Chapter PH

SHERIDAN 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. Power3

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
PH-ii

Contents

Structural and Stratigraphic Setting .................................................................................PH-1

Depositional Setting............................................................................................................PH-3

Wyodak-Anderson coal zone—Sheridan coalfield......................................................PH-3

Sheridan coalfield Structural Cross Section A-A’.................................................PH-3

Sheridan coalfield Structural Cross Section B-B’..................................................PH-5

Sheridan coalfield Structural Cross Section C-C’..................................................PH-6

Coal Quality..........................................................................................................................PH-7

Coal Resources—Wyodak-Anderson coal zone.......................................................PH-10

Resource Estimates—An Overview......................................................................PH-10

Wyodak-Anderson Coal Resources.......................................................................PH-11

Confidence Limits for Wyodak-Anderson Coal Resources in the

Sheridan coalfield .......................................................................................................PH-13

References..........................................................................................................................PH-16

Figures

PH-1. Geologic map of the Sheridan coalfield, Powder River Basin.

PH-2. Coal beds of the Wyodak–Anderson coal zone and overlying carbonaceous

shale, mudstone, siltstone, and sandstone in the highwall of the Big Horn

mine.

PH-3. Fluvial channel sandstone above the Wyodak–Anderson coal zone in the

highwall of the Big Horn Mine.

PH-4. Interbedded mudstone, siltstone, and sandstone deposited in crevasse splay

and floodplain environments in the highwall of the Big Horn mine.
PH-iii

Figures—continued

PH-5. The Wyodak-Anderson coal zone consisting of the Dietz and Monarch coal

beds in the highwall of the Big Horn mine.

PH-6. Index map showing locations of Sheridan coalfield cross sections and

mines.

PH-7. Sheridan coalfield structural cross section A-A’.

PH-8. Sheridan coalfield structural cross section B-B’.

PH-9. Sheridan coalfield structural cross section C-C’.

PH-10. Index map showing coal quality data distribution in the Wyodak-Anderson

coal zone, Sheridan coalfield, Powder River Basin, Wyoming.

PH-11. Ash yield in the Wyodak-Anderson coal zone, Sheridan coalfield, Powder

River Basin, Wyoming.

PH-12. Sulfur content in the Wyodak-Anderson coal zone, Sheridan coalfield,

Powder River Basin, Wyoming.

PH-13. Moist, mineral-matter-free Btu/lb in the Wyodak-Anderson coal zone,

Sheridan coalfield, Powder River Basin, Wyoming.

PH-14. Pounds of sulfur dioxide per million Btu in the Wyodak-Anderson coal

zone, Sheridan coalfield, Powder River Basin, Wyoming.

PH-15. Antimony concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-16. Arsenic concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-17. Beryllium concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-18. Cadmium concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.
PH-iv

Figures—continued

PH-19. Chromium concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-20. Cobalt concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-21. Lead concentration in the Wyodak-Anderson coal zone, Sheridan coalfield,

Powder River Basin, Wyoming.

PH-22. Manganese concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-23. Mercury concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-24. Nickel concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-25. Selenium concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-26. Uranium concentration in the Wyodak-Anderson coal zone, Sheridan

coalfield, Powder River Basin, Wyoming.

PH-27. Location of 7.5-minute quadrangle maps in the Sheridan coalfield.

PH-28. Federal coal and Federal surface ownership in the Sheridan coalfield.

PH-29. Wyodak-Anderson net coal isopach map in the Sheridan coalfield.

PH-30. Wyodak-Anderson overburden isopach map in the Sheridan coalfield.

PH-31. Wyodak-Anderson mine and lease areas, clinker, and resource area in the

Sheridan coalfield.
PH-v

Tables

PH-1. Summary data for Wyodak-Anderson coal zone in the Sheridan coalfield,

Powder River Basin, Wyoming

PH-2. Wyodak-Anderson coal resources in the Sheridan coalfield, Sheridan

County, Wyoming, reported by overburden thickness, net coal thickness, and

reliability categories

PH-3. Wyodak-Anderson coal resources in the Sheridan coalfield, Sheridan

County, Wyoming reported by Federal coal and Federal surface ownership

PH-4. Wyodak-Anderson coal resources in the Sheridan coalfield, Sheridan

County, Wyoming reported by 7.5-minute quadrangle map area

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

categories for the Wyodak-Anderson coal zone in the Sheridan coalfield,

Powder River Basin, Wyoming

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

Wyoming
PH-1

STRUCTURAL AND STRATIGRAPHIC SETTING

The Sheridan coalfield contains the Fort Union Formation (Paleocene) that includes,

from bottom to top, the Tullock, Lebo, and Tongue River Members (fig. PH-1).

