Geologic Setting and Coal Mining

     The Powder River Basin covers about 22,000 sq miles in northeastern Wyoming and southeastern Montana (Figure 1) and is located in the Northern Great Plains physiographic province. The structural axis of the basin trends northwest and is near the western edge of the basin. The Powder River Basin has a narrow, steeply dipping western side and a broad, gently dipping eastern side. The Paleocene Fort Union Formation along the eastern side of the Powder River Basin contains some of the thickest and most extensive deposits of low-sulfur subbituminous coal in the world (Molnia and Pierce, 1992), including the thick Wyodak coal bed found in the Hilight quadrangle.

     The Powder River Basin of Wyoming was chosen as the study site for the first western coal availability study because of its vast coal resources and its importance in U.S. coal production -- nine of the ten coal mines with the largest production in the United States in 1995 are located in the Powder River Basin (Keystone Coal Industry Manual, 1997, p. 730). All the coal mines in the Powder River Basin are surface mines. The Wyoming portion of the Powder River Basin provides about 20 percent of the coal produced annually in the United States (Weakly, 1994).

     The study site -- the Hilight 7 1/2-minute quadrangle (Figure 2) -- is an area of about 52 sq miles and is located in Campbell County, Wyoming, about 35 miles south of Gillette. The Hilight quadrangle is situated between the Coal Creek mine (owned by Thunder Basin Coal Co., a subsidiary of ARCO Coal Co.) and the Jacobs Ranch mine (owned by Kerr-McGee Coal Co.) (Figure 2). The northern limit of the Jacobs Ranch mine tract extends into the very southeastern edge of the Hilight 7 1/2-minute quadrangle. The formerly proposed Keeline coal mine (Neil Butte Co.) lies fully within the quadrangle (see Figure 3); that Federal coal lease has been relinquished. The productive capacity of that mine would have been as much as 12 million short tons annually. The Hilight quadrangle was chosen for our study because of its location between two active mines, the interest that had been shown in developing the coal deposit at one time, and the issues in multiple-use land management of the area.

     The Eocene Wasatch Formation is at the surface everywhere in the Hilight quadrangle except along the east-central edge, in the drainage of Black Thunder Creek, where the Paleocene Fort Union Formation crops out (Coates, 1977; IntraSearch, 1979).

     The main coal bed in the Hilight quadrangle is the Wyodak coal bed of the Tongue River Member of the Fort Union Formation. Figure 4 shows a composite columnar section that is typical of the Hilight area. In the quadrangle, the Wyodak coal bed is up to 120 ft thick, and, in many places, has 1-5 partings that vary in thickness. Overburden thickness in the quadrangle for this coal bed ranges from 15 to more than 600 ft. (See Major Coal Zones Studied section for a more detailed description of occurrence of the Wyodak coal bed.)

     According to the information in the mine and reclamation plan for the Keeline mine (Neil Butte Company, 1985), the Wyodak coal bed in the Keeline lease area is a non-agglomerating subbituminous class C coal which averages approximately 9,150 Btu/lb on a moist, mineral-matter-free basis. On an as-received basis, the heating value of the coal ranges from 7,905 to 8,960 Btu/lb with an average value of 8,350 Btu/lb. As-received moisture ranges from 24.9 to 31.6 percent by weight with an average of 27.7 percent; as-received ash content ranges from 4.9 to 12.4 percent by weight with an average value of 7.9 percent; and as-received sulfur content ranges from 0.3 to 2.0 percent by weight with an average value of 0.6 percent. These quality values are for the coal in-place and will vary slightly from the coal as-mined due to atmospheric exposure (Neil Butte Company, 1985). (See Keystone Coal Manual, 1997, pgs. 687 - 696, for further information about the Wyodak coal bed and other major coal beds in the Powder River Basin.)

Factors Affecting Availability of Coal Resources

     There are many factors which can affect the availability of coal for mining. The three general groups of factors or considerations in Powder River Basin coal development are: legal unsuitability criteria, land-use conflicts, and technological factors. Table 1 shows a listing of the factors we considered under each of these groups. It is important to note that not every factor became a restriction within the Hilight quadrangle.

