SR

59

SR 450

I 90

I 90 & US 16

14 & 16

SR 50

0 10 20 Miles
Figure PG-3. Map showing clinker

in the Gillette coalfield, Powder

River Basin.

Gillette

Coalfield

44°00'

43°30'

Powder

River

Basin

(part)

Weston

Co.

Campbell

Co.

Converse

Co.

Crook

Co.

Railroad

Road

Clinker

Mine and lease

boundaries

Gillette

Wright

SR 387

S

R

59

SR 450

I 90

I 90 & US 16

SR 50

0 10 20 Miles
Figure PG-4. Fluvial channel sandstone above the Wyodak–Anderson coal zone. Photograph by R.M. Flores.
Figure PG-5. Vertically stacked, multierosional, fluvial sandstone above the Wyodak-Anderson coal zone. 

Photograph by R.M. Flores.
Figure PG-6. Stacked fluvial channel sandstone containing crossbeds. Photograph by R.M. Flores.
Figure PG-7. Crevasse-splay sandstone, siltstone, and mudstone above the Wyodak–Anderson coal zone. 

Photograph by R.M. Flores
Figure PG-8. Crevasse channel sandstone lateral to crevasse-splay sandstone, siltstone, and mudstone 

above the Wyodak–Anderson coal zone. Photograph by R.M. Flores.
Figure PG-9. Thick fluvial channel sandstone with deep erosional base above the Wyodak–Anderson coal 

zone. Photograph by R.M. Flores.
Figure PG-10. Lateral accretion units in the fluvial channel sandstone. Photograph by R.M. Flores.
Figure PG-11. Depositional model of anastomosed streams for the Wyodak–Anderson coal zone in 

the Gillette coalfield. Adapted from Flores and others (1989).
Figure PG-12. A peat deposit near the margin of a raised mire in Central Kalimantan, Borneo, Indonesia. 

Photograph by C. Blaine Cecil.
Figure PG-13. A river channel at the edge of a raised mire in Sumatra, Indonesia. Photograph by C. 

Blaine Cecil.
Datum = Sea Level

Wyodak-Anderson

Coal Zone

Wyodak-Anderson

Coal Zone

Wyodak-Anderson

Coal Zone

Wyodak-Anderson

Coal Zone

Wyodak-Anderson

Coal Zone

80.05 Miles

Caballo mine Belle 

Ayr

mine

Black

Thunder

mine

North

Rochelle mine Rochelle mine

5.01 Miles

Antelope

mine

2.20 3.72 Miles Miles 2.29

Miles

1.27 Miles 5.30 Miles

Cordero

4.66 Miles

Coal Creek

5.71 Miles 10.63 Miles

Caballo Rojo mine

4.19 Miles 2.86 Miles

East

Gillette

mine

0.88 Miles

Union mine

86 Miles

Clovis Point mine

0.85 Miles

Wyodak mine

2.36 Miles

A'

South

8.38 Miles 2.24 Miles 2.23 Miles

Jacobs

Ranch

mine

2.92 Miles

{

4300

4100

Elevation

(feet)

4500

4700

4900

3900

3700

3500

4100

4300

4700

4900

3900

3700

3500

4300

4100

4500

4700

4900

3900

3700

3500 Figure PG-14. Gillette coalfield structural cross section A-A'.

D-D'

E-E'

F-F'

G-G'

G-G'

H-H'

I-I'

I-I'

Wyodak-Anderson

Coal Zone
Figure PG-15. Modern analog of a peat forming, raised swamp in the Atchafalaya River Basin, Louisiana. 

Photograph by R.M. Flores. 
Figure PG-16. Modern analog of a peat-forming swamp in the Atchafalaya River

 Basin, Louisiana. Photograph by R.M. Flores.
Figure PG-17. Lateral migration of a fluvial channel caused by avulsion and abandonment of the Santee 

River in South Carolina. Photograph by C. Conner.
Coal

Quaternary rocks

Sandstone

Mudstone

"Rock"

No record

B B

Wyodak-Anderson Coal Zone

Explanation

00

00

ation

feet)

00

00

00

00

00

00

00

00

Datum = Sea Level

Figure PG-18. Gillette coalfield structural cross section B-B'. A-A'

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.
Coal

Sandstone

Mudstone

No record

Wyodak-Anderson

Coal Zone

4300

4100

(feet)

4500

3900

3700

3500

3300

3100

2900

2700

2500

Datum = Sea Level

Explanation A-A'

Figure PG-19. Gillette coalfield structural cross section C-C'.
Coal

Sandstone

Mudstone

"Rock"

