Chapter DB

DATABASE CREATION AND RESOURCE EVALUATION

METHODOLOGY

By R.M. Flores

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

Introduction..........................................................................................................................DB-1

Database Construction.......................................................................................................DB-2

Drill-hole Location and Stratigraphic Data..............................................................DB-3

Coal Zone.......................................................................................................................DB-4

Correlation......................................................................................................................DB-5

Coal Analytical Data.....................................................................................................DB-7

Coal Resource Evaluation Methodology.......................................................................DB-8

Volumetrics....................................................................................................................DB-9

Coal Resource Categories.........................................................................................DB-10

Computerized Methods.............................................................................................DB-11

Conclusions........................................................................................................................DB-13

References..........................................................................................................................DB-14

Figures

DB-1. Cross section of the Wyodak-Anderson coal zone south of Gillette,

Wyoming.

DB-2. Diagrammatic cross section of a series of interconnected coal zones and

related coal beds. Not to scale.
DB-1

INTRODUCTION

The objectives of the coal resource assessment of the Fort Union Formation and

equivalent rock units in the northern Rocky Mountains and Great Plains region are:

(1) compile the information needed to assess selected coal beds and zones of the

Fort Union Formation and its equivalent formations that are potentially mineable in

the next few decades; (2) identify clean and compliant coal that meets standards of

the Environmental Protection Agency for sulfur, ash, and trace elements of

environmental concern; (3) create a publicly available digital database of this coal

that can be rapidly accessed and analyzed to provide information critical to decisionmaking by government, industry, and the public; and (4) produce widely available

digital products accessible in a variety of interpretive and interactive forms. In order

to accomplish these objectives, it was necessary to create a computerized database

that contains a vast amount of geographic, stratigraphic, and analytical information.

Data were stored, retrieved, manipulated, and analyzed utilizing StratiFact software

(GRG Corporation, 1996). This software consists of a relational database manager

and a geographic information system (GIS).

The data were acquired through cooperation with the U.S. Bureau of Land

Management, State geological surveys, U.S. Office of Surface Mining, and coal

companies. The USGS National Coal Resources Data System (NCRDS) also

provided digital data. Proprietary and non-proprietary data from these sources

consist of drill-hole information and coal quality and coal geochemical analyses

provided to the USGS as hard copies or in digital format. The data from hard

copies were entered manually using spreadsheet software (for example, Access,

QuattroPro, and Excel). The digital files were transformed into processible formats

(ASCII) and downloaded into the StratiFact database manager. When all datasets
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from each drill hole were edited, correlated, and checked for quality control, the

completed StratiFact database was queried to retrieve information to generate digital

files for processing in other software (for example, ARC/INFO, ArcView,

earthVision) for calculation of coal resources.

DATABASE CONSTRUCTION

Data for the Fort Union Formation and equivalent units were obtained mainly from

drill holes and subordinately from measured sections in outcrops used as control

points. Data from 18,207 drill holes were collected from government and private

industry sources for this assessment. More than seventy-five percent of these drill

holes are from coal exploratory drilling and development, and the rest are from oil

and gas exploration. Thus, the basic entity of the database is a drill hole bored

vertically from a surface location. Information from this drill hole is either in the

form of driller’s or geologist’s lithology logs from rotary drill cuttings or cores, and

geophysical logs (for example, gamma ray, density, neutron, resistivity, and

spontaneous potential). The digital files of the geophysical logs are not included in

the database.

The geophysical logs, which make up a large part of the drill-hole information,

require special analysis to measure the thickness of coal beds and related rocks. The

precision and accuracy of measurement of thickness of coal beds and adjacent rocks

on geophysical logs depend on the speed of logging, scale of the

log, type of log, type of equipment, and instrument settings (Vaninetti and

Thompson, 1982; Wood and others, 1983). Perhaps the most important part of the

log analysis is the ability of the user to identify the top and bottom of the beds by

using the points of inflection and mid-point of inflection methods (Wood and
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others, 1983, p. 55). The points of inflection method requires picking the top and

bottom of the beds where the curves change directions. The mid-point inflection

method requires picking the top and bottom of the beds at points midway between

the points of inflection and the initial peak. Thus, the value and accuracy of the

geophysical logs in measuring the thickness of coal beds and adjacent rocks varies

with the experience of the users. However, because the Fort Union coal beds are

very thick, the “operator’s error” with the use of either method is considered

negligible. In addition, the mining practice for these thick coal beds is to discard

some of the top of the bed and leave the bottom of the bed in place during mining,

both of which consist commonly of a “dirty” coal or carbonaceous shale considered

uneconomic.