Unsuitability Criteria Determinations for the Hilight quadrangle

     The coal unsuitability criteria are listed in the Federal Regulations, Title 43, Subpart 3461 (43 CFR 3461). These 20 specific legal criteria are used to determine if an area can be mined by surface mining methods. The 43 CFR 3461 regulations are issued under the authority of, and implement several major provisions of, Public Law 95-87, which is the Surface Mining Control and Reclamation Act of 1977 (30 U.S.C. 1201 et seq.). The 20 unsuitability criteria involve consideration of scenic areas, natural and historic values, wildlife, flood plains, alluvial valley floors, and other special values (Bureau of Land Management, 1984).

     The following sections (a) and (b) are discussions of particular unsuitability criteria and how they might affect mining within the Hilight quadrangle.

          a) Unsuitability Criteria that are Restrictions to Mining:

     RAILROAD CORRIDOR: There is a 300 ft buffer, shown in figure 5, along the main and trunk lines of the existing rail routes through the Hilight quadrangle. This area is determined to be unsuitable for mining at the present time.

     CEMETERY: There is a small cemetery (Kintz Cemetery) in the northern part of the quadrangle. The cemetery and its buffer cover a circular area that is 600 ft in diameter; this area is shown on figure 5. This area would be considered unsuitable for mining.

     Note: It is conceivable that both the railroad and the cemetery could be relocated to allow mining to proceed, once the appropriate agreements and permits are acquired. But for the purposes of this study, we will consider them to be restrictions to coal mining.

          b) Unsuitability Criteria that are Considerations in Mining and Mine Planning:

     In these cases (alluvial valley floors, raptor sites, roads, lands in certain federal land systems, dwellings), an area could be declared unsuitable for coal mining; alternatively, a mitigation measure could be defined to limit the effects of mining, and the area could be mined with the appropriate mitigation. Detailed studies, which would determine unsuitability or mitigation, would be made at a later time if an expression of interest was received for coal development in the area (Bureau of Land Management, 1984). Economic analyses by the coal developer would help to determine whether costs for mitigation would preclude mining.

     ALLUVIAL VALLEY FLOORS (AVF): There is one potential AVF (as currently defined in the coal-screening process) along Black Thunder Creek (Figure 6). All lands identified as AVF's where mining would interrupt, discontinue, or preclude farming, are unsuitable for surface coal mining. Additionally, when mining Federal lands outside an AVF would damage the quality or quantity of water in surface or underground systems that would supply AVF's, the land shall be considered unsuitable.

     These determinations have not been made concerning the Black Thunder Creek area within the Hilight quadrangle; the area will need AVF determinations by the Wyoming State Department of Environmental Quality. The area is currently open to coal leasing until a negative determination has been made.

     RAPTOR SITES: In the proposed final environmental impact statement for the Buffalo Resource Area (Bureau of Land Management, 1985), golden eagle sites (with buffers) were identified as unsuitable for mining. However, currently, these and other raptor sites (with buffers) are considered open to leasing and coal mining, pending further study; the mining effects at the sites could probably be mitigated. Each site with its buffer covers a circular area about 1 mile in diameter, as shown in figure 6.

     ROADS: County roads (gravel) cross the quadrangle. They are shown on figure 7 with a buffer; the roads, including buffers on each side, are about 150 ft across. These gravel roads can be moved and should not affect coal mining. There are no State Highways in the quadrangle. [A few miles south of the Hilight quadrangle, State Highway 450 and its buffer have been determined to be unsuitable for coal mining.]

     FEDERAL LAND SYSTEMS: None of the Federal land systems that are unsuitable for coal leasing are present in the Hilight quadrangle. The quadrangle does contain a portion of the Thunder Basin National Grassland (TBNG), a large area in northeastern Wyoming that includes scattered Federal lands under the jurisdiction of the U.S. Forest Service (USFS); but TBNG is not part of a National Forest. The same unsuitability criteria and land use considerations discussed in this report apply to coal mining on the Thunder Basin National Grassland. Where the mineral ownership in the National Grassland is Federal, the Bureau of Land Management develops the coal-leasing and mining stipulations in conjunction and cooperation with the USFS. Figure 7 shows the boundary of the TBNG within the Hilight quadrangle.

     DWELLINGS: The area is sparsely populated and relatively undeveloped. The few dwellings that exist would probably be bought by the coal company and would not prohibit mining. The dwellings and their buffers are shown in figure 6; each site and buffer cover a circular area about 600 ft in diameter.