These members are mainly distributed in the western part of the coalfield (see fig.

PH-1; Barnum, 1974, 1975; Ebaugh, 1976; Kanizay, 1978; Law and others, 1979).

The Eocene Wasatch Formation, which conformably overlies the Fort Union

Formation, is distributed in the easternmost part of the coalfield (see fig. PH-1;

Culbertson, 1975; Culbertson and Mapel, 1976). Undifferentiated Fort Union

Formation is in the southwestern part of the coalfield. The contact between these

formations is not delineated in the subsurface because they have basically the same

lithology and are difficult to correlate. However, the surface contact between the

Wasatch and Fort Union Formations is placed at the top of the Roland coal bed,

following Baker (1929). These rocks dip 1-3 degrees eastward.

The Sheridan coalfield boundary approximately follows the area defined by Taff

(1909), who investigated the mining areas in the Fort Union Formation north and

northwest of Sheridan, and mapped Wasatch coal east of Sheridan. In this study, the

Sheridan coalfield (see fig. PH-1) is delineated by the surface contact line of the

Lebo and Tongue River Members on the west, the surface contact line of the

undifferentiated Fort Union and Tongue River Members on the south, the 106-

degree, 45-minute longitude line on the east, and the Wyoming and Montana State

line on the north.

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

the Fort Union Formation are mined in the Sheridan coalfield and are locally named,
PH-2

from top to bottom, Dietz 2, Dietz 3, and Monarch (Canyon) coal beds (Law and

others, 1979). These coal beds, including the Carney coal (more than 20 ft below

the Monarch or Canyon coal), were extensively mined by underground methods

(Taff, 1909; Dunrud and Osterwald, 1980).

These coal beds merge, split, or pinch out into mudstone, siltstone, and sandstone.

These rocks are undifferentiated in the cross sections. The Fort Union rocks are

displaced by numerous northeast-southwest-trending normal faults.

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

sandstone as seen in the highwalls of the Big Horn coal mine (figs. PH-1 and PH2). Here, the sandstone is fining-upward, shows an erosional base, and contains

trough crossbeds that indicate a fluvial channel deposit (fig. PH-3). The sandstone

is interbedded with, and laterally grades into, a tabular, rippled silty sandstone,

which in turn is interbedded with mudstone and siltstone; these rocks are crevasse

splay and floodplain deposits (fig. PH-4). The fluvial deposits are interbedded with

the Dietz and Monarch (Canyon) coal beds (fig. PH-5) that accumulated in raised

swamps and total as much as 54 ft in thickness.

The Wyodak-Anderson coal zone of the Tongue River Member is in the westnorthwest part of the Sheridan coalfield near the city of Sheridan, Wyoming.

Clinker, resulting from burned coal and related rocks of this zone, is well developed

northwest of Sheridan (Kanizay, 1978).

The Anderson, Dietz, and Canyon (Monarch) coal beds of the Wyodak-Anderson

coal zone were mined from 20 underground mines from 1894 to 1976 (Dunrud and
PH-3

Osterwald, 1980). Surface subsidence pits and troughs mark locations of these

abandoned underground mine workings.

These coal beds were strip mined in the Big Horn mine (see fig. PH-1) from 1962

to present. Annual mine production from 1989 to 1997 has declined from 106,000

to 43,000 short tons (Resource Data International, Inc., 1998).

DEPOSITIONAL SETTING

The stratigraphic variation, thickness, distribution, and shape of the WyodakAnderson coal zone (see cross sections; fig. PH-6) reflect its deposition in raised

mires formed in meandering and anastomosed fluvial systems.

