Computer Techniques

     The study involved compilation of three basic types of data. The first is point-source data on coal-bed thickness, elevation above sea level, and coal quality. BLM supplied digital files of publicly available drill-hole data in and around the Hilight area; these files contained coal-bed names and correlations, especially for the Wyodak coal bed, and were used after minor revisions. Additional public drill-hole data, which also contained coal-bed thickness, bed name, and elevation (from the Coal Resource Occurrence -- Coal Development Potential studies of Federal coal resources) were retrieved from the USGS National Coal Resources Data System for the Hilight quadrangle and for the surrounding eight quadrangles, and were used after minor revisions.

     The second data type consists of line data that define coal outcrops, boundaries, and areas that pose potential restrictions to mining, as well as other land-use considerations. Many of these data were also provided by the BLM in digital format. Other line data were plotted on 7 1/2-minute topographic maps and digitized by the USGS. Most of the line data which define areas of technological restrictions, i.e., coal too deep or too thin, were generated by our public-domain GIS software, GRASS (Geographical Resource Analysis Support System, U.S. Army Construction Engineering Research Laboratory), from grids of coal thickness and overburden thickness.

     The third type of data is the digital elevation models (DEM's), which are digital files of surface topography, produced by the National Mapping Division of the USGS. The DEM surface-elevation raster is used to calculate overburden thickness. A computer program subtracted one raster (the elevation of the top of the coal) from a second raster (the DEM raster of surface elevation) to generate a raster of overburden thickness.

     The overall steps required to calculate coal resources for this study included: (1) acquisition of coal stratigraphic and analytical data, and their transfer into a point-data management system; (2) correlation and grouping of coal beds by bed or zone; (3) transfer of point-source and line data into a geographic information system (GIS); (4) conversion of point-source and line data into rasters using GIS programs; (5) calculation of original coal resources from rasters; and, finally, (6) calculation of restricted coal tonnages and coal tonnages available to mining, by overburden thickness and by Category 1 and 2 restrictions. The methodology for coal-resource calculations used in this study follows the Coal Resource Classification System of the USGS (Wood and others, 1983).

     Computerized techniques are used to facilitate visualization of coal-bed correlations and calculation of original, restricted, and available resources. The point-source geologic data were initially processed using StratiFact software (GRG Corporation) to store, manipulate, and graphically display cross sections throughout the quadrangle; to correlate coal beds between drill holes; and to group coal beds and partings by assigning coal bed designations. Then, coal-bed data were retrieved by bed designations and brought into GRASS, which contains the USGS coal resource calculation programs.

     Digital line data obtained from BLM was processed using a GIS called ARC/INFO (Environmental Systems Research Institute, Inc.) to clean the data (delete dangles, intersect lines, and create topology) and to reformat the BLM digital files so they also could be imported into GRASS. Line data digitized by the USGS were also brought into GRASS. GRASS volumetric programs were run to calculate original, remaining, restricted, and available coal tonnages, by coal bed. Table 3 is a summary of these calculations; complete results are shown in Tables 4 through 13. The GRASS software generated estimates of restricted and available coal tonnages for the Wildcat and Moyer coal beds (Tables 7-8a,b,c and 12-13a,b,c); however, these coal beds are considered entirely unavailable because of their depth. No rounding to significant numbers is done by the GRASS programs.

     Each of these resource estimates in the tables are subdivided into categories by overburden thickness, coal thickness, and reliability of estimate. Reliability categories used were: measured (coal within 1/4 mile of a coal-thickness measurement); indicated (coal 1/4- to 3/4- mile from a coal-thickness measurement); inferred (coal 3/4- to 3 miles from a coal-thickness measurement); and hypothetical (coal more than 3 miles from a coal-thickness measurement).

     These coal tonnage estimates can be compared to previous resource estimates for this quadrangle and can indicate the amount of available coal in other parts of the Powder River Basin that have similar geologic and land-use conditions.

Results

     Coal Availability Calculation Using Category 1 Restrictions
(Likely restrictions to mining)

     The Hilight quadrangle contains 4.4 billion tons of total coal resources (Figure 12). Original and remaining coal resources for the quadrangle are the same because no commercial mining has yet taken place. Under Category 1 restrictions (railroad, power line, cemetery, oil and gas field and producing wells, Hilight gas plant) about 60 percent or 2.7 billion tons of the original coal resource are considered available for development in the Hilight quadrangle (Figure 13). This 60 percent represents the sum of the available Rider, Lower, and Main Wyodak coal resources (totaling approximately 2.7 billion tons) as a proportion of the total 4.4 billion tons of original coal in the quadrangle (Figure 14; Table 3). The coal resources of the Wildcat and Moyer beds are considered unavailable because of depth.

     The Main Wyodak coal bed in the Hilight quadrangle contains 2.9 billion tons of coal, of which 1.9 billion tons (67 percent) are considered available for development. Figure 15 shows the total available resources of the Main Wyodak coal bed, and the amount of coal that is restricted by land-use considerations (24 percent) and by technological considerations (9 percent). The relative proportions of these land-use considerations are depicted in figure18. The large oil and gas field is the technological consideration which limits the availability of the Wyodak coal bed.