     Other Considerations to Mining (in addition to those in the Unsuitability Criteria):

          a) Multiple-Use Issues

     The Bureau of Land Management (1985) Resource Management Plan (RMP) for the Buffalo Resource Area covers this part of the Powder River Basin. The RMP provides planning and guidance, in accordance with Federal laws and regulations, concerning energy and mineral development, cultural resources, grazing management, wildlife habitat, recreation, and other uses of public lands. Within the Hilight quadrangle, multiple-use considerations that might affect coal availability were identified in the RMP and other documents; these considerations include: pipelines, the Hilight oil and gas field and gas plant, power lines, gravel pits, archaeological sites, and surface- and mineral-estate ownership. Certain multiple-use conflicts could be mitigated to allow for the surface mining of coal; other situations may render some coal unavailable for mining. Economic analyses by the coal developer would determine whether an area could be profitably mined, especially if mitigation measures are mandated. Individual factors are discussed below.

     PIPELINES: There is a network of oil and gas pipelines throughout the Hilight quadrangle (Figure 6). Most likely, these pipelines would be moved so that surface mining could proceed, but moving and restoring them would represent an added economic consideration to mining.

     HILIGHT OIL AND GAS FIELD: This large oil and gas field (Figure 5) extends beyond the boundaries of the quadrangle. The entire field had 125 producing wells as of December, 1992, (Wyoming Oil and Gas Conservation Commission, 1993) and a water-flood project is underway to enhance recovery. As of June 1994, there were about 40 active producing wells within the Hilight quadrangle. Total cumulative production of the entire field (to 1992) was 77 million barrels of oil and 255 million thousand cubic feet of gas. The main producing formation is the Lower Cretaceous Muddy Sandstone, about 9,000 ft below ground level (Wyoming Geological Association, 1981).

     How land-use conflicts between coal mining and the oil and gas field development would be resolved will depend on economic conditions, regulations, and negotiations between oil developers and coal developers. Perhaps an area around a major cluster of active wells would be eliminated from mining activities until these wells are no longer actively producing. Or, mining activities might proceed around individual active wells that are given a buffer zone. Conversely, specific wells might be plugged and then reestablished after mining.

     HILIGHT GAS PLANT: The plant, operated by Western Gas Processors, has a capacity of about 60 million cubic feet per day (De Bruin and Boyd, 1991). The plant (Figure 5) connects to several major pipelines for gas and crude oil, as well as to a pipeline for gas-processing-plant products. This installation, with a 500 ft buffer, would probably be considered a restriction to coal mining during the time that the neighboring oil and gas fields are still producing.

     POWER LINES: A major power line (Figure 5) crosses the southwest corner of the quadrangle. At the time of mining, the implications of moving this power line would be determined. If it were not moved, there would be a 300 ft buffer placed on both sides of the power line, and the coal in that area would be considered unavailable.

     GRAVEL PITS: There are three gravel pits in the quadrangle (Figure 7). They are developed in clinker and would not preclude mining. There are numerous other clinker deposits in this part of the Powder River Basin.

     ARCHAEOLOGICAL AREAS: No major archaeological areas that prevent mining are known in the quadrangle. There are several minor archaeological sites and also several minor historic sites within the Hilight quadrangle. A mitigation plan would be developed before these areas are disturbed by coal mining.

     SURFACE OWNERSHIP: Almost the entire surface of the Hilight quadrangle is privately owned. Surface-owner consultation would be necessary before mining on this land. There are about 2.5 sq miles of State-owned surface and less than 1 sq mile of Federally owned surface, in a quadrangle whose area is about 52 sq miles.

     COAL OWNERSHIP: The Federal Government owns all of the coal in the Hilight quadrangle except that beneath about 1/4 sq mile of privately owned land, and beneath the 2.5 sq miles of State-owned surface.

          b) Technologic Factors:

     These are geologic and mining considerations that could affect the development of coal in the Hilight quadrangle.

     ACTIVE MINES: There are no active mines within the quadrangle. The northernmost segment of the Jacobs Ranch coal lease extends into the southern edge of the Hilight quadrangle (Figure 7), but this area has not yet been mined.

     MINED-OUT AREAS: None.