Wyodak-Anderson

Coal Zone

0

0

0

0

eet)

0

0

0

0

0

0

0 Datum = Sea Level

Explanation

Figure PG-20. Gillette coalfield structural cross section D-D'. A-A'

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.
Wyodak-Anderson

Coal Zone

Coal

Quaternary Rocks

Sandstone

Mudstone

"Rock"

E E

4400

4200

4600

4800

evation

(feet)

4000

3800

3600

3400

3200

3000 Datum = Sea Level

Explanation

Figure PG-21. Gillette coalfield structural cross section E-E'. A-A'

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.
Wyodak-Anderson

Coal Zone

Coal

Sandstone

Mudstone

"Rock"

F F'

n

)

Datum = Sea Level

Explanation

A-A'

Figure PG-22. Gillette coalfield structural cross section F-F'.

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.
Coal

Sandstone

Mudstone

"Rock"

13.31 Miles

G G'

Wyodak-Anderson

Coal Zone

}

tum = Sea Level

Explanation

igure PG-23. Gillette coalfield structural cross section G-G'.

A-A'

A-A'

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.
?

{

Coal

Sandstone

Mudstone

"Rock"

No Record

17.49 Miles

H H'

Anderson

 Zone 4300

4100

Elevation

(feet)

4500

4700

4900

5100

3900

3700 Datum = Sea Level

Explanation

Figure PG-24. Gillette coalfield structural cross section H-H'.

A-A'

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.
Coal

Quaternary Rocks

Sandstone

Mudstone

"Rock"

I I'

Wyodak-Anderson

Coal Zone

on

et)

Datum = Sea Level

Explanation

Figure PG-25. Gillette coalfield structural cross section I-I'. A-A' A-A'

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.

















igure PG-43. Location of 7.5-minute quadrangle maps in the Wyodak-Anderson study limit of the 

illette coalfield.

Miles

Kilometers

0 10 20

0 10 20 30 40

1 WILDCAT

2 CALF CREEK

3 WESTON SW

4 ORIVA NW

5 RAWHIDE SCHOOL

6 MOYER SPRINGS	

7 ORIVA

8 GILLETTE WEST

9 GILLETTE EAST

10 FORTIN DRAW

11 FOUR BAR J RANCH

12 APPEL BUTTE

13 THE GAP

14 COYOTE DRAW

15 COON TRACK CREEK

16 PLEASANTDALE

17 SCAPER RESERVOIR

18 THE GAP SW

19 SADDLE HORSE BUTTE

20 PEPSSON DRAW

21 THREEMILE CREEK RESERVOIR

22 EAGLE ROCK

23 NEIL BUTTE

24 ROUGH CREEK

25 GREASEWOOD RESERVOIR

26 ROCKY BUTTE GULCH	

27 RENO JUNCTION

28 HILIGHT

29 OPEN A RANCH

30 BAKER SPRING 

31 RATTLESNAKE DRAW

32 LITTLE THUNDER RESERVOIR

33 RENO RESERVOIR

34 PINEY CANYON NW

35 TURNERCREST

36 RENO FLATS

37 TECKLA SW

38 TECKLA

39 PINEY CANYON SW

40 MACKEN DRAW

41 COAL DRAW NORTH

42 BETTY RESERVOIR

43 DUGOUT CREEK NORTH

44 COAL BANK DRAW

1 2 3

4 5 6

7 9

12 13 15

16 17 18 19

20 21 22 24

25 27 29

23

26 28

14

10

11

30 31 32 33 34

8

35

44

39

40

36 37 38

41 42 43
No Federal coal or 

surface

No Federal coal, but 

Federal surface

Federal coal, but no 

Federal surface

Federal coal and 

surface

Powder River 

Basin (part)

Wyodak-Anderson 

coal extent (part)

Gillette Coalfield

Federal Coal = Federal 

subsurface management of 

all minerals, coal only, or oil, 

gas, and coal 

Federal Surface = Federal 

surface ownership

Figure PG-44. Federal coal and Federal surface ownership in the 

Gillette coalfield.

43 30'

44

44 30'

105 45' 105 30'

O

Miles

Kilometers

0 10

0 10 20

5 15

O O

O

O
0 5 10 15 20

0 10 20 30

Miles

Kilometers

0

2.5

5

10

20

30

40

50

75

100

125

150

175

105 30'

44 30'

44

43 30'

o

o

o

o

Campbell

Converse

Net coal thickness

 (ft)

Wyodak-Anderson

coal extent (part)

Gillette Coalfield

County line

Figure PG-45. Wyodak-Anderson net coal isopach map for the Gillette 

coalfield.

