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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



Coal resource estimation systems on a worldwide basis generally lack illustrations amplifying written instructions as to how to calculate coal resources.

The following illustrations are intended to bridge this gap and to show how coal resources data are utilized to construct bed maps. Many more illustrations could be prepared, but the accompanying figures are believed to be representative.

Figure 9 illustrates how to determine areas of reliability (areas based on distance of from points of thickness measurements-- measured, indicated, inferred, and hypothetical coal) where a coal bed has been mapped on the surface and the thickness of coal has been measured at five points. The solid outcrop line indicates where the bed has been mapped with assurance as to its intersection with the ground surface; the dashed line indicates uncertainty as to the exact location. Utilizing radii originating at the points of thickness measurements, arcs are constructed at appropriate distances according to the criteria for the measured, indicated, inferred, and hypothetical reliability categories. The arcs enclose areas theoretically underlain by coal for each reliability category.

Figure 10 uses the same coal bed outcrop as figure 9 and shows the effect of additional thickness information from drill-hole measurements and a small mine. A comparison of figures 9 and 10 illustrates how the additional data derived from drill-holes and mining expand the areas designated in figure 10 as measured and indicated and modify the inferred and hypothetical areas.

The objective of figure 11 is to show how drill-hole data combined with outcrop data on a continuously exposed strip-mined coal bed result in the coalescing of measured and indicated coal and in an expansion of the area of the inferred reliability category. It also shows how a single point of thickness of coal measurement defines an area of measured, indicated, and inferred coal.

The primary objective of figure 12 is to show how to construct a minimum thickness-of-coal isopach (14 inches) based solely on outcrop data. A secondary objective is to illustrate how to locate a coal outcrop and boundaries such as county lines on a bed map from a geologic map. Generally, as a bed thins along an outcrop, it is increasingly more difficult to trace. Figure 12 illustrates a simple, nearly straight coal bed outcrop with three thickness-of-coal measurements. The three measurements are all thicker than the 14-inch minimum thickness of coal considered when calculating anthracite and bituminous coal resources. The thickness gradients between the 26-inch and each of the 20-inch points of measurement of coal are extrapolated along the crop line to the east and west of the 20-inch measurements to points A and B where the coal gradients predict coal thicknesses of 14 inches. A straight line is then drawn between the predicted 14-inch points A and B on the crop line and subsequently is bisected at mid-point O. An arc AB is drawn between the predicted 14-inch points using either AO or BO as a radius. The arc AB locates the 14-inch isopach according to the available data on thickness and length of outcrop.

Figure 13 displays areas of reliability, thickness-of-overburden contours, political subdivisions, and a completed coal bed map ready for determinations of acreages and weighted average thickness of coal in each area. Areas A to P are ready for planimetry of acreages, for calculation of coal tonnages after weighted average thickness of coal for the areas are determined, and for subsequent entry on tonnage tables according to their assignment to a thickness of coal, thickness of overburden, reliability category, and to a county. Table 4 lists the areas of reliability by thickness of overburden category, by a single thickness of coal category, and by counties. To avoid confusion, figure 13 and table 4 were constructed with only one thickness of coal category, 14 to 28 inches. Subsequent figures are more complicated because more thickness categories are introduced.

Figures 14 and 15 show a diagram of a ridge underlain by coal; they also show the geometric method of determining the 14-inch minimum thickness of bituminous coal isopach and the areas of reliability constructed from the points of thickness of coal measurement. The 28- and 42-inch coal isopach lines were constructed by using the gradients between the various thicknesses of coal measurements. This method of determining the area of coal under a ridge is commonly necessary when thickness measurements are sparse and an estimation of coal resources beneath a ridge is required. Table 5 enumerates the areas assigned to the varous thicknesses of coal and overburden and reliability categories.

Figures 16 and 17 and table 6 show how to construct the 14-inch minimum thickness limit of anthracite or bituminous coal resources, other coal isopachs, and the areas of reliability and overburden isopachs for a simple valley reentrant in a coal bed outcrop. Figures 16 and 17 and table 6 are important because they show how to treat a valley reentrant in a coal outcrop characterized by minimal thickness data.

Figures 18 and 19 and table 7 show how to determine the location of the minimum coal isopach (14 inches for bituminous coal) in a valley and ridge underlain by a coal bed that is locally less than the minimum thickness. Figure 19 also illustrates the areas of reliability, over-burden categories, and coal isopachs. The location of the minimum coal isopach (14 inches) on a ridge which has thickness data on only one side is a common problem facing geologists estimating coal resources.