     LIMIT OF COAL: The "Coal resource occurrence map of the Hilight quadrangle" (IntraSearch, 1979) shows an inferred outcrop trace for the Wyodak coal bed on the eastern edge of the quadrangle. This is the only area of Wyodak outcrop within the Hilight quadrangle. Part of that area has formed clinker from the burning of the coal bed at or near the outcrop. The limit of coal (Figure 7) is drawn so that these clinkered areas are not included in the area considered for resource assessment and mine planning.

     OVERBURDEN GEOCHEMISTRY: The only data on overburden geochemistry came from the Keeline mine permit application (Neil Butte Company, 1985), which covered an area in the east-central part of the quadrangle (see Figure 3). There, potentially toxic heavy metals and minor elements were present in relatively small concentrations. Salinity (based on electrical conductivity measurements) and sodium adsorption ratios (SAR) were within acceptable limits with only a few isolated exceptions. Only one component, potentially acid-producing overburden, was encountered in significant areal extent on parts of the former lease area. However, there is abundant overburden without these components; those strata would be blended with the poor-quality overburden to produce an acceptable mixture. No more than five percent of all overburden and parting materials was estimated to require special management because of a variety of chemical parameters encountered in unsuitable concentrations (Neil Butte Company, 1985). This will be an additional expense for the mining operation but probably will not prevent mining of any area.

     OVERBURDEN THICKNESS AND MINING CONSIDERATIONS: We assumed that 300 ft of overburden would be the limit for surface mining, based on general mining practice in the western United States and the Powder River Basin. Figure 8 shows the areas of the Hilight quadrangle where the overburden on the Wyodak coal bed is less than 300 ft thick. This 300-ft cut-off is not an absolute rule, especially in the case of a coal bed as thick as the Wyodak bed. Overburden exceeds 600 ft in the quadrangle, but where the Wyodak coal is about 80 ft thick, the mining ratio (8 feet of overburden : 1 foot of coal) could be attractive under certain economic conditions. At the present time, however, coal mines in the Powder River Basin are not surface mining beyond approximately 300 ft of overburden.

     We also assumed that underground mining would occur where overburden depths range from 300 ft to 1000 ft. We realize that there are no underground coal mines presently operating in the Powder River Basin and there are not likely to be in the near future. However, there has been a feasibility study regarding future longwall mining of a coal bed that is over 200 ft thick in places (the Big George coal bed) which has a minimum of 1000 ft of overburden (Ahcan and others, 1991). (Additional discussion of underground mining for thick coal beds in the western United States is found in Hackett and others, 1990.) The Big George coal bed is down-dip from the Wyodak coal bed and closer to the center of the Basin; subsurface evidence indicates that the Big George coal bed correlates with the Wyodak coal bed (Molnia and Pierce, 1992; Keystone Coal Industry Manual, 1997, p. 693).

     We assumed that subsidence (local lowering and deformation of the land surface) could be likely if underground mining were to occur, depending upon the depth to coal, thickness of coal removed, type of overburden, and other engineering factors. Dunrud and Osterwald (1980) discuss the higher likelihood of subsidence in instances where the overburden is less than about 10-15 times the thickness of the coal that is mined underground -- a situation that could exist with the Big George coal bed or the Wyodak coal bed.

Grouping of Constraints to Mining in the Hilight Quadrangle

     The actual constraints or restrictions that were used in the coal availability calculation for the quadrangle were grouped in many overlapping ways. Those included: (1) whether the constraint was a land-use restriction or a technologic restriction; (2) whether the land-use restriction arose from the Unsuitability Criteria or from other multiple-use management plans; (3) whether the restriction was located and applicable where overburden thickness is 0-300 ft., 300-1,000 ft., or both; and (4) whether the constraint was likely to restrict a mining operation (as judged by common local practice) or could be mitigated in some way to allow mining to proceed. Table 2 shows the restrictions that were used for the coal availability calculation in the Hilight quadrangle. The following discussion explains the terms used and how the groupings were determined.

Table 2. Constraints to coal mining in the Hilight quadrangle. [* indicates overburden thickness. “Yes” indicates actual situations that occur in the quadrangle. “Unsuitability” refers to the coal Unsuitability Criteria listed in the Federal Regulations (43 CFR 3461)]

     The software that we use for coal availability calculations (see Computer Techniques section) divides the availability restrictions into two types: land-use and technologic. Land-use restrictions are placed upon mining by societal policies to preserve those surface features or entities that could be adversely affected by mining (Carter and Gardner, 1989). Land-use restrictions, therefore, may change if societal interests change. Typically, land-use restrictions apply to surface mining, but may also affect underground mining.

