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Coal Resource Classification System of the U.S. Geological Survey

By Gordon H. Wood, Jr., Thomas M. Kehn, M. Devereux Carter, and William C. Culbertson



These general guidelines are required for uniform classification and reporting of coal into the different resource categories. They are modeled after the guidelines in Circular 831 (U.S. Geological Survey, 1980, p. 3-4) for all minerals.

1. All naturally occurring concentrations of coal can be distributed into one or more of the classification categories.
2. Where the term "reserves" is used without a modifying adjective, for example, marginal, indicated, restricted, low-sulfur, or inferred, it is to be considered synonymous with the demonstrated-economic category.
3. Quantities and qualities of coal may be expressed in a variety of terms and units to suit different purposes but must be clearly stated and defined.
4. A reserve base is a resource category delineated only by physical and chemical criteria. A major purpose for the recognition and discrimination of a reserve base is to aid in long-range public and commercial planning. A reserve base estimate for specific rank, thicknesses of coal and overburden, quality, usage, geologic formation, age, depositional environment, and many other factors can be specified for any given deposit or area, or for the Nation. The position of the lower boundary of a reserve base is intended to extend into the subeconomic category. The intention of this extension is to define quantities of in-place material, parts of which may become economic depending on the extraction plan finally utilized. As a result of any given extraction plan, the reserve base can be subdivided into component parts--reserves, marginal reserves, and a remnant of subeconomic resources. For the purpose of Federal (USGS) assessment, criteria for the reserve base are listed on page 29 of this report.
5. Undiscovered resources should be subdivided in accordance with the definitions of hypothetical and speculative resources or they may be subdivided in terms of relative probability of occurrence (see figs.
1 and 2.).
6. Inferred reserves and the inferred reserve base represent postulated extensions of reserves and the reserve base. They are identified resources but are quantified with a relatively low degree of certainty. Postulated quantities of resources based on geologic inference alone should be assigned to the undiscovered categories.
7. Locally, limited quantities of coal may be produced from beds that are of insufficient thickness or are too deeply buried to be classified as reserves. This situation arises when production facilities are already established or when favorable local circumstances, such as particular coal qualities or removal of overburden for other purposes, make it possible to produce coal that otherwise could not be extracted profitably. Where such production is occurring, the quantity of in-place coal (including coal for in situ gasification) shall be included in a reserve base and the quantity that is potentially producible shall be documented as reserves. The profitable production of such coal, however, should not be used as a rationale to assign a reserves classification to coal in other areas having similar overburden, thickness of coal, and qualities.
8. Coal resources classified as reserves must be considered as economically producible at the time of classification. Conversely, coal not currently producible at a profit cannot be classified as reserves. However, there are situations in which mining plans are being made, lands are being acquired, or mines and plants are being constructed to produce coal that does not meet economic criteria for reserve classification under current costs and prices but that would do so under reasonable future projections and expectations. The marginal reserve category applies to such situations. When economic production appears certain, coal classified as marginal reserves will be reclassified as reserves. Marginal reserves also may include any other coal whose economic producibility borders on being profitable.
9. Tonnage of coal that is too impure, too thin, too deep, or for other reasons not considered to be potentially economic may be estimated, but is not to be classified as a resource. These tonnages may be classified in the box labeled "other occurrences" in figures
1, 2, and 3.
10. Rank classes of coal, for example, bituminous coal as distinct from lignite, should be separately quantified.
11. The amount of cumulative production is not a part of the remaining coal resources. However, an understanding of what has been produced in the past is important to resource understanding, both in terms of the amount of production and the amount of coal remaining in place. Separate accounting for cumulative production should be made for each report area, county, State, and the Nation.
12. By-passed coal in large blocks left in the ground during mining, or planned to be left in the ground during current or future extraction, should be recorded in an appropriate resource category depending upon its economic recovery potential. Coal lost-in-mining should also be recorded in an appropriate resource category if there is a potential for further recovery.
13. In classifying reserves and resources it may be necessary to recognize locally that some coal deposits derive their economic viability from coproduct or byproduct relationships with other commodities.
14. Factors other than economic and geologic considerations, including legal, regulatory, environmental, and political concerns, may restrict or prohibit the utilization of all or part of a coal deposit. Reserve and resource quantities known to be restricted should be recorded as such in an appropriate category.
15. This classification system allows for the presentation of more subdivisions than will commonly be reported or for which data are available. Where appropriate, subdivisions may be aggregated or omitted.
16. Data supporting resource estimates are to be documented and preserved.