WYODAK-ANDERSON COAL ZONE—SHERIDAN COALFIELD

SHERIDAN COALFIELD STRUCTURAL CROSS SECTION A-A’

• The north-south oriented structural cross section A-A’ (fig. PH-7) is more than

18 mi long. The Wyodak-Anderson coal zone, which is 300-ft thick, shows

merging and splitting of as many as five coal beds. The coal beds dip southsoutheast. This cross section was drawn using data from 19 drill holes, which

are not labeled on the cross section.

• The coal beds of the Wyodak-Anderson coal zone are interbedded with, and split

by, undifferentiated mudstone, siltstone, and sandstone (shown in green) that

were deposited in floodplains, crevasse splays, and fluvial channels.
PH-4

• The coal beds of the Wyodak-Anderson coal zone commonly pinch out into

fluvial deposits.

• The coal beds of the Wyodak-Anderson coal zone tend to merge and pinch out in

the central part of the cross section, pinch out to the south, and thicken to the

north.

• The coal beds of the Wyodak-Anderson in the northern part of the cross section

display a tabular geometry.

• The coal beds of the Wyodak-Anderson coal zone in the southern part of the

cross section exhibit lenticular geometry.

• The lateral and vertical variations and geometry of the coal beds are controlled by

the depositional environments of the coal and associated detrital sediments.

• Merging and splitting of the coal beds reflect absence or presence of fluvial

channel deposits, respectively. Deposits of fluvial channels, such as sandstone,

tend to split and expand the interval between coal beds due to compaction effects.

During burial, sandstone is less compactible by overlying sediment than

mudstone. Thus coal beds interbedded with mudstone tend to merge upon

compaction by overlying sediments.

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

created “want areas” where the coal beds of the Wyodak-Anderson coal zone

pinch out. Such areas are shown in the central part of the cross section.
PH-5

• Local and regional subsidence, in combination with deposition in rain-fed raised

mires that formed away from fluvial channels, controlled the accumulation of

thick Wyodak-Anderson coal beds, and kept their ash content low.

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

below the Wyodak-Anderson coal zone are each more than 500 ft thick. The

rocks include undifferentiated fluvial-channel, overbank, floodplain, and

lacustrine deposits.

SHERIDAN COALFIELD STRUCTURAL CROSS SECTION B-B’

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

mi long. The Wyodak-Anderson coal zone, as much as 250 ft thick, shows

merging and splitting of as many as three coal beds that dip east. This cross

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

cross section.

• Individual coal beds of the Wyodak-Anderson coal zone locally split, merge, and

pinch out into undifferentiated mudstone, siltstone, and sandstone (shown in

green) that were deposited in floodplains, crevasse splays, and fluvial channels.

• Splitting, merging, and pinching out of coal beds of the Wyodak-Anderson coal

zone are common in the central and eastern parts of the cross section.

• Toward the west, the coal beds of the Wyodak-Anderson coal zone are laterally

uniform and are interbedded with moderately thick, undifferentiated mudstone,

siltstone, limestone, and sandstone (shown in green).
PH-6

• The coal beds and the Wyodak-Anderson coal zone are uniformly thick from

west to east. This indicates the absence of major fluvial-channel deposits that

would disrupt and expand the interval.

• The lateral and vertical variations and geometry of the coal beds are controlled by

the depositional environments of the coal and associated detrital sediments.

• The uniform distribution of the coal beds and coal zone suggests that precursor

peat accumulated on interchannel raised mires, which probably occupied

abandoned fluvial-channel and floodplain deposits.

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

below the Wyodak-Anderson coal zone are as much as 600 ft thick and more

than 400 ft thick, respectively. The intervals are composed of undifferentiated

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

siltstone.

SHERIDAN COALFIELD STRUCTURAL CROSS SECTION C-C’

• The west-east oriented structural cross section C-C’ (fig. PH-9) is more than 15

mi long. The Wyodak-Anderson coal zone, as much as 250 ft thick, contains

merging and splitting coal beds that dip eastward. This cross section was drawn

using data from 13 drill holes, which are not labeled on the cross section.

• The coal beds merge in the central part of the cross section to form a bed about

50 ft thick.
PH-7

• Toward the west and east, the coal beds of the Wyodak-Anderson coal zone are

interbedded with and split by undifferentiated mudstone, siltstone, and sandstone

(shown in green) that was deposited in floodplains and floodplain lakes, fluvial

channels, and crevasse splays.