     Figures 16 and 17 show the amount of available coal and the amounts of coal restricted because of land-use and technological restrictions for the Rider Wyodak coal bed and the Lower Wyodak coal bed, respectively, in the Hilight quadrangle.

Coal Availability Calculation Including Category 2 Restrictions
(Considerations that probably will be mitigated)

      If Category 2 restrictions (dwellings, alluvial valley floor, pipelines, raptor areas, inactive oil and gas wells) for the Rider, Lower, and Main Wyodak beds are added to Category 1 restrictions, an additional 807 million tons of coal would be restricted from mining (this figure is approximate because of overlap between some Category 1 and Category 2 considerations). Thus 42 percent (1,851 million tons) of the Wyodak coal beds would be available for mining.

Comparison to Other Coal Availability Studies

     This coal availability study -- the first in the Western United States -- indicates that about 67 percent of the coal in the Main Wyodak bed is available for mining, and about 60 percent of the total coal in the quadrangle is available for mining. Studies in the Appalachian coal region indicate that, overall, only 50 percent of the original coal resource in that region is available for development (Carter and Gardner, 1994). In many areas of the Appalachian region, much of the original resource is already mined-out; whereas in the Hilight quadrangle, there has been no mining. Of the remaining Appalachian coal resource, no more than 60 percent is considered available for future development, because of restrictions to mining (Carter and Gardner, 1994).

     There are significant differences in several coal quality parameters between Appalachian coals and Powder River coals; these differences must be considered in any comparison of available resources of different regions. In general, the Powder River Basin coals are lower in rank, higher in moisture content, and lower in sulfur content than the Appalachian coals.

     There are also major differences between Appalachian and Powder River Basin coal development. Different land-ownership patterns, mineral-ownership patterns, environmental regulations, mining methods, topography, and land-management policies exist in the two regions. Powder River Basin coal development occurs in an area which is relatively undeveloped and contains no large population centers. Powder River Basin topography is relatively flat; there are numerous, gently-dipping, relatively shallow, thick coal beds. No underground mining is planned. Coal mining in the Powder River Basin involves Federally-owned coal resources and Federal coal mining laws and development regulations. All of these factors influence the amount of coal that is available, and how that coal will be developed.

     The U.S. Bureau of Mines coal recoverability studies of the Appalachian region have shown that less than 10 percent of the original resource can be mined and marketed at a profit (Rohrbacher and others, 1994). The coal recoverability study of the Hilight quadrangle is presently being conducted by the U.S. Geological Survey, to determine what percent of the available coal is economically recoverable, through design of a theoretical mine plan for the quadrangle. This mine plan takes into account the restricted areas within the quadrangle and mining practices common in the Powder River Basin.

Comparison to Other Coal Resource Calculations for the Quadrangle

     Two earlier calculations were compared to our assessment:

     (1) IntraSearch (1979) evaluated the coal resources of all unleased Federal coal beds in the quadrangle which are 5 feet or greater in thickness and occur at depths down to 3000 ft. Using these criteria, IntraSearch reported that there are 3.7 billion short tons of unleased Federal coal resources in the Hilight quadrangle. The IntraSearch study computed no resources or reserves for leased Federal coal, State coal, fee (private) coal, or lands encompassed by coal prospecting permits and preference right lease applications.

     Our estimate of 4.4 billion tons of coal as the total coal resources in the quadrangle compares well with this previous estimate of 3.7 billion tons by IntraSearch (1979), in view of the fact that the IntraSearch estimate did not include all of the coal in the quadrangle.

     (2) A second resource estimate for the area (Berryhill and others, 1950) provided a calculation of total original reserves of subbituminous coal in Wyoming by township, by overburden thickness and coal-bed thickness. Coal-tonnage estimates given for the four townships that occur within the Hilight quadrangle are as follows:

T. 44 N., R. 70 W. 303 million short tons
T. 44 N., R. 71 W. 485 million short tons
T. 45 N., R. 70 W. 390 million short tons
T. 45 N., R. 71 W. 520 million short tons

     These estimates of original reserves, which total 1.7 billion tons, are for coal beds greater than 2.5 ft thick and with overburden less than 1000 ft; and are the sum of measured, indicated, and inferred reserve estimates (as defined by Berryhill and others, 1950) for each township.  

     Because the Hilight quadrangle does not include any one of these townships in its entirety, we used a percent (based on surface area) of each of the above township reserve estimates to approximate a coal reserve figure for the quadrangle. That total came to 780 million short tons.

     We believe that this resource figure derived from Berryhill and others (1950) for the Hilight quadrangle is much smaller than later estimates because very few surface coal mines were operating in the Powder River Basin at that time, and thus a limited amount of data on subsurface coal was available. Also, the resource estimates for this area by Berryhill and others (1950) did not include coal resources with over 1000 feet of overburden.