Where coal of more than one rank class or rank group is covered by an individual report, the resource data shall be reported separately for each major rank class and when possible for each rank group listed below:

Class Rank group Abbreviation
Anthracite Meta-anthracite ma
Do Anthracite an
Do Semianthracite sa
Bituminous coal Low-volatile bituminous coal lvb
Do Medium-volatile bituminous coal mvb
Do High-volatile A bituminous coal hvAb
Do High-volatile B bituminous coal hvBb
Do High-volatile C bituminous coal hvCb
Do Subbituminous A coal subA
Subbituminous coal Subbituminous B coal subB
Do Subbituminous C coal subC
Lignite Lignite A ligA
Do Lignite B ligB

Abbreviations can be used wherever appropriate in reports and tables.


Tonnage estimates shall be reported according to thickness of overburden:

Mandatory underground mining categories: Mandatory and optional surface mining categories**1:
0 to 500 feet (0 to 150 m) 0 to 500 feet (0 to 100 m) mandatory use
500 to l,000 feet (150 to 300 m) 0 to 100 feet (0 to 30 m)optional use
l,000 to 2,000 feet (300 to 600 m) 100 to 200 feet (30 to 60 m) optional use
2,000 to 3,000 feet (600 to 900 m) 0 to 200 feet (0 to 60 m) optional use
3,000 to 6,000 feet (900 to 1,800 m) 200 to 500 feet (60 to 150 m) optional use
Optional other occurrence category:
>6,000 feet (> 1,800 m)

**1 Use of optional surface mining categories requires the complete coverage of the 0-500 feet (0-150m) category.
Other categories may be used if they are in increments of 100 feet (30m) and do not exceed 500 feet (150m).

Resources of currently and potentially strippable and underground minable coal beds and coal zones shall be computed for the 0-500 feet (0-150 m) overburden category. Use of the surface mining optional categories allows tonnage estimates to be related to overburden. When optional categories are used, the sum of their tonnage estimates must be equal to the tonnage estimate for the 0-500 feet (0-150 m) category.

In addition, other criteria such as the ratio of overburden to coal thickness or the ratio of cubic yards of overburden to tonnage of coal may be used to outline and evaluate strippable coal deposits. Such departures from the standard categories and criteria are advisable only where there is adequate data.


Tonnage estimates shall be reported by rank and thickness of coal:
Anthracite and bituminous coal Subbituminous coal and lignite
14 to 28 inches (35 to 70 cm)**1 2.5 to 5 feet (75 to 150 cm)**1
28 to 42 inches (70 to 105 cm) 5 to 10 feet (150 to 300 cm)
42 to 84 inches (105 to 210 cm) 10 to 20 feet (300 to 600 cm)
84 to 168 inches (210 to 420 cm) 20 to 40 feet (600 to 1,200 cm)
168 inches or thicker (420 cm+) 40 feet or thicker (1,200 cm+)

**1 Centimeter thicknesses may be reported in meters.


For future planning by Federal and State governments, industry, and the public, coal resources such as identified resources, reserve base, and reserves, are to be estimated for relatively small areas. Unit areas the size of townships, 7. 5-minute quadrangles, and 15-minute quadrangles generally are satisfactory. For specific purposes such as tract assessment, evaluation, and management, however, it may be desirable and (or) necessary to report estimates for areas as small as an individual land section, or smaller.

In addition, there are many requirements for estimating the remaining tonnages and type of coal for each coal-bearing county, State, and the Nation. Although tonnage estimates for most currently known coalbearing counties and States exist, many are inadequate and out of date. Numerous requests for information emphasize the need for modern reliable county and State estimates. Such modern estimates are to be prepared as information becomes available and can be assembled. The unit area for such re-evaluations is logically a completed county, the smallest unit universally employed by State and National agencies. Therefore, all resource estimates regardless of size of area (except for State and national estimates) must be reported by the county in which they occur.