The basic information required for the StratiFact database manager from each drill

hole includes: (1) point identification and geographic location; (2) stratigraphy; and

(3) depth-based coal analytical data. More information on file designs or

configuration of the database manager can be found in the guide to StratiFact

published by GRG Corporation (1996).

DRILL-HOLE LOCATION AND STRATIGRAPHIC DATA

The drill-hole identification is typically a unique hole number or a hole name. Drillhole location coordinates were entered in coordinate systems (Universal Transverse

Mercator system, latitude and longitude system, or state plane survey). Drill-hole

elevation data were entered as part of the location format. The rock types (for

example, coal, sandstone, siltstone, mudstone, etc.) were entered along with the top

and bottom depth of the lithology and lithology name. A “rock” name was entered

when the lithology between the coal is either undifferentiated sandstone, siltstone,
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and mudstone or unidentified. Additionally, the stratigraphic data contain formation

names (for example, Fort Union Formation), subdivisions labeled as coal zones (for

example, Wyodak-Anderson) consisting of one or more coal beds, and unassessed

zones above and below this zone. The assessed and unassessed zones are identified

according to the names of coal beds contained within the zones and/or members of

the formation in which they exist. A drill hole may penetrate one or more

stratigraphic formations or members of a formation that contain alternating coal

beds and rock types. The coal beds are recognized by standard nomenclature and

may exist in one or more members and in a formation.

COAL ZONE

Throughout a basin of coal deposition, individual coal beds may either thicken or

thin, merge or split into thinner beds separated by rock units (for example,

sandstone, siltstone, mudstone), or pinch out. Coal splits into two or more beds that

gradually thin or pinch out or interfinger with clastic rocks. Thus, a “zone” or an

interval of coal beds and interbedded rock units will contain coal in one or more

beds, and this “zone” is more correlatable over a wide area than are the component

coal beds.

The term coal zone is used in this investigation to define related coal beds that are in

stratigraphic proximity to each other but may not be all as one unit. Coal-zone

names are either adopted from accepted nomenclature or are selected from the coalbed names that are inclusive, from bottom to top of the interval, of contained coal

beds. Figure DB-1 is a diagrammatic representation of variation of a coal zone that

reflects the study areas in the Powder River, Williston, Hanna, Carbon, and Greater
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Green River Basins. A coal zone in these basins typically contains from one to as

many as eleven coal beds (for example, Wyodak-Anderson coal in the Powder

River Basin). As exemplified by the Wyodak-Anderson coal zone in the Powder

River Basin, the coal beds, from west to east, are split by fluvial deposits, merge

into one thick bed, resplit, and remerge into another thick bed (fig. DB-1). In

addition, the associated coal beds thin westward and pinch out or abut against “want

areas” or fluvial channel deposits. In this example, separate coal beds may exist

within the coal zone at some places, but some beds may be missing elsewhere. In

addition, a coal zone in one area may be interconnected with the same coal zone in

an adjoining area by either a coal bed at the top, in the middle, or at the bottom of the

coal zone (fig. DB-2). Thus, the laterally juxtaposed coal zone forms either a series

of “onlapping” or “zigzagging” patterns (fig. DB-2) throughout the basin of

deposition. Consequently, because of very complicated correlation and noncorrelation of coal beds within the coal zone, the coal resource assessment is more

easily performed on the coal zone rather than on the coal bed. The continuity of the

coal zone over a large area makes it more amenable to more accurate calculations of

coal resources than would assessment of individual beds.