• The coal beds of the Wyodak-Anderson coal zone thin and pinch out toward the

west, suggesting the extent of the raised mire in which the coal formed.

• The peat probably accumulated in an interchannel raised mire flanked by fluvial

channels and/or crevasse splays. The thickening of detrital sediments associated

with the split and pinched out coal beds may represent levee, overbank, or

crevasse deposits that periodically intruded the adjoining mire.

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

below the Wyodak-Anderson coal zone are as much as 1,100 ft thick and more

than 450 ft thick, respectively. The rocks consist of interbedded fluvial-channel

sandstone, and overbank, floodplain, and lacustrine mudstone.

COAL QUALITY

Actively mined coal in the Sheridan coalfield, Powder River Basin, Wyoming is

considered to be a “clean coal.” Coal from the Sheridan coalfield is developed from

one mine and utilized for electric power generation. The coal is a low-contaminant,

compliant, subbituminous coal resource.

The Sheridan coalfield has one assessment unit, the Wyodak-Anderson coal zone.

This coal zone has the following arithmetic mean values (on an as-received basis)
PH-8

for coal that is not presently being mined or under lease to be mined in the future:

moisture–20.40%, ash yield–5.30%, total sulfur–0.70%, calorific value–9,540

(Btu/lb), lb SO2 per million Btu–1.48, and moist, mineral-matter-free

Btu–10,120. Arithmetic mean concentration (in parts per million and on whole-coal

and remnant-moisture basis) of elements of environmental concern for the WyodakAnderson coal zone are: antimony–0.24, arsenic–3.2, beryllium–0.21,

cadmium–0.09, chromium–3.6, cobalt–1.2, lead–1.8, manganese–21,

mercury–0.08, nickel–4.8, selenium–0.45, and uranium–0.58. Table PH-1 is a

summary of coal quality in this coalfield.

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

calculated for the Wyodak-Anderson coal zone. 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 (American Society for Testing and Materials, 1994b, designation D3180-89).

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 as much

as 80 hours. However, some samples may have been dried only long enough to

allow grinding (to less than 100 mesh). Moisture content in the samples is
PH-9

unknown, although moisture contents were probably similar to that which would

remain after air-dry loss determination (American Society for Testing and Materials,

1994c, D3302-91). 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, pounds of mercury per trillion

Btu).

The locations of public data points used in the summary table are shown in figure

PH-10. When more than one analysis was available per location, the analytical

values were weight averaged on coal sample thickness. Figures PH-11 through PH26, show values of the variables listed in the summary table for coal in the WyodakAnderson coal zone, except for calorific value and moisture content. For ash yield

(fig. PH-11) and total sulfur content (fig. PH-12), the values are coded to low,

medium, and high, following guidelines established in U.S. Geological Survey

Circular 891 (Wood and others, 1983). For moist, mineral-matter-free Btu

(apparent rank) (fig. PH-13), the apparent rank designations established by

American Society for Testing and Materials, (1994a), designation D388-92a were

utilized. For pounds of sulfur dioxide per million Btu (lbSO2) (fig. PH-14), values

are coded according to the U.S. Environmental Protection Agency’s Phase I (low),

Phase II (medium), and non-compliant (high) limits for sulfur emission from coalfired power plants (U.S. Environmental Protection Agency, 1996).

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

referred to as “hazardous air pollutants” or “HAPs”). Analytical values for these

elements (fig. PS-15 to PS-26) are color keyed based on the following parameters:

(1) each element of environmental concern was ranked from the lowest to highest
PH-10

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 quartiles (two quartiles

representing 50 percent of the values or between the 25th to 75th percentile). High

represents those values that are in the upper .25 quartile (or greater than the 75th

percentile).

COAL RESOURCES—WYODAK-ANDERSON COAL ZONE

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 Sheridan 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 Federal coal and

surface ownership, 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 is extrapolated.
PH-11

• 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.