Estimates of the total coal tonnages in the following resource categories are required, where data are available, for adequate inventorying of county, State, and national coal resources: measured, indicated, inferred, and hypothetical resources; reserve base and inferred reserve base; and original resources and remaining resources. Additional divisions of total resource estimates are not required but are desirable to document more thoroughly the quantities of coal available for particular uses.


A map of each coal bed or coal zone known to contain coal resources must be prepared to document the estimate for coal resources. A coal bed map will show (1) the trace of a bed's outcrop; (2) all points where the thickness of coal was measured at outcrops and other surface exposures; (3) all wells, drill holes, and subsurface points where the thickness of coal was measured; (4) county, State, and national boundaries; (5) land lines (townships and ranges) and latitude and longitude coordinates; (6) boundaries of Federal and State Parks, forests, wildlife refuges, grasslands, military bases, and areas known to be environmentally or legally restricted from mining; (7) reliability categories; (8) isopachs of thicknesses of coal and overburden; (9) structure contours; (10) areas where coal is absent because of erosion or nondeposition; (11) structural features affecting coal, such as fold axes and traces of faults; and (12) boundaries of prior surface and subsurface mining. Information derived from proprietary data can be deleted where required by the author in preparing a bed map for publication. Such proprietary information may include, for example, points of thicknesses of coal beds and overburden, drill-hole locations, mine boundaries, and other data furnished by mining companies or land proprietors. Coal beds may be so numerous in some areas that only selected bed maps of the major deposits would be published, even though bed maps of all coal beds would be prepared in order to estimate the resources.

Figures 9 through 23 show how to prepare parts of a coal bed map. Tables 4 through 7 show listings of areas of reliability by thicknesses of coal and overburden categories derived from figures 9 through 23 (see figs above). The areal interrelations of data depicted on a coal bed map define a series of areas, each of which represent a separate coal resource category. The areal extent, average thickness of coal, and the depth, rank, and weight of coal per unit-volume in each area must be determined in order to estimate the tonnages of coal underlying each area.


The stratigraphic thickness of coal is measured at outcrops, in trenches and prospect pits, at mine faces either underground or in surface pits, and in drill holes by direct measurement or geophysical logging. (See "Geophysical Logs * * *,"p.46.) Measurements may be made in inches or in feet and inches, in feet and tenths of feet, or in meters and centimeters. Partings in a coal bed greater than 3/8 inch (1 cm) should be excluded from the thickness measurements of a coal bed when estimating coal resources (see thickness of coal for resource calculations, p. 31, and specific instruction No. 3, p. 34) but should be recorded as stratigraphic information. Field measurements of the thicknesses of coal beds should be made to the nearest 1 inch, tenth of a foot, or metric equivalent. Thicknesses determined from core and drill-hole logs and from geophysical logs should be reported to the smallest practical unit of measurement. The weighted average thickness of a coal bed based on field measurements in an area should be reported to the nearest inch, tenth of a foot, or metric equivalent. An average thickness should be determined by using all measurements of coal, by the thickness gradients between measurements, and by isopaching. The thickness of coal at specific points should be recorded on coal bed maps, and then the bed should be isopached using the points of measurements and gradients between the points. An isopach should be drawn along each of the standard coal thickness category limits pertinent to the rank and thickness of coal in the area to be estimated. The weighted average thickness of an area of coal is calculated by estimating the size of each isopach unit according to the percent or proportion of the area covered by the unit. Where mining has been extensive, the thickness of coal in unmined areas can be extrapolated from data on thicknesses of coal obtained from mine maps and from adjacent mines.


The distribution of data points is exceedingly important in the estimation of coal resources because distribution (including spacing) is universally considered as the ultimate control governing the reliability, accuracy, and precision of any estimation. In most areas the coal resource worker must rely on existing outcrops, trenches, prospect holes, mine workings, and drill-hole data. If drilling is possible, a drilling pattern should be established, on the basis of geologic knowledge, to supplement the existing data so as to raise the assignment of coal resources to a higher degree of reliability.