CORRELATION

The coal zone is established in order to correlate in detail the various coal beds and

associated rock types. Correlation of coal beds is guided by the regional and local

geologic structures and lithofacies association or depositional settings of associated

rock types. That is, differences in depths of coal beds may be explained by the

structural dip of rocks and/or by structural faults and folds (Whitacker and others,

1978; Broadhurst and Simpson, 1983; Weisenfluh and Ferm, 1984). In addition,

occurrences of coal beds near the surface may be controlled by ancestral (for
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example, glacial) and modern (for example, river) erosion. More importantly, the

environments and related processes at the time of deposition or accumulation of the

coal beds (as peat deposits) influenced their lateral extent or continuity (Wanless,

1955; Ferm, 1970; Ferm and Staub, 1984; Flores, 1986). That is, highly dynamic

environments such as deltas and rivers, which laterally switched back and forth

(avulsion process) during their existence, are prone to develop very discontinuous

associated peat swamps that form coal. This process of avulsion makes correlation

of coal beds difficult unless it is aided by very closely spaced drill holes. In general,

inactive areas associated with these environments, such as floodplains, interdeltas,

and abandoned deposits, are prone to development of associated laterally extensive

peat swamps and subsequent coal beds. This allows uncomplicated correlation of

coal beds requiring less closely spaced drill holes. However, when these

environments were overrun by rivers and/or deltas, the accumulation of associated

peat deposits was interrupted, resulting in nondeposition of coal beds. Once these

rivers and deltas shifted elsewhere (for example, topographic low areas) the same

area may have been reoccupied by peat swamps and the coal deposition may

resume. These processes may explain the variable continuity of the coal beds as

well as their splitting and merging to form coal zones within the same area of

deposition. When these processes are repeated contemporaneously basinwide, the

result is “onlapping” and “zigzagging” of coal beds and zones. Thus, continuity of

coal beds and/or zones and associated rock types is a function of their depositional

environments, and the degree of reliability of correlation is determined by the

spacing of drill holes that indicate the lateral variations imposed by these geological

factors.

Palynology has been applied throughout the assessment region to provide a

biostratigraphic (“palynostratigraphic”) framework for correlation of coal beds and
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zones. A palynostratigraphic zonation was developed from reference sections in

selected outcrops, coal mines, and cores that are correlated to subsurface drill-hole

data. The Fort Union Formation and equivalent rocks were divided into six

palynostratigraphic zones designated P1 (lowermost Paleocene) through P6

(uppermost Paleocene) by Nichols (1994; 1996). The palynostratigraphic zonation

is the basis for age determinations of individual coal beds and zones, and for

correlations of coal-bearing rocks between basins in the assessment region.

COAL ANALYTICAL DATA

Each coal sample locality and/or drill hole is identified by a unique number and

name. When possible the original location number and name were retained from

the drill hole where the coal sample was collected. Location coordinates were

recorded in Universal Transverse Mercator, latitude and longitude, and state plane

coordinate systems. The coal analytical data consist of proximate and ultimate

analyses from drill holes and mine locations. Guidelines for sampling to ascertain

the rank and chemical, mineralogical, petrographic, and geophysical and physical

properties of coal are discussed by Swanson and Thompson (1976). The proximate

analysis includes the moisture, volatile matter, fixed carbon, and ash contents (on an

as-received, moisture-free or dry, and ash-free basis). The ultimate analysis consists

of the hydrogen, carbon, nitrogen, sulfur, oxygen, and ash contents (on an asreceived, moisture-free or dry, and ash-free basis). The calorific or heat value

(Btu/lb), forms of sulfur (sulfate, pyritic, and organic), and chemistry/mineralogy of

the ash (for example, aluminum, calcium, manganese, potassium, silicon, and/or

sodium oxides) are also included. Prescribed methods for analyses of these

physical and chemical properties of the coal are discussed in American Society for

Testing and Materials (1997).
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The geochemical dataset includes the 12 trace elements of environmental concern

named in the 1990 Clean Air Act Amendments (antimony, arsenic, beryllium,

cadmium, chromium, cobalt, lead, manganese, mercury, nickel, selenium, and

uranium). These data were entered in a spreadsheet format and each data entry

includes the name of the coal bed or zone from which the coal sample was collected.

The coal quality and geochemistry data from each sample location was linked to

either the actual drill hole from which the sample was taken or to the nearest

stratigraphic drill hole using the point identification number and the name of the coal

bed/zone. This method relates the coal quality and geochemistry data to the coal

stratigraphy, permitting spatial analysis and ultimately the use of coal quality (for

example, rank, total sulfur, SO2 per million Btu) in reporting the coal resources.