• 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 Sheridan 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 western extent (study limit) of the Wyodak-Anderson coal zone in the Sheridan

coalfield is based on the Wyodak-Anderson outcrop or the Tertiary/Cretaceous

contact from published maps by Baker (1929), Love and Christiansen (1985), and

Green and Drouillard (1994). The western study limit was generalized in some areas
PH-12

to include small areas of coal outside of the main boundary. The eastern boundary

was placed at the 106-degree, 45-minute longitude line; the southern boundary was

delineated by the 44-degree, 45-minute latitude line; and the northern boundary was

placed at the Montana/Wyoming state line. The total study area is 161,898 acres

(65,518 hectares) in size.

Wyodak-Anderson coal resources in the Powder River Basin of Wyoming and

Montana 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 Sheridan coalfield we

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

coal beds in the Wyodak-Anderson coal zone, including the Anderson, Dietz,

Canyon, and Monarch beds (see the explanation of coal bed nomenclature in chapter

PS of this CD-ROM), were correlated in the database. We used a custom program

to calculate the 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. PH-27, U.S. Geological Survey National Mapping

Division unpublished data, undated), Federal coal and surface ownership (fig. PH28, Biewick and others, 1998), net coal thickness (fig. PH-29), and overburden
PH-13

categories (fig. PH-30)) 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

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. PH-31) (Kanizay, 1978), Wyodak-Anderson mine

or lease areas (fig. PH-31) (Dunrud and Osterwald, 1980), 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 PH-1 through PH-3). The final

resource area (fig. PH-31) (area that met all coal resource criteria) was 149,662 acres

(60,566 hectares) in area.

*Commercial software package.

CONFIDENCE LIMITS FOR WYODAK-ANDERSON COAL

RESOURCES IN THE SHERIDAN COALFIELD

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.
PH-14

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

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

Sheridan 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 193 locations. This data set only included locations that

contained Wyodak-Anderson coal (no 0 coal thickness 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). Spatial

correlation, as determined from the semivariogram of residual thickness, was

negligible. Standard deviation of coal thickness was computed from residual

thickness. 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 WyodakAnderson 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 PH-5 and PH-6. A detailed description of
PH-15

the methodology used is given in Schuenemeyer and Power (in press), and in Ellis

and others (1999; in press).
PH-16

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 AsDetermined 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.

Baker, A.A., 1929, The northward extension of the Sheridan coal field, Big Horn

and Rosebud Counties, Montana: U.S. Geological Survey Bulletin 806-B, p.

15-67.

Barnum, B.E., 1974, Preliminary geologic map and coal resources of the

Ranchester quadrangle, Sheridan County, Wyoming, and Big Horn County,

Montana: U.S. Geological Survey Open-File Report 74-35, 4 p., 2 sheets.

_____1975, Geologic map and coal resources of the Ranchester quadrangle,

Sheridan County, Wyoming, and Big Horn County, Montana: U.S.

Geological Survey Coal Investigations Series Map C-75, scale 1:24,000.

Biewick, Laura R.H., Urbanowski, Shayne R., Cain, Sheila, and Neasloney, Larry,

1998, Land status and Federal mineral ownership in the Powder River Basin,
PH-17

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.

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.

Culbertson, W.C., 1975, Preliminary geologic map and coal sections of the Wyarno

quadrangle, Sheridan County, Wyoming: U.S. Geological Survey

Miscellaneous Field Studies Map MF-723, scale 1:24,000.

Culbertson, W.C., and Mapel, W.J., 1976, Coal in the Wasatch Formation,

northwest part of the Powder River basin near Sheridan, Sheridan County,

Wyoming: Wyoming Geological Association, 28th Annual Field

Conference, p. 193-201.

Dunrud, C.R. and Osterwald, F.W., 1980, Effects of coal mine subsidence in the

Sheridan, Wyoming area: U.S. Geological Survey Professional Paper 1164,

p.7, figure 4.

Dynamic Graphics, Inc., 1997, EarthVision, v.4, 1015 Atlantic Ave., Alameda, CA

94501.

Ebaugh, W.F., 1976, Preliminary surficial and bedrock geologic map of the Big

Horn quadrangle, Sheridan County, Wyoming: U.S. Geological Survey

Miscellaneous Field Studies Map MF-801, scale 1:24,000.

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.
PH-18

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

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.