Coal-bearing areas, as determined from coal bed maps (see figs 9-23) are to be measured to a precision of 2 percent or less. Such determinations may be made with a planimeter, with graph paper, with equally spaced dots, or with a computer. The most common instrument used for area determinations is the polar planimeter. In recent years the digital electronic planimeter has become increasingly popular. Prior to determining areas with a planimeter, a planimeter factor for acres or hectares, which depends upon the scale of the map used, must be ascertained. This is done according to instructions that accompany the planimeter. A similar factor must be determined if the graph paper technique of determining acreages is employed. After either factor is ascertained, the user is prepared to start measuring areas on the map and to convert the measurements using the appropriate planimeter or graph paper factor into acres or hectares. On many 7.5-minute quadrangles, several dozen to several hundred areas must be measured that are based on the many parameters into which coal-bearing areas can be categorized. These parameters may include thicknesses of coal and overburden; distance from points of coal thickness measurements (reliability categories); quality; physical characteristics; rank, land ownership by Federal, State, Indian, and local governments, companies, individuals, and other nations; county, State, and townships and ranges of the land classification system; quadrangle, coal field, basin, region, and province; legally and environmentally restricted areas, and others as desired.

As the areas are measured, mean results ascertained by averaging the planimeter or graph paper readings are recorded so that each area's acreage can be computed

A planimeter is accurate in measuring map areas ranging from several square inches to 20-30 square inches. Generally, planimetric measurements are repeated several times and then averaged. However, if readings are in disagreement by more than 2 percent, they should be repeated until an agreement of 2 percent, or less, is achieved. Map areas of less than 1 square inch commonly are not as precisely measurable with a planimeter as are larger map areas and must be remeasured and the planimeter vernier read many times to obtain an agreement within a 2-percent error. Repetition of planimetric measurements with only two vernier readings, one at the beginning and one at the end, can be accomplished by continuously tracing the borders of the area being measured a predetermined number of times in a manner similar to the repetitive turning of angles with a transit by a surveyor. The difference between the two readings should be divided by the number of times the borders are traced; the answer multiplied by the planimeter factor is the size of the area. This repetitive measurement of an area is a good method of obtaining precise planimetric readings.


A tonnage estimate for any coal deposit can be made if the thickness of coal, areal extent, and weight of coal per unit volume are known. The weight of coal per unit volume (density) or specific gravity varies with rank, ash content, and the amount of each macerals group (such as vitrinite, inertinite, and exinite) in the coal. Ideally the density of coal in a deposit should be determined from numerous specific gravity determinations on unbroken coal, but there are rarely sufficient determinations to characterize a deposit. Therefore, it is recommended that tonnage calculations be based either on the average specific gravity or the average weight of unbroken coal per unit volume of the different ranks shown in table 2.


After the area underlain by coal, the average thickness of coal, and the weight of coal per unit volume for each category shown on a coal bed map have been determined, the tonnage can be estimated. The tonnage is estimated by the following formula:

A x B x C = tonnage of coal


A = weighted average thickness of coal in inches, feet, centimeters, or meters,
B = weight of coal per appropriate unit volume in short or metric tons, and
C = area underlain by coal in acres or hectares.


The rounding of tonnage figures shall be done only after all calculations have been completed using data for areas (acres or hectares), coal thicknesses, and weight of coal per unit volume and summed for each coal category. The tonnage estimates for each coal category are then rounded to significant numbers so as to not eliminate tonnage estimates for small areas of measured and indicated coal. For example, the tonnage estimates for small areas such as A, B, C, F, G, H, K, M, O, P, Q, and R in figure 17 and A, B, C, D, E, F, G, H, L, Q, W, Y, AA, HH, and OO in figure 19 are not rounded out of the totals and are included in the summing of the total coal resources for all areas in figures 17 and 19. After summing, the resultant total coal resources estimates for a whole map area (a quadrangle, county, or basin) are not rounded because their component parts have been rounded.


Resource estimation is difficult in those localities where a coal bed bifurcates or splits into tongues, each of which exceeds the minimum thickness for resource estimation. The difficulty is caused by the necessity to delineate a boundary between the area where the resources of the main coal bed are estimated and the areas where the resources of the tongues are individually estimated.