     Technologic restrictions affect the economics, safety, or resource extraction during mining and coal preparation, and are determined by current mining industry practice. These restrictions change with advances in science and engineering or with changes in economic conditions. Technologic restrictions affect both surface and underground mining but are generally more prohibitive to underground mining (Carter and Gardner, 1989).

     The USGS coal resource calculation program computes these resources by overburden thickness categories; for this study (see Figure 8) we divided the overburden thickness into two categories: 0-300 ft and 300-1000 ft (surface and underground mining, respectively).

     We further grouped the factors affecting the availability of coal into two sets: Category 1, those factors that were likely to restrict a mining operation; and Category 2, those factors that probably could be mitigated in some way. The two Unsuitability Criteria that are restrictions to mining (railroad corridor and cemetery) are included in Category 1. Category 1 considerations would result in a certain amount of coal tonnage being unavailable for coal mining; in contrast, Category 2 considerations would perhaps increase the cost and complexity of the mining operation, but, through mitigating measures, might allow for mining of the coal involved.

     The following are the Category 1 considerations (areas unavailable because of present land-use and technologic conflicts). These areas are depicted on Figure 5.

Category 1

1. Railroad corridor

2. Power line

3. Cemetery

4. Active oil and gas wells

5. Hilight gas plant

     Because of the concentration of active oil and gas wells in the southern half of the quadrangle, we assumed that an area around this entire cluster of actively producing wells would be eliminated from mining until these wells are no longer producing. The area delineated is shown as "Active oil field" on Figure 5. We felt that, because there was such a large quantity of actively producing wells in a small area, it would not be efficient to try to develop a plan to mine around each well, and thus we outlined an enclosing area that would be unavailable for coal mining.

     We considered that the railroad, power line, cemetery, and gas plant would be land-use restrictions to mining at both overburden categories (0-300 ft [surface mining] and 300-1000 ft [underground mining]). Because Powder River Basin overburden is typically weak, low in rock strength (Ahcan and others, 1991; Dunrud and Osterwald, 1980) and susceptible to subsidence, we restricted mining below these surface features.

     We considered the actively producing oil and gas wells (individual wells in the north part of the quadrangle and the cluster of wells in the south part) to be a land-use restriction at 0-300 ft overburden category [surface mining] and a technologic restriction at the 300-1000 ft overburden category [underground mining]. The latter determination is because of the technologic difficulties involved in underground mining where producing oil and gas wells intersect the mine.

     The following are the Category 2 considerations (may allow for the mining of coal, through mitigating measures). These areas are depicted on Figure 6.

Category 2

1. Dwellings

2. Potential alluvial valley floor of Black Thunder Creek

3. Pipelines

4. Raptor sites

5. Inactive oil and gas wells

     We grouped the alluvial valley floor, raptor sites, dwellings, and pipelines as land-use restrictions for surface mining operations. They were also considered to be land-use restrictions for underground mining operations because of the likelihood that they could be disturbed by surface subsidence. (We included the raptor sites here because of the possibility that their flora and hydrology might be destroyed or disrupted by subsidence.) Inactive (but not abandoned) oil and gas wells would be a land-use restriction for surface mining and a technologic restriction for underground mining.

     For the purpose of the coal availability resource calculation, we assumed that the smaller number of inactive oil and gas wells (in contrast to the actively producing oil and gas wells) would not need to be grouped together into a single area unavailable to mining, but rather could be considered on an individual basis and factored into the mine plan, so that mining could still proceed through them (if they are adequately plugged) or around them.

     Resource and availability calculations were completed for each of the major coal zones, for the considerations in Category 1 [restrictions to mining] and in Category 2 [additional considerations to mining] -- by land-use and technologic designations, and by overburden thickness. (See Computer Techniques and Results sections.)

Major Coal Zones Studied

     Resources were calculated on five Fort Union Formation coal intervals (Figure 4). Not all drill holes encountered all five coal intervals. If a drill hole started and (or) ended in a coal bed of interest, we included that partial measurement in our resource calculations. We did not include coal beds whose thickness is less than 2.5 ft, because: 1) these beds, although common in the Fort Union Formation, are of limited extent and cannot be correlated over a significant distance; and 2) USGS Circular 891 (Wood and others, 1983) defines 2.5 ft as the minimum thickness of subbituminous coal for resource calculations.