Green G.N. and Drouillard, P.H., 1994, The digital geologic map of Wyoming in

ARC/INFO format: U.S. Geological Survey Open-File Report 94-0425.

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

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

Kanizay, S.P., 1978, Preliminary geologic map of the Sheridan area, northwestern

Powder River Basin, Wyoming: U.S. Geological Survey Miscellaneous

Field Studies Map MF-1043, scale 1:50,000.

Law, B.E., Barnum, B.E., and Wollenzien, T.P., 1979, Coal bed correlations in the

Tongue River Member of the Fort Union Formation, Monarch, Wyoming,

and Decker Montana, areas: U.S. Geological Survey Miscellaneous

Investigations Series Map I-1128.

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

Geological Survey, scale 1:500,000.

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., in press, Uncertainty Estimation For Resource

Assessment—An Application To Coal: Mathematical Geology.
PH-19

Taff, J.A., 1909, The Sheridan coal field: U.S. Geological Survey Bulletin 341, p.

123-150.

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.

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.

Figure PH-2. Coal beds of the Wyodak–Anderson coal zone and overlying carbonaceous shale, mudstone, 

siltstone, and sandstone in the highwall of the Big Horn mine. Photograph by R.M. Flores.
Figure PH-3. Fluvial channel sandstone above the Wyodak–Anderson coal zone in the highwall of the Big Horn 

mine. Photograph by R.M. Flores.
Figure PH-4. Interbedded mudstone, siltstone, and sandstone deposited in crevasse splay and floodplain 

environments in the highwall of the Big Horn mine. Photograph by R.M. Flores. 
Figure PH-5. The Wyodak–Anderson coal zone consisting of the Dietz and Monarch coal beds in the highwall of 

the Big Horn mine. Photograph by R.M. Flores.

?

entiated mudstone,

 limestone and sandstone

d

South

A'

WYODAK-ANDERSON

COAL ZONE

18.06 Miles

Figure PH-7. Sheridan coalfield structural cross section A-A'.

C-C'
?

?

?

?

East

B'

West

B

WYODAK-ANDERSON

COAL ZONE

WYODAK-ANDERSON

COAL ZONE

Coal

Undifferentiated mudstone,

siltstone, limestone and sandstone

No record

3800

Elevation

(feet)

4100

3600

4000

3400

3900

3200

3800

3000

3700

2800

3600

2600

3500

4700

4800

4600

4600

4500

4400

4400

4200

4300

4000

4200

2900

2800

2700

2600

3400

4600

3300

4400

3200

3100

3000

Datum = Sea Level

18.39 Miles

A-A'

Figure PH-8. Sheridan coalfield structural cross section B-B'.
?

?

?

?

um = Sea Level

Figure PH-9. Sheridan coalfield structural cross section C-C'.

A-A'

CC

WYODAK-ANDERSON

COAL ZONE

ODAK-ANDERSON

COAL ZONE

Coal

Undifferentiated mudstone,

siltstone, limestone and sandstone

No record

















1

0 2 4 6 8

0 2 4 Miles

Kilometers

Wyodak-Anderson 

coal extent

Sheridan coalfield

Powder River Basin 

(part)

Quadrangle boundary

2 3 4

5 6 7

1 RANCHESTER

2 MONARCH

3 ACME

4 BAR N DRAW

5 WOLF

6 HULTZ DRAW

7 SHERIDAN

8 WYARNO

7.5-minute 

quadrangle maps

1

Figure PH-27. Location of 7.5-minute quadrangle maps in the Sheridan coalfield.

8

Montana

Wyoming
0 2 4 6 8 10

0 2 4 6 Miles

Kilometers

107

45

44 45'

106 45'

o

o o

o

Federal Coal = Federal 

subsurface management of 

all minerals, coal only, or oil, 

gas, and coal 

Federal Surface = Federal 

surface ownership

No Federal coal or surface

Federal coal, but no Federal 

surface

Federal coal and surface

Powder River Basin (part)

Figure PH-28. Federal coal and Federal surface ownership in the Sheridan coalfield.

Wyodak-Anderson coal extent 

(part)

Sheridan coalfield study limit

Wyoming

Montana
0

2.5

5

10

20

30

40

50

60

70

80

90

100

110

Wyodak-Anderson

coal extent (part)

Sheridan Coalfield

gure PH-29. Wyodak-Anderson net coal isopach map in the Sheridan coalfield.