Powder 

River Basin 

(part)
0 5 10 15 20

0 10 20 30

Miles

Kilometers

100

200

300

400

500

600

700

800

900

1000

1250

Figure PG-46 Wyodak-Anderson overburden isopach map for the Gillette coalfield.

Powder 

River Basin 

(part)

0

Overburden 

thickness (ft)

105 30'

44 30'

44

43 30'

o

o

o

o

Campbell

Converse

County line

Wyodak-Anderson

coal extent (part)

Gillette Coalfield
Wyodak-Anderson

coal extent (part)

County line

0 5 10 15

0 10 20

Miles

Kilometers

clinker

Gillette coalfield

(part)

(part)

Campbell

Converse

re PG-47. Wyodak-Anderson mine and lease areas, clinker, and resource area in the Gillette coalfield.

Campbell

Converse

44

43 30'

o

o

Index map showing area

included in resource calculations

(does not include areas of mines, leases,

 clinker, or net coal < 2.5 ft thick)
Table PG-1. Summary data for Wyodak-Anderson coal zone in the Gillette coalfield, Powder River Basin,

Wyoming. Modified from unpublished U.S. Geological Survey coal quality database (USCHEM), February, 1992;

Bragg and others (1994); and data from proprietary source(s)

Variable Number Range Mean

of samples Minimum Maximum

Moisture 1108 14.50 42.30 27.47

Ash 187 3.50 25.06 7.45

Total Sulfur 187 0.20 1.16 0.48

Heat Value 285 3,740 9,950 8,220

lb SO2

3

85 0.44 3.27 1.25

MMMFBtu 485 4,580 10,560 8,910

Antimony 549 0.01L 17 0.72

Arsenic 562 0.20L 19 2.3

Beryllium 564 0.078L 3.3 0.35

Cadmium 556 0.007L 3.0 0.39

Chromium 565 0.59L 50 7.0

Cobalt 565 0.38L 27 2.3

Lead 566 0.50L 17 2.9

Manganese 566 0.18 210 22

Mercury 566 0.006L 27 0.17

Nickel 566 0.71L 35 5.0

Selenium 556 0.08L 16 1.4

Uranium 561 0.11L 12 1.5

1

 Values are in percent and on an as-received basis.

2 Value is in British thermal units (Btu).

3 Value is in pounds per million Btu and on an as-received basis.

4 Value is in British thermal units on a moist, mineral-matter-free basis.

5 Values are in parts per million (PPM) on a whole coal and remnant moisture basis and ‘L’ denotes less than value shown.
Table PG-2. Wyodak-Anderson coal resources in the Gillette coalfield, reported by county, overburden thickness

(fig. PG-46), net coal thickness (fig. PG-45), and reliability categories. Resources are reported in millions of short

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

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

clinker. Columns will not sum due to independent rounding

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)

CAMPBELL 0-100 ft 2.5-5 ft 0.94 6.5 2.1 0.099 9.6

5-10 ft 4.7 23 10 0.45 39

10-20 ft 32 100 50 0.74 190

20-30 ft 48 110 80 0 240

30-40 ft 95 320 180 0 600

40-50 ft 190 690 360 140 1,400

50-100 ft 570 1,800 1,800 280 4,400

100-150 ft 190 240 56 0 480

150-200 ft 0 5.1 0 0 5.1

0-100 ft total 1,100 3,300 2,500 420 7,400

100-200 ft 2.5-5 ft 0.11 0.43 0.11 0 0.65

5-10 ft 0.58 0.96 1.6 0 3.1

10-20 ft 4.8 14 6.4 0 25

20-30 ft 2.8 28 15 0 46

30-40 ft 24 110 27 0 160

40-50 ft 100 460 86 0 650

50-100 ft 880 2,600 950 0 4,500

100-150 ft 380 660 5.8 0 1,000

150-200 ft 28 56 0 0 84

100-200 ft total 1,400 4,000 1,100 0 6,500
Table PG-2. Wyodak-Anderson coal resources, Gillette coalfield, Powder River Basin—continued

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)