COAL RESOURCE EVALUATION METHODOLOGY

Coal resource evaluation employs digital computer technology (for example,

software), similar to manual methods, for isopaching and planimetering coal beds

and zones, and overburden to categorize and calculate quantities of coal. These

manual and computer methods determine the thickness and volume of coal,

overburden thickness of rocks above the coal, and reliability categories; however, the

computer method is deemed more efficient, more readily repeatable, more detailed,

and faster than the manual method. Detailed methods for data preparation and

computer calculation of coal resources for the assessed Fort Union and equivalent

coal in the Powder River Basin is discussed by Ellis and others (1999; in press).

See specific chapters in this CD-ROM for a general discussion of the methodology

used to assess the coal resources of other basins in this study (Williston Basin WN;
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Greater Green River Basin GN; Hanna and Carbon Basins HN). The coal resource

evaluations in these basins consist of estimating the volumetrics of 18 coal beds and

zones (Wyodak-Anderson, Rosebud, Knobloch, Harmon, Hansen, Hagel, BeulahZap, Ferris 23, 25, 31, 50, and 65, Hanna 77-79 and 81, Johnson-107, and

Deadman coal beds or zones). These selected Tertiary coal beds and zones, except

the Knobloch and Johnson-107, were mined and produced more than 30 percent of

the total U.S. coal production in 1997. Thus, these selected coal deposits are

important to development in the next few decades.

VOLUMETRICS

In order to calculate the volume for each of the 18 coal beds and zones, it is

necessary to determine the thickness and areal distribution of the coal. The volume

of coal is calculated using these parameters and is then converted from acre-ft to

short tons by factoring in the density (number of short tons per acre-ft for the

apparent coal rank) of the coal. Thus the formula used for resource calculation is:

short tons = acres x coal thickness x coal rank or density conversion factor (for

example, 1,750 for lignite, 1,770 for subbituminous, 1,800 for bituminous, and

2,000 for semianthracite and anthracite) (Wood and others, 1983).

As described under the section on construction of the database, coal beds existing in

vertical proximity were assigned to and correlated over large areas as a coal zone

throughout their depositional extent. Throughout this extent an individual coal zone

may contain one or more coal beds recorded in each drill hole. Thus, given the

number of coal beds and their continuity per coal zone, a method was devised to

measure and combine the net thickness of the coal beds of one zone at each datapoint location. This method was applied after the coal beds in the coal zone were
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correlated, taking into account all the geological factors discussed under the section

on database construction. Correlations defined the coal beds included in the coal

zone and the elevations of the top and base of the coal zone. The amount of

overburden (thickness of rocks above the coal zone) was calculated by subtracting

the grid of the surface elevations from digital elevation models (DEMs) and the grid

of the elevations of the top of the coal zone. As a part of this method, the rock in the

coal zone from each drill hole was also divided into partings and splits as defined by

Wood and others (1983). Rock partings exist when the thickness of the rock

between coal beds in the zone is less than the thickness of the underlying and

overlying coal beds. Rock splits exist when the thickness of the rock between the

coal beds in the zone is more than the thickness of either the underlying or overlying

coal beds. These criteria are tailored to the estimation of coal resources where a coal

bed bifurcates into two or more beds, as in the coal zone. This classification is

required in order to accurately calculate the net coal thickness of coal within a coal

zone.

COAL RESOURCE CATEGORIES

A standardized and universally accepted method for reporting reliability categories

of coal resources has been established for the U.S. Geological Survey (USGS) by

Wood and others (1983). This coal resource classification system is an expansion

of the system adopted by the U.S. Geological Survey (1976) and Averitt (1975) in

reporting the 1974 U.S. coal resources. In 1976, the USGS and the U.S. Bureau of

Mines modified the system used by Averitt (1975), which served as the standard

reference for coal resource/reserve assessment by many Federal and State agencies.

