Structural Features

Several structural geology features are prominent along the eastern margin of the Greater Green River Basin within the study area. The beds comprising the Overland Member generally dip from east to west across the study area, with the steepest dips observed along the eastern edge of the area. Erosion-resistant rock units form prominent ridges in the areas where these steeply dipping rocks crop out. One such ridge, known as the Red Rim, is present just to the east of the study area where the Upper Cretaceous Lance Formation crops out (Hettinger and others, 2008). The study area encompasses parts of the Great Divide and Washakie subbasins, which are separated by a roughly west to east trending anticlinal feature known as the Wamsutter arch (fig. 2). South of the Wamsutter arch, the rocks dip to the southwest into the Washakie Basin; north of the arch, the rocks dip to the north into the Great Divide Basin. The structural features are drawn on the base elevations of the main bench of the Cherokee coal bed. The generalized stratigraphy of the study area is shown in figure 3. The benches of the Cherokee coal bed shown in figure 3 are defined as follows:

The structural geology features and base coal elevations for the main bench of the Cherokee coal bed in the study area are shown in figure 4.

Depositional Environment

The depositional environment of the Overland Member of the Fort Union Formation is interpreted to be fluvial dominated, with fluvial channels, flood plains, and paludal areas (Hettinger and others, 2008), which is reflected by the variable thicknesses of the partings that split the Cherokee coal bed into multiple benches. As interpreted from Hettinger and others (2008), the thickest partings are dominated by fine-grained sandstones that compose the core of fluvial channels flowing through the study area during the Paleocene. Away from the channel core, the partings become thinner and are composed of sandy siltstones, silty mudstones, and mudstones representing floodplain deposits. The part of the floodplain nearest the fluvial channels contains very fine-grained sandstones, sandy shales, and siltstones that represent levee, crevasse splay, and overbank sediments. Away from the fluvial channels, the floodplain sediments become thinner and continue to decrease in overall grain size. The distal parts of the floodplain are mainly comprised of mudstones and silty mudstones, with rare siltstone or very fine-grained sandstone lenses, representing sporadic flooding events. The transition from the floodplain to the paludal environment is marked by continued thinning of the sediments owing to the absence of any sand or silt in the mudstones and an increase in the carbonaceous content of the sediments. The distal parts of the floodplain at the transition to the paludal environment are composed of black carbonaceous clay shales. The paludal environment is represented by the central core of the thick Cherokee coal bed interval, with little or no sediment input and uninterrupted plant growth, allowing for the thick accumulation of peat.

Stratigraphy

The Cherokee coal bed lies stratigraphically in the upper part of the Paleocene Fort Union Formation (fig. 3). The Fort Union Formation was subdivided into three members by Honey and Hettinger (2004). These three members, from youngest to oldest, are the Overland Member, the Blue Gap Member, and the China Butte Member. Both the Overland Member and the China Butte Member are coal-bearing units. The Blue Gap Member consists mainly of fine-grained sediments (mudstone and claystone, with lesser amounts of sandstone and siltstone) but pinches out in southern Wyoming just north of the Wyoming/Colorado State line (Honey and Hettinger, 2004). The China Butte Member, the lowermost unit in the Fort Union Formation, comprises interbedded sandstones, siltstones, and mudstones. It also contains several coal intervals with locally thick coal beds (Honey and Hettinger, 2004).

The stratigraphic position of the Cherokee coal bed places it in the middle to upper part of the Overland Member (fig. 3). The Overland Member contains several laterally extensive coal beds, including, from stratigraphically youngest to oldest, the High Point, the Cherokee, the Cow Butte, and the Horse Butte coal beds.

An unconformity separates the overlying Eocene Wasatch Formation from the Overland Member. The thickness of the stratigraphic interval from the unconformity to the High Point coal bed varies, with the thinnest interval in the eastern and southeastern part of the study area and the thickest interval in the northwest part of the study area.

The High Point coal bed overlies the Cherokee coal bed within the Overland Member. The High Point bed is generally 30–100 feet above the Cherokee bed (fig. 3) and in some places may be present in two or more benches. In some areas, it is difficult to interpret whether coal beds above the Cherokee coal correlate to the High Point coal or are sporadic rider coal beds of limited areal extent associated with the Cherokee coal.

The Cherokee coal bed is the thickest and most laterally extensive of the coal beds in the Overland Member, as shown in figure 3. The Cherokee coal bed attains a maximum thickness of greater than 50 feet near its outcrop on the eastern side of the study area. A parting-free coal zone greater than 30 feet thick is present in the east-central part of the study area along the cropline. Partings are present to the north, west, and south of this parting-free zone and split the Cherokee coal bed into multiple benches. These partings can range in thickness from one inch to greater than 100 feet. In many areas, two or more of the individual benches may be parting-free and appear to be combined into a single, thick bench. At least four benches of the Cherokee coal bed, separated by partings, can be correlated and mapped throughout the study area. These four benches can range in thickness from less than one foot to more than 10 feet. The benches generally thin from east to west across the study area, with several of the benches pinching out towards the western edge of the study area. The overall cumulative thickness of the benches of the Cherokee coal bed, excluding non-coal partings, are shown in figure 5.

Immediately below the Cherokee coal interval, a thick, but laterally discontinuous coal bed may be present, especially in the eastern part of the study area in association with the thickest occurrence of Cherokee coal. This coal bed, which locally may be more than 10 feet thick, is named the Cherokee leader coal. It splits into multiple benches away from these locally thick areas and quickly pinches out. The bed is rarely present in the central and western parts of the study area. The Cherokee leader coal bed appears to be separate from the Cherokee coal. Correlations of the two coal beds between available drill holes, based on comparisons of coal thicknesses in the Cherokee and Cherokee leader coal intervals, make it difficult to include the Cherokee leader coal bed as an individual bench of the Cherokee coal bed.

Below the Cherokee leader coal are the Cow Butte and Horse Butte coal bed intervals. The Cow Butte interval generally lies 90 to 125 feet below the Cherokee main bench (fig. 3) and in most places consists of two coal beds – the Cow Butte and upper bench. The Cow Butte coal beds may attain thicknesses of more than 10 feet locally, but they typically split and thin laterally away from these locally thick areas and in most places consists of two coal beds—a lower and upper bench.

The Horse Butte coal interval generally lies 150 to 220 feet below the Cherokee coal interval (fig. 3). The Horse Butte coal bed may attain thicknesses of more than 10 feet locally and is present in most of the study area as two persistent benches. The upper bench of the Horse Butte ranges in thickness from 5 to 8 feet. The lower bench may be 50 to 75 feet below the upper bench and in most areas is less than 2 feet thick. These multiple benches of the Horse Butte coal bed form the Horse Butte coal interval.

Below the Horse Butte coal interval, the lower part of the Overland Member contains a basal sandstone, up to 5 feet thick, containing lenses of very coarse-grained sandstone and isolated pebbles (Honey and Hettinger, 2004). The Overland Member of the Fort Union Formation lies unconformably on the China Butte Member (or the Blue Gap Member, where present).

Geologic cross sections A–A’, B–B’, and C–C’ show the Overland Member containing the coal beds described above. The locations of the cross sections in the study area are shown in figure 6. Some drill hole numbers shown on the cross sections have letter prefixes; these letter prefixes are defined as follows:

Cross section A–A’ (fig. 7) is a west to east cross section showing the thick Cherokee coal bed along the eastern edge of the section splitting into individual benches as the section trends downdip to the west. The Cow Butte coal interval is also persistent in the eastern half of the cross section, but pinches out to the west, whereas the Horse Butte coal interval becomes more persistent to