As stated in the glossary, a parting is "a layer or stratum of non-coal material in a coal bed which does not exceed the thickness of coal in either the underlying or overlying benches" of coal. Where the non-coal material exceeds the thickness of either the underlying or overlying parts of the coal bed, the coal bed is considered for the purpose of resource estimation to have split into two coal beds (each of which is depositionally a tongue from a thicker main coal bed).

In estimating resources in such a geologic situation, it is necessary to delineate on a coal bed map the areas where resources will be separately calculated; to do so, a line is used to connect all points where one of the tongues becomes thinner than the intervening parting. It is also necessary to locate on the coal bed map all points where the thickness of the main coal bed and the tongues were measured. Measured, indicated, and inferred reliability circles should be drawn on the bed map from each point of thickness of coal measurement on the main bed and the tongues. The circles should be drawn, according to the appropriate distance for each reliability category, across the boundary line for resource estimation. After the circles are drawn, the thickness variations of the main coal bed and each tongue (bed) are to be isopached separately. Tonnage estimates should then be calculated for each tongue (bed) and the main bed using procedures outlined elsewhere in this report for separate coal beds.


The estimation of hypothetical coal resources in areas where geologic, thickness, rank, and areal size data are sparse or absent is necessary to promote exploration for poorly known and undiscovered coal areas. Currently about 2.24 trillion short tons of the Nation's 3.68 trillion tons of remaining coal resource inventory are classified as undiscovered (hypothetical) (Averitt, 1975). Much additional unknown coal may be concealed in the central parts of basins and is not as yet included in the Nation' s coal inventory. This additional unknown coal must be identified, as must the 2.24 trillion tons currently remaining in the inventory, because knowledge of the quantity, quality, and rank of the unknown and hypothetical coal could influence the Nation's energy usage plans. Therefore, a question commonly asked by government and industry estimators responsible for inventorying coal resources is: How do you estimate resources where there are no nearby thickness, depth, and rank data? Another way of stating this question is: How do you estimate the resources of central parts of poorly explored or unexplored basins?

The problem of estimating hypothetical resources in the central parts of basins and elsewhere has been approached by (1) not attempting to calculate them; (2) assuming that the centers of the basins or coal-bearing areas where control is lacking are barren of coal; (3) assuming that the better known marginal areas of coal-bearing basins are representative of the average quantity of coal per square mile in the central parts of the basins; (4) assuming an average thickness of coal and applying this factor to the volume of coal-bearing rocks in an area; and (5) making an outright guess. None of these solutions are satisfactory. A sixth solution has been to extrapolate measurement data from surrounding areas into the unknown area. This approach is better than the others but commonly is inadequate because it does not consider geologic factors that would control unsystematic distribution of coal in an unknown area.

The following discussion describes two methods, based on geologic principles, for estimating hypothetical resources. Both are acceptable if the supporting basic geologic and measurement data are documented and presented for evaluation. These alternative approaches can be termed the "extrapolated bed map method" and the "extrapolated coal zone method. "Both methods are based on the extrapolation of geologic knowledge and measurement data into an area of unknown coal resources. In some problem areas, a mix of the two methods may be desirable.

Much of the data called for in the descriptions of the extrapolated bed map and extrapolated coal zone methods will not be available in many areas where hypothetical resources must be estimated. The descriptions of the methods are written for the ideal amount of control adjacent to an unknown area; thus, the methods will have to be modified on the basis of the type, availability, and amount of control data.