     The five coal zones for which resources were calculated are the Rider Wyodak, the Main Wyodak, the Lower Wyodak, the Wildcat, and the Moyer.

     Resources were not calculated for the Oedekoven coal bed (Figure 4) because of insufficient data within the quadrangle.  Figure 9 shows some representative sections from the Hilight study area which include the Rider Wyodak, Main Wyodak, and Lower Wyodak coal beds.

     The Main Wyodak coal bed is herein defined as that part of the Wyodak coal interval that occurs as one bed according to the definition in USGS Circular 891 (Wood and others, 1983, p. 36). Figures 10 and 11 show the variability of the Main Wyodak coal bed; it can contain many partings, but as long as the partings are not as thick as either of the coal benches they separate, the Main Wyodak coal bed is considered to be one bed (Wood and others, 1983). Using this criteria, the Main Wyodak coal bed contains 5-120 ft of coal in an interval that is 5-156 ft thick. We used two thickness categories for the Main Wyodak coal bed: 5-40 ft and greater than 40 ft. Overburden thickness for the Main Wyodak coal bed is 15- 625 ft.  

     If benches of the Wyodak bed are separated by partings which exceed the thickness of either adjacent bench, then the bench must be considered a separate bed for the purposes of resource calculation (Wood and others, 1983), and its thickness is not included in the thickness of the Main Wyodak coal bed. These separated beds (they do not occur in every drill hole) were grouped as discussed below.

     (a) Wyodak benches above the Main Wyodak bed were called the Rider Wyodak bed (see figures 9, 10, and 11). The Rider Wyodak bed can include multiple benches of Wyodak coal above the Main Wyodak, and is designed to include all the rest of the benches of the Wyodak coal bed above the Main Wyodak, regardless of parting thicknesses. The analysis, currently underway, of recoverable coal in the Hilight quadrangle will determine where the Rider Wyodak coal beds can be economically mined in conjunction with mining of the Main Wyodak coal bed.

     Total coal thickness of the Rider Wyodak bed is 3-24 ft, in a stratigraphic interval of 3-90 ft. We used two coal thickness categories for the Rider Wyodak coal bed: 3-5 ft and 5-40 ft. [This second category was chosen to parallel the 5-40 ft thickness category for the Main Wyodak bed, even though total coal in the Rider Wyodak bed in the Hilight quadrangle is no more than 24 ft thick.] Overburden thickness for the Rider Wyodak coal bed is 0-400 ft.

     (b) Wyodak coal benches below the Main Wyodak bed were called the Lower Wyodak coal bed (figures 9, 10, and 11). The Lower Wyodak bed can include multiple benches of Wyodak coal below the Main Wyodak, and is designed to include all the rest of the benches of the Wyodak coal bed below the Main Wyodak bed, regardless of parting thicknesses. The analysis, currently underway, of recoverable coal in the Hilight quadrangle will determine where the Lower Wyodak coal beds can be economically mined in conjunction with mining of the Main Wyodak coal bed.

     Total coal thickness of the Lower Wyodak bed is 3-25 ft, in a stratigraphic interval of 3-90 ft. We used two coal thickness categories for the Lower Wyodak coal bed: 3-5 ft and 5-40 ft. Overburden thickness for the Lower Wyodak coal bed is 75-600 ft.

     The Wildcat coal bed is 3-16 ft thick and typically occurs in one bench. We used two thickness categories for the Wildcat coal bed: 3-5 ft and 5-40 ft. Overburden thickness for the Wildcat bed is 500-1300 ft.

     The Moyer coal bed is 3-11 ft thick and typically occurs in one bench. The two thickness categories that we used for the Moyer coal bed are: 3-5 ft and 5-40 ft. Overburden thickness for the Moyer bed is 650-1370 ft.

     The data set of stratigraphic information used for resource calculations and for the determinations discussed above includes data points within the Hilight quadrangle and data points within a three-mile-wide band surrounding the quadrangle. The total was approximately 350 data points (Figure 3). The data in the three-mile band around the Hilight quadrangle were used to guide and control the computer-generated grids of coal thickness and overburden thickness in the quadrangle, and to complete the calculation of measured, indicated, and inferred coal resources for data points within the Hilight quadrangle but close to the quadrangle border.