Our coal resource reporting categories generally follow the 1983 USGS

methodology (Wood and others, 1983), but with the addition of coal quality.
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The basic concept in the USGS coal resource classification system is based on the

geologic assurance that it is directly related to the distance from drill holes (control

points) where coal thickness and overburden are measured. The specified distances

from drill holes (control points) are reliability circles: 0–0.25 mi radius for

measured coal, 0.25–0.75 mi radius for indicated coal, 0.75–3.0 mi radius for

inferred coal, and beyond 3 mi radius for hypothetical coal. Thus, the classification

system is designed to quantify the amounts of coal (1) that are known or identified

resources (measured, indicated, and inferred reliability categories), and (2) that

remain to be identified (hypothetical reliability category). These distance reliability

categories, as well as the net coal thickness category (2.5–5 ft, 5–10 ft, 10–20 ft,

20–40 ft, and >40 ft) and the overburden thickness category (0–100 ft, 100–200 ft,

200–500 ft, and 500–1,000 ft), are required for reporting resources for the Fort

Union lignite and subbituminous coal (>500 ft is used because no or little

overburden exceeds 1,000 ft). In addition, coal resources were reported by county,

state, 7.5-minute quadrangle, Federal versus non-Federal surface and coal

ownership, coal quality (using ash and sulfur contents, and pounds of SO2/million

Btu), and apparent coal rank (as indicated by moist, mineral-matter free Btu;

American Society for Testing and Materials, 1997) categories. The coal resources

reported by these categories do not include mined out and leased areas, and areas of

burned coal and associated rocks or clinker. Also, coal resources of Indian tribal

lands were not calculated.

COMPUTERIZED METHODS

Several computer software programs were utilized to create digital information for

the calculation of the coal resources. ARC/INFO and ArcView software were used
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to create layers of spatial digital information or coverages (for example, state

boundaries, counties, geological boundaries, mine and lease boundaries, quadrangle

maps, clinker, point locations, etc.). These coverages are in Lambert Conformal

Conic projection, Clarke Spheroid 1866, with parameters of first standard parallel of

33°, second standard parallel of 45°, and central meridian of 106°.

The computer method for calculating coal resources involved compensating for the

irregular distribution (x and y values) of drill-hole data (dense versus sparse

distribution) within the study basins. In order to compensate for this distribution, a

rectangular grid was superimposed over the data area and a Z value (for example,

top and bottom of coal beds, net thickness, etc.) was interpolated by computer. This

gridding procedure was performed by using the earthVision software. Different

grid sizes and algorithms were used to test either the “coarseness” or “fineness” of

the grids in order to generate the appropriate isopach maps. Thus, resource

calculations were accomplished by using grid nodes and sub-nodes values in the

earthVision program (Ellis and others, 1999, in press; Roberts, 1998). Coal

thickness isopach and overburden maps generated from this procedure were utilized

to produce unioned coverages using ARC/INFO. These coverages are composed of

combined layers or polygons of spatial information containing attributes (for

example, counties, State, Federal versus nonFederal ownerships). After unioning all

the coverages, the resulting coverage was clipped to the coal zone areal extent. The

ARC/INFO union coverage polygon files were imported into earthVision for

volumetric calculations (assigned in short tons). This method, which used the net

coal thickness grid node and sub-grid node values of the coal thickness, was

determined to be the most accurate method for estimation of coal resources by

computer (Ellis and others, 1999; in press).
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The coal resources are reported in tables, which contain rows such as overburden,

coal thickness, and distance reliability categories. The tables also include columns

such as coal quality (percent sulfur and ash), lbs SO2/million Btu, and apparent coal

rank. Coal resources within each of these categories are reported as short tons (in

millions to billions) in two significant figures. Schuenemeyer and Power (in press)

developed a procedure for the estimation of uncertainty or measurement of error in

the volume of coal resources as a part of the USGS National Coal Resource

Assessment.

CONCLUSIONS

After drill hole and outcrop data are obtained and entered into a database, the process

of evaluating coal resources involves retrieving drill-hole data from the StratiFact

database manager for each identified and correlated coal zone in each study basin.

The data are used to create grids of measured values and finally to calculate coal

tonnages (in millions to billions short tons) categorized according to specific

intervals of depths or overburden, net thickness of coal, and degree of assurance of

existence of the coal (relation of distribution and quantity of drill holes). Additional

reporting categories include coal resources by counties, state, surface and subsurface

ownerships, coal quality, and apparent coal rank. Coal tonnages are reported in

Excel tables.
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American Society for Testing and Materials, 1997, Annual book of ASTM

standards; volume 05.05, gaseous fuels; coal and coke; atmosphere analysis:

West Conshohocken, Pennsylvania, p. 137-491.