     Comparison of these Hilight resource estimates show the usefulness of periodically recalculating coal resources for an area when there have been increases in data quantity and quality over time. The coal availability calculation is a further refinement -- it determines how much of the total coal resource has already been mined, and how much would actually be accessible for development. These kinds of determinations can better guide us as an economy and a society as we develop our natural resources and the lifestyles that depend on them

References

Ahcan, Rudi, Zerdin, Franc, and Lutzens, W.W., 1991, Longwall mining in ultrathick U.S. coal seams, in Longwall USA International Exhibition and Conference; Pittsburgh, Pennsylvania, June, 1991.

Berryhill, Henry L., Jr., Brown, Donald M., Brown, Andrew, and Taylor, Dorothy A., 1950, Coal resources of Wyoming: U.S. Geological Survey Circular 81; 78 p.

Bureau of Land Management, 1984, Second draft resource management plan and draft environmental impact statement for the Buffalo Resource Area, Casper District, Wyoming.

Bureau of Land Management, 1985, Proposed final environmental impact statement for the Buffalo Resource Area, Casper District, Wyoming.

Carter, M. Devereux, and Gardner, Nancy K., 1989, An assessment of coal resources available for development: U.S. Geological Survey Open File Report 89-362; 52 p.

Carter, M. Devereux, and Gardner, Nancy K., 1994, Coal availability studies -- The impact of restrictions on the development potential of coal resources, in Chiang, Shiao-Hung, ed., Coal -- Energy and the Environment: 11th Annual International Pittsburgh Coal Conference, 1994. p. 1-4.

Coates, Donald A., 1977, Surficial geologic map of the Hilight quadrangle, Campbell County, Wyoming: U.S. Geological Survey Miscellaneous Field Studies Map MF-894. 1:24,000 scale; 1 plate.

De Bruin, Rodney H., and Boyd, Cynthia S., 1991, Oil and gas map of Wyoming: Geological Survey of Wyoming Map Series 35.

Dunrud, C. Richard, and Osterwald, Frank W., 1980, Effects of coal mine subsidence in the Sheridan, Wyoming, area: U.S. Geological Survey Professional Paper 1164; 49 p.

Eggleston, Jane R., Carter, M. Devereux, and Cobb, James C., 1990, Coal resources available for development -- a methodology and pilot study: U.S. Geological Survey Circular 1055; 15 p.

Hackett, T. D., Boreck, D. L., and Clarke, D. R., 1990, Multislice mining for thick western coal seams: U.S. Bureau of Mines Information Circular 9239; 27 p.

IntraSearch, 1979, Coal resource occurrence and coal development potential maps of the Hilight quadrangle, Campbell County, Wyoming: U.S. Geological Survey Open File Report 79-69.

Keystone Coal Industry Manual, 1997, Chicago, Ill., Intertec Publishing Company, 800 p.

Molnia, Carol L., and Pierce, Frances Wahl, 1992, Cross sections showing coal stratigraphy of the central Powder River Basin, Wyoming and Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-1959-D, scale 1:500,000.

Neil Butte Company, 1985, (unpublished), Mine and reclamation plan for the Keeline Mine. Prepared by Hydro-Engineering, Casper, Wyoming.

Rohrbacher, Timothy J., Teeters, Dale D., Osmonson, Lee M., and Plis, Matthew N., 1994, Coal recoverability and the definition of coal reserves, Central Appalachian Region, 1993: U.S. Bureau of Mines Open File Report 10-94; 36 p.

Weakly, Alan, 1994, Powder River Basin: Mother Lode of the nation's compliance coal, in Mining Engineering, vol. 46, no. 8, August, 1994.

Wood, Gordon H., Jr., Kehn, Thomas M., Carter, M. Devereux, and Culbertson, William C., 1983, Coal resource classification system of the U.S. Geological Survey: U.S. Geological Survey Circular 891; 65 p.

Wyoming Geological Association, 1981, Powder River Basin Oil and Gas Fields: Wyoming Oil and Gas Fields Symposium Series.

Wyoming Oil and Gas Conservation Commission, 1993, Wyoming oil and gas statistics, 1992.

Contact
Carol L. Molnia (clmolnia@usgs.gov) 303-236-7769
Laura R. H. Biewick (lbiewick@usgs.gov) 303-236-7773
Dorsey Blake (dblake@usgs.gov) 303-236-7746
U.S. Geological Survey
MS 939, Box 25046 Denver Federal Center
Denver, Colorado 80225-0046

Susan J. Tewalt (stewalt@usgs.gov) 703-648-6437
M. Devereux Carter (mdcarter@usgs.gov) 703-648-6413
U.S. Geological Survey
MS 956, National Center
Reston, Virginia 20192

Charlie Gaskill (wycgaskill@wy.blm.gov) 307-261-7663
Bureau of Land Management
1701 East E Street
Casper, Wyoming 82601-2167