0 2 4 6 8 10

0 2 4 6 Miles

Kilometers

Powder River Basin 

(part)
0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

Wyodak-Anderson

coal extent (part)

Sheridan Coalfield

Powder River Basin 

(part)

Faults

ure PH-30. Wyodak-Anderson overburden isopach map in the Sheridan coalfield.

0 2 4 6 8 10

0 2 4 6 Miles

Kilometers
Wyodak-Anderson 

coal extent (part)

Mine and lease areas

Wyodak-Anderson 

clinker

Faults

0 2 4 6 8 10

0 2 4 6 Miles

Kilometers

Resource area

Sheridan Coalfield

Powder River Basin 

(part)

(does not include areas of 

Wyodak-Anderson mines, leases, 

clinker, or net coal < 2.5 ft thick)

e PH-31. Wyodak-Anderson mine and lease areas, clinker, and resource area in the Sheridan coalfield.

Index map showing 

area included in 

resource calculations

yg
Table PH-1. Summary data for Wyodak-Anderson coal zone in the Sheridan coalfield, Powder River Basin,

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

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

Variable Number Range Mean

of samples Minimum Maximum

Moisture 11 20.40 20.40 20.40

Ash 11 5.30 5.30 5.30

 Total Sulfur 11 0.70 0.70 0.70

Calorific Value 21 9,540 9,540 9,540

lb SO2

3

1 1.48 1.48 1.48

MMMFBtu 41 10,120 10,120 10,120

Antimony 52 0.23 0.25 0.24

Arsenic 52 3.1 3.3 3.2

Beryllium 5 2 0.20 0.23 0.21

Cadmium 52 0.09 0.10 0.09

Chromium 52 3.6 3.7 3.6

Cobalt 52 1.1 1.2 1.2

Lead 52 1.5 2.1 1.8

Manganese 52 11 30 21

Mercury 52 0.08 0.09 0.08

Nickel 52 4.3 5.4 4.8

Selenium 52 0.40 0.50 0.45

Uranium 52 0.41 0.75 0.58

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 as-received basis
Table PH-2. Wyodak-Anderson coal resources in the Sheridan coalfield, Sheridan County, Wyoming reported by

overburden thickness (fig. PH-30), net coal thickness (fig. PH-29), 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 resources in mine or lease areas, or areas containing

Wyodak-Anderson clinker. Columns and rows will not sum due to independent rounding

Overburden Net Reliability categories (distance from data point) Total

thickness coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

(MST)

0-100 ft 2.5-5 ft 0.16 7.6 39 0 47

5-10 ft 5.4 33 140 7.7 190

10-20 ft 17 120 220 5.1 360

20-30 ft 7.2 83 56 0 150

30-40 ft 11 36 3.6 0 51

40-50 ft 29 34 0 0 63

50-100 ft 38 20 34 0 92

0-100 ft total 110 330 490 13 940

100-200 ft 2.5-5 ft 0.20 5.0 1.6 0 6.8

5-10 ft 4.4 40 41 0 85

10-20 ft 17 85 120 7.5 230

20-30 ft 38 190 84 0 310

30-40 ft 55 100 3.0 0 160

40-50 ft 79 76 2.1 0 160

50-100 ft 100 170 12 0 280

100-200 ft total 290 660 260 7.5 1,200

200-300 ft 2.5-5 ft 0.11 2.5 0.57 0 3.1

5-10 ft 3.3 14 6.4 0 24

10-20 ft 29 68 78 8.9 180

20-30 ft 56 160 130 0.75 350

30-40 ft 34 140 56 0 230

40-50 ft 15 120 9.8 0 150

50-100 ft 150 640 53 0 840

100-150 ft 0 9.6 0 0 9.6

200-300 ft total 280 1,200 330 9.6 1,800
Table PH-2. Wyodak-Anderson coal resources, Sheridan coalfield, Powder River Basin—continued

Overburden Net Reliability categories (distance from data point) Total

thickness coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

(MST)