Averitt, P., 1975, Coal resources of the United States, January 1, 1974: U.S.

Geological Survey Bulletin 1412, 131 p.

Broadhurst, F.M., and Simpson, I.M., 1983, Syntectonic sedimentation, rigs and

fault reactivation in coal measures of Britain: Journal of Geology, v. 91, p.

330-337.

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, 49 p.

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

Schuenemeyer, J.H., in press, Preliminary report on the methodology for

calculating coal resources in the Wyodak-Anderson coal zone, Powder River

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

98-789B.

Ferm, J.C. 1970, Allegheny deltaic deposits, in Morgan, J.P., ed., Deltaic

sedimentation; modern and ancient: Society of Economic Paleontologists and

Mineralogists Special Publication 15, p. 246-255.

Ferm, J.C., and Staub, J.R., 1984, Depositional controls of mineable bodies, in

Rahmani, R.A. and Flores, R.M., eds., Sedimentology of coal and coalbearing sequences: International Association of Sedimentologists Special

Publication No. 7, p. 275-290.
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Flores, R.M., l986, Styles of coal deposition in Tertiary alluvial deposits, Powder

River Basin, Montana and Wyoming, in Lyons, P.C. and Rice, C.L., eds.,

Paleoenvironmental and Tectonic Controls in Coal-forming Basins of the

United States: Geological Society of America, Special Paper 2l0, p. 79-l04.

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Corporation, 4175 Harlan Street, Suite 200, Wheat Ridge, Colorado 80033-

5150, 241 p.

Nichols, D.J., 1994, Palynostratigraphic correlation of Paleocene rocks in the Wind

River, Bighorn, and Powder River Basins, Wyoming, in Flores, R.M.,

Mehring, K.T., Jones, R.W., and Beck, T.L., eds., Organics and the Rockies

Field Guide: Wyoming State Geological Survey Public Information Circular

No. 33, p. 17-29.

_____1996, Palynological zonation of the Paleocene of the Rocky Mountain

intermontane basins: Geological Society of America, 1996 Annual Meeting,

Abstracts with Programs, v. 28, no. 7, p. 372.

Roberts, L. R., 1998, Flow chart of steps used to calculate and report coal tonnage

estimates for the National Coal Assessment Fifteenth Annual International

Pittsburgh Coal Conference.

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

resource assessment—an application to coal: Mathematical Geology.

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analytical methods used in the U.S. Geological Survey for determining

chemical composition of coal: U.S. Geological Survey Circular 735, 11 p.

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Bulletin 1450-B, 7 p.

Vaninetti, J., and Thompson, R.M., 1982, Geophysical well-logging and related
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subsurface data useful in coal exploration and development program:

American Association of Petroleum Geologists, Short Course Notes, 34 p.,

82 figs.

Wanless, H.R., 1955, Pennsylvanian rocks of eastern Illinois Basin: American

Association of Petroleum Geologists Bulletin, v. 39, 1753-1820.

Whitacker, S.H., Irvine, J.A., and Broughton, P.L., 1978, Coal resources of

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Survey of Canada Economic Geology Report 30, 151 p.

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Pratt seam, Black Warrior Basin, Alabama, U.S.A., in Rahmani, R.A. and

Flores, R.M., eds., Sedimentology of coal and coal-bearing sequences:

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317-332.

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

Geological Survey Circular 891, 65 p.
WA

{

}WA

Coal

Quaternary Rocks

Sandstone

Mudstone

Undifferentiated "rock"

Pinch out of coal bed

Want area

Split

coal bed

Merged coal bed

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.

8.26 Miles

West East

WA = Wyodak-Anderson Coal Zone

100 Feet

0

Figure DB-1. Cross section of the Wyodak-Anderson coal zone south of Gillette, Wyoming.
Coal Beds

Undifferentiated "rock"

Coal

The term "rock" indicates undifferentiated

sandstone, siltstone, mudstone, and limestone.

Figure DB-2. Diagrammatic cross section of a series of interconnected coal zones

and related coal beds. Not to scale.