CAMPBELL 200-300 ft 10-20 ft 2.5 0.97 0 0 3.5

20-30 ft 2.9 25 2.8 0 31

30-40 ft 5.7 78 27 0 110

40-50 ft 84 390 130 0 600

50-100 ft 1,300 4,500 1,100 0 6,900

100-150 ft 510 860 88 0 1,500

150-200 ft 0 11 0 0 11

200-300 ft total 1,900 5,800 1,400 0 9,100

300-400 ft 5-10 ft 0.16 0.21 0 0 0.37

10-20 ft 1.3 4.9 18 0 25

20-30 ft 1.0 3.8 50 0 55

30-40 ft 9.4 10 96 0 120

40-50 ft 48 170 80 0 300

50-100 ft 1,600 5,300 1,700 0 8,600

100-150 ft 380 1,000 270 0 1,700

150-200 ft 11 20 0 0 31

300-400 ft total 2,100 6,500 2,200 0 11,000

400-500 ft 5-10 ft 0.6 4.0 3.9 0 8.4

10-20 ft 0.72 2.2 5.3 0 8.3

20-30 ft 1.4 6.9 0.97 0 9.3

30-40 ft 4.7 22 32 2.0 61

40-50 ft 7.0 19 100 17 150

50-100 ft 1,400 4,600 2,800 0 8,800

100-150 ft 610 1,600 390 0 2,600

150-200 ft 3.0 11 0 0 14

400-500 ft total 2,000 6,300 3,400 19 12,000
Table PG-2. Wyodak-Anderson coal resources, Gillette coalfield, Powder River Basin—continued

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)

CAMPBELL 500-1,000 ft 5-10 ft 0 1.3 2.7 0 4.0

20-30 ft 9.1 12 0 0 21

30-40 ft 11 15 1.6 0 27

40-50 ft 20 63 260 86 430

50-100 ft 2,800 13,000 19,000 380 35,000

100-150 ft 1,500 5,800 10,000 90 18,000

500-1,000 ft total 4,400 19,000 29,000 560 53,000

1,000-1,500 ft 20-30 ft 8.2 26 52 0 86

30-40 ft 0.37 48 300 0 340

40-50 ft 0 1.4 390 19 410

50-100 ft 57 450 5100 540 6,100

100-150 ft 92 850 4200 35 5,200

1,000-1,500 ft total 160 1,400 10,000 590 12,000

CAMPBELL total 13,000 46,000 50,000 1,600 110,000

CONVERSE 0-100 ft 2.5-5 ft 0.091 0.79 0 0 0.88

5-10 ft 0.58 4.5 2.3 0 7.4

10-20 ft 32 200 47 0 270

20-30 ft 47 200 140 0 390

30-40 ft 19 100 14 0 130

40-50 ft 9.6 60 39 0 110

50-100 ft 27 28 49 0 100

0-100 ft total 140 590 290 0 1,000

100-200 ft 2.5-5 ft 0.13 0.82 0 0 0.95

5-10 ft 0.63 4.1 15 0 20

10-20 ft 30 120 110 1.8 260

20-30 ft 44 190 120 0 360

30-40 ft 21 110 36 0 170

40-50 ft 8.3 70 12 0 90

50-100 ft 44 110 6.4 0 160

100-200 ft total 150 610 300 1.8 1,100
Table PG-2. Wyodak-Anderson coal resources, Gillette coalfield, Powder River Basin—continued

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)

CONVERSE 200-300 ft 5-10 ft 0.13 5.3 31 0 37

10-20 ft 16 67 110 1.1 200

20-30 ft 12 29 36 0 76

30-40 ft 11 62 29 0 100

40-50 ft 7.8 82 73 0 160

50-100 ft 59 170 30 0 260

200-300 ft total 110 410 310 1.1 830

300-400 ft 5-10 ft 1.3 3.4 41 3.5 49

10-20 ft 0 1.8 45 0.75 47

20-30 ft 0 5.7 31 0 36

30-40 ft 0 0.31 44 0 44

40-50 ft 5.2 14 17 0 37

50-100 ft 50 160 54 0 260

300-400 ft total 56 180 230 4.3 470

400-500 ft 5-10 ft 1.2 11 42 1.5 56

10-20 ft 0 0 17 0 17

20-30 ft 0 0 3.3 0 3.3

50-100 ft 0 7.6 0 0 7.6

400-500 ft total 1.2 18 62 1.5 83

500-1,000 ft 5-10 ft 4.3 31 70 0 110

10-20 ft 0 0 2.2 0 2.2

500-1,000 ft total 4.3 31 72 0 110

CONVERSE total 450 1,800 1,300 8.6 3,600

Grand total (MST) 14,000 48,000 51,000 1,800 110,000
Table PG-3. Wyodak-Anderson coal resources in the Gillette coalfield, Wyoming, reported by county (fig. PG-47)

and by Federal coal and surface ownership (fig. PG-44). Coal resources are reported in millions of short tons