300-400 ft 2.5-5 ft 0.061 1.1 0 0 1.2

5-10 ft 8.3 16 10 0 34

10-20 ft 15 62 47 2.3 130

20-30 ft 36 150 230 2.2 420

30-40 ft 36 210 190 0.8 430

40-50 ft 16 120 58 0 190

50-100 ft 220 610 78 0 900

100-150 ft 9.3 5.0 0 0 14

300-400 ft total 340 1,200 610 5.3 2,100

400-500 ft 2.5-5 ft 0.22 0.76 0 0 0.98

5-10 ft 9.2 36 4.2 0 49

10-20 ft 11 100 73 3.7 190

20-30 ft 14 85 220 8.5 320

30-40 ft 17 50 140 5.6 210

40-50 ft 14 55 41 0 110

50-100 ft 54 250 72 0 370

100-150 ft 4.1 2.5 0 0 6.6

400-500 ft total 120 580 540 18 1,300

500-1,000 ft 2.5-5 ft 1.5 2.2 0 0 3.7

5-10 ft 4.6 9.7 0 0 14

10-20 ft 15 140 170 0.16 330

20-30 ft 15 95 300 22 430

30-40 ft 7.7 80 190 16 290

40-50 ft 22 110 220 35 390

50-100 ft 100 480 670 13 1,300

500-1,000 ft total 170 9 10 1,500 86 2,700
Table PH-2. Wyodak-Anderson coal resources, Sheridan coalfield, Powder River Basin—continued

Overburden Net Reliability categories (distance from data point) Total

thickness coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

(MST)

1,000-1,500 ft 20-30 ft 0 0 12 4.5 17

30-40 ft 0 1.0 68 18 87

40-50 ft 0 14 100 15 130

50-100 ft 15 74 250 0 340

1,000-1,500 ft total 15 89 430 37 580

Grand total (MST) 1,300 4,900 4,200 180 11,000

Table PH-3. Wyodak-Anderson coal resources in the Sheridan coalfield, Sheridan County, Wyoming reported by

Federal coal and Federal surface ownership (fig. PH-28) (Biewick and others, 1998). Coal resources are reported in

millions of short tons (MST) with two significant figures. The table does not include resources in mine or lease

areas, or areas containing Wyodak-Anderson clinker. Column will not sum due to independent rounding

Federal ownership Total (MST)

No Federal coal or surface ownership 5,300

Federal coal, but no Federal surface ownership 5,200

Federal coal and Federal surface ownership 150

Grand total (MST) 11,000
Table PH-4. Wyodak-Anderson coal resources in the Sheridan coalfield, Sheridan County, Wyoming reported by

7.5-minute quadrangle map area (fig. PH-27). Coal resources are reported in millions of short tons (MST) and with

two significant figures. The table does not include resources in mine or lease areas, or areas containing WyodakAnderson clinker. Column will not sum due to independent rounding

7.5-minute quadrangle map Total (MST)

ACME 2,700

BAR N DRAW 2,900

HULTZ DRAW 280

MONARCH 650

RANCHESTER 98

SHERIDAN 1,300

WOLF 42

WYARNO 2,700

Grand total (MST) 11,000
Table PH-5. Data used for computation of confidence intervals within reliability categories for the WyodakAnderson coal zone in the Sheridan coalfield, Powder River Basin, Wyoming. Volume refers to the calculated

resource in millions of short tons (MST). NA, not applicable

Parameter Reliability category Entire

Measured Indicated Inferred Hypothetical area

Area (in square meters) 68,915,982 264,330,666 258,679,514 13,736,407 605,662,568

Percent of area 11 44 43 2 100

Acres (area x 0.0002471) 17,030 65,318 63,921 3,394 149,662

SD (standard deviation (in ft) from residual

thickness)

15.74 15.74 15.74 15.74 NA

Acre feet (acres x SD) 268,044 1,028,098 1,006,118 53,427 NA

Volume standard deviation (MST) 41 239 948 95 1,322

Pseudo n 136 58 4 1 NA

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

the Sheridan 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)) 1,333 4,904 4,221 176.0 10,640

Lower 90% confidence limit (MST) 1,266 4,510 2,662 21.00 8,459

Upper 90% confidence limit (MST) 1,400 5,298 5,780 332.0 12,810