One of the most difficult problems in estimating resources of coal is where the beds are moderately to steeply dipping or are highly deformed. A simplified problem is illustrated in figures 20A, B, and C by a bed that dips uniformly 30 degrees into the subsurface. The problem as illustrated can be solved by two methods. The first method is to estimate all areal resource tonnages in the plane of the coal bed and then to project all pertinent areal categories to the ground surface. For example, the measured, indicated, inferred, and hypothetical reliability circles, coal isopachs, overburden contours, and structure contours should be drawn in the plane of the coal bed (fig. 20A). Then the areas of each of these categories are planimetered and coal tonnages estimated. This procedure provides an adequate estimate of coal tonnages for all values of dip. However, it is difficult to project the areas from the plane of the coal bed to the surface area overlying any particular category (fig. 20B). Although this method gives an adequate tonnage estimate, it distorts the ground surface depiction of areal categories (fig. 20C).

The second method is to draw the areal boundaries of the various categories on a surface map, measure the areas underlain by the various categories, and estimate the coal tonnages for each category. After estimation of the resources as if they were flat-lying, the estimates are divided by the cosine of the dip. The resultant answer is the tonnage in the plane of the bed. This method will provide correct tonnage estimates in each resource category underlying the surface but will horizontally exaggerate the areas of measured, indicated, and inferred coal and should be used only where the dip is more than 10 degrees and less than 30 degrees. For dips above 30 degrees, it is recommended that resource estimates be made by the first method. For dips less than 10 degrees, all estimation of resources should be done as if the bed was flat-lying.

The authors suggest that geologists involved with estimating the coal resources of highly deformed areas consult with coal geologists who have worked on similar areas and estimation problems. Currently, resource problems in highly deformed areas have not been solved except locally for small areas. Although methods have been developed, they are generally very time consuming and as yet unproven by mining. Figure 21 illustrates a coal bed that has been mined from shafts at some distance from outcrops. All data are derived from the mines or drill holes. The figure shows measured, indicated, inferred, and hypothetical coal; county lines; thickness of coal measurements; coal thickness categories; and the 0-200 feet, 200-300 feet, and the 300-500 feet overburden categories. It also shows a national park, a national bird refuge, a State park, and a large lake, all of which merit assignment of the underlying coal in those areas to restricted categories.

Figure 22 demonstrates a small coal basin where many resource classification categories are distinguished, including two large areas of hypothetical coal. The figure also shows two coal beds that intertongue with an intervening linear sandstone body. The coal beds have been strip-mined at six localities and underground mined at three localities. The coal beds have been tested by 15 drill holes which revealed the existence of two coal beds separated by sandstone. Numerous surface measurements of coal thickness support the widespread extent of at least one bed, and three measurements revealed the relations of two coal beds with the sandstone body. A bed map of the basin revealed two coal-bearing areas separated stratigraphically and areally by an intervening sandstone body. The completed bed map in figure 22 allows the segregation and subsequent estimation of the resources of the two coal beds into a large number of categories enumerated in the caption for the figure. The two coal beds are subdivided by their heat values on a moist, mineral-matter-free basis into lignite and subbituminous coal. The 8,300-Btu isolines that represent the boundary between subbituminous coal and lignite are drawn so that tonnage estimates can be made for proper rank classification.

Figure 23 depicts a large but simple subbituminous coal and lignite basin that contains predominantly hypothetical resources traversed by three synclines and two anticlines. The total thickcness of coal at each point of surface measurement and in each drill hole penetrating the coal zone is obtained by summing the thicknesses of all coal beds that are thick enough to be considered as resources (2.5 feet, or 75 cm). These thicknesses are isopached at 40-foot thick increments, and the overburden, heat value, and reliability categories are plotted on the map. Estimates of coal tonnage should be calculated for each rank category. The total tonnage estimate for hypothetical resources in the example is the sum of each of these estimates. Generally, the size of the thickcness of coal increments is left to the geologist to determine. Figure 23 is appropriate to an analysis based on a coal-zone approach to estimation of hypothetical resources as outlined on p. 37-38.

Figures 24 and 25 show suggested forms for recording coal resource data. These forms, or similar forms, are necessary for recording coal bed names; overburden and thickness of coal categories; planimeter readings; calculated areas; and weighted average thickness of coal for the area being calculated. The forms also provide columns for tabulating tonnage estimates in the measured, indicated, and inferred reliability categories, by coal thickness categories, and miscellaneous information such as planimeter factor, persons responsible for steps of work, and total tonnages by thickness and overburden categories. The data recorded on such forms and the calculations made therefrom represent the culmination of coal resource work starting in the field and continuing through the preparation of bed maps such as those in figures 9 through 23. The forms for data entry and calculation should be designed with care so the data entered on the forms from accompanying bed maps contain the necessary information for estimating all types of coal resources and reserves. This information can be combined with other information such as pertinent data on costs, transportation, and marketing to provide a documented analysis of the coal resources/reserves in a coal bed, field, region, basin, province, county, State, and (or) the Nation.


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