Step 1.--Assemble all available geologic data in the areas adjacent to the unknown area for the coal bed whose resources are to be estimated.
Step 2.--Assemble and plot on a base map all pertinent data on thicknesses of coal and overburden, quality of coal, reports of coal in drill holes and wells, and rank of coal.
Step 3.--Analyze all data collected in steps 1 and 2 to ascertain trends of coal deposition by (a) constructing isopach maps of the coal bed in known areas; (b) constructing isopleth maps of heat values, ash contents, and contents of trace elements that may be related to sources of sediment; and (c) identifying trends of persistent thick or thin coal, or other parameters of coal.
Step 4.--Identify depositional and erosional trends and features in the known areas of rocks adjacent to the coal bed and in the coal bed itself that may indicate ancestorial through-flowing rivers in swamps, coastal environments, unconformities, directions of delta building, paleogeomorphic locations of swamps on a coastal plain or delta, and directions and distances to sediment sources. Plot all pertinent data on a map.
Step 5.--Extend stratigraphy from known areas into and across the unknown area by utilizing all surface and subsurface information. Collect surface stratigraphic data and existing petroleum and water well information. Evaluate and analyze all data and determine best correlations so that coal bed extensions and correlations can be made. Determine from available data probable geologic model of deposition. Plot all data on the map of step 4.
Step 6.--Construct structure sections across the unknown area so that changes in the thickness of overburden and rank of the coal bed can be considered and (or) postulated.
Step 7.--Construct a structure contour map of the coal bed in the unknown area.
Step 8.--Combine all pertinent data from steps 2, 3, 4, 5, and 7 onto a single coal bed map.
Step 9.--Fill in the unknown area by projecting pertinent coal and other data across or through the unknown area. This projection should include isopachs for thicknesses of coal and overburden, isopleths for rank and (or) heat values, predicted erosional channels cutting through the coal, and other stratigraphic, depositional, or structural features affecting the coal bed.
Step 10.--Place land lines and boundaries of political subdivisions on the map.
Step 11.--Define areas of hypothetical coal by plotting the outer limits of inferred coal as determined from points of thickness control in the known areas. Determine acreages of hypothetical coal for the following resource categories: thicknesses of coal and overburden; rank; quadrangle; townships and ranges; counties; and State.
Step 12.--Determine the average coal bed thickness for each acreage unit in each category.
Step 13.--Calculate and sum estimated tonnages for each category, round estimated tonnages to significant figures as per specific instruction No. 12, p. 36, and sum into a final hypothetical tonnage. These estimated tonnages and the total summed tonnage probably represent the best information that can be estimated for a bed that extends from a known area into an unknown area where control points are absent or sparse.


If the procedure for an extrapolated bed map analysis of hypothetical coal resources is impractical because of time limitations or sparsity of data it is suggested that the approach to estimating hypothetical resources be by an extrapolated coal zone method.

Step 1.--Determine if a coal zone exists by analyzing the available data.
Step 2.--Assemble and plot on a base map all available geologic data in areas adjacent to the unknown area.
Step 3.--Construct an isopach map of the cummulative thicknesses of all coal beds in the coal zone (less partings) that exceed 14 inches for anthracite and bituminous coal beds and 30 inches for lignite and subbituminous coal beds in known areas.
Step 4.--Identify trends and geologic features in the rocks of the coal zone in the known area that may indicate persistent ancient through-going rivers, coastal environments, unconformities, directions of sediment transport, intertonguing sediments and coal beds, regressions and transgressions, and so on. Plot pertinent data on a map.
Step 5.--Extend stratigraphy of the coal zone and related rocks into and across the unknown area on the map of step 4 by utilizing all surface and subsurface data. Determine geologic model of deposition of the coal zone. Construct stratigraphic diagram of the coal zone and fit diagrammatic data to the map.
Step 6.--Construct structure sections across the unknown area so that thickness of overburden and rank changes of coal zone can be considered and (or) postulated.
Step 7.--Construct a structure contour map of the coal zone in the unknown area.
Step 8.--Combine all pertinent data from steps 2, 3, 4, 5, and 7 onto a single coal zone map.
Step 9.--Fill in unknown area by projecting pertinent coal zone and other data across or through unknown area. Step 10.--Place land lines and boundaries of political subdivisions on map.

Step 11.--Define areas of hypothetical coal by plotting an outer limit of inferred coal as determined from points of thickness control in the known areas. Determine acreages of hypothetical coal for the following categories: thicknesses of overburden, cummulative thicknesses of coal, rank, quadrangle, townships and ranges, counties, and State.
Step 12.--Calculate and sum tonnages for each category after determining a cummulative thickness of coal for each acreage unit in each category and calculating tonnage. Round tonnages to significant figures as per specific instruction No. 12, p. 36, and sum into a total hypothetical tonnage amount.

The extrapolated coal zone solution for hypothetical coal eliminates the estimation of tonnages for many individual beds and, in the view of some workers, provides a more valid estimate than the bed-by-bed approach. Despite this view, the bed-by-bed method is herein recommended for use where practicable and (or) feasible.


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