(MST) with two significant figures. Resources were not calculated for areas containing net coal less than 2.5 ft

thick, in mine or lease areas, or in areas containing mapped clinker. Column will not sum due to independent

rounding

County Federal ownership Total (MST)

CAMPBELL No Federal coal or Federal surface ownership 9,000

No Federal coal, but Federal surface ownership 1,900

Federal coal, but no Federal surface ownership 94,000

Federal coal and Federal surface ownership 5,900

CAMPBELL total 110,000

CONVERSE No Federal coal or Federal surface ownership 340

No Federal coal, but Federal surface ownership 30

Federal coal, but no Federal surface ownership 2,400

Federal coal and Federal surface ownership 810

CONVERSE total 3,600

Grand total (MST) 110,000
Table PG-4. Wyodak-Anderson coal resources in the Gillette coalfield, Wyoming, reported by 7.5-minute

quadrangle map area (fig. PG-43) in each county. Coal resources are reported in millions of short tons (MST) and

with two significant figures. Resources were not calculated for areas containing coal less than 2.5 ft thick, in mine

or lease areas, or in areas containing mapped clinker. Column will not sum due to independent rounding

County 7.5-minute quadrangle map Total (MST) County 7.5-minute quadrangle map Total (MST)

CAMPBELL APPEL BUTTE 5,000 CAMPBELL PLEASANTDALE 2,900

BAKER SPRING 1,900 RATTLESNAKE DRAW 4,000

BETTY RESERVOIR 110 RAWHIDE SCHOOL 4,300

CALF CREEK 1,000 RENO FLATS 4,300

COAL BANK DRAW 1.0 RENO JUNCTION 6,000

COAL DRAW NORTH 85 RENO RESERVOIR 3,100

COON TRACK CREEK 74 ROCKY BUTTE GULCH 6,400

COYOTE DRAW 1,400 ROUGH CREEK 1,400

DUGOUT CREEK NORTH 52 SADDLE HORSE BUTTE 390

EAGLE ROCK 4,200 SCAPER RESERVOIR 5,900

FORTIN DRAW 370 TECKLA 3,700

FOUR BAR J RANCH 2,600 TECKLA SW 4,900

GILLETTE EAST 2,400 THE GAP 3,900

GILLETTE WEST 5,000 THE GAP SW 3,200

GREASEWOOD RESERVOIR 2,100 THREEMILE CREEK RESERVOIR 6,400

HILIGHT 3,500 TURNERCREST 1,200

LITTLE THUNDER RESERVOIR 5,800 WESTON SW 55

MACKEN DRAW 2.9 WILDCAT 590

MOYER SPRINGS 510 CAMPBELL total 110,000

NEIL BUTTE 2,200 CONVERSE BETTY RESERVOIR 1,600

OPEN A RANCH 450 COAL BANK DRAW 36

ORIVA 2,900 COAL DRAW NORTH 1,000

ORIVA NW 3,300 DUGOUT CREEK NORTH 730

PEPSSON DRAW 2,800 MACKEN DRAW 200

PINEY CANYON NW 110 CONVERSE total 3,600

PINEY CANYON SW 500 Grand total (MST) 110,000
Table PG-5. Data used for computation of confidence intervals within reliability categories for the WyodakAnderson coal zone in the Gillette coalfield. Volume refers to the calculated resource in millions of short tons (MST).

Rows will not sum due to independent rounding. NA, not applicable

Parameter Reliability category Entire

Measured Indicated Inferred Hypothetical area

Area (in square meters) 416,538,074 1,530,925,727 1,602,753,691 60,295,123 3,610,512,615

Percent of area 12 42 44 2 100

Acres (area x 0.0002471) 102,929 378,300 396,049 14,899 892,177

SD (standard deviation (in ft) from

semivariogram model)

19.65 23.33 26.33 26.33 NA

Acre feet (acres x SD) 2,022,084 8,825,319 10,426,851 392,255 NA

Volume standard deviation (MST) 125 854 3,945 694 5,618

Pseudo n 819 334 22 1 NA

Table PG-6. Estimates of uncertainty (calculated with measurement error) for Wyodak-Anderson coal resources in

the Gillette coalfield, Powder River Basin, Wyoming. To show detail, resources are reported in millions of short tons

(MST) with 4 significant figures

Parameter Reliability category Entire

Measured Indicated Inferred Hypothetical area

Total calculated resource (MST) 13,520 48,160 51,260 1,597 114,500

Lower 90% confidence limit (MST) 13,310 46,760 44,770 455.0 105,300

Upper 90% confidence limit (MST) 13,730 49,570 57,750 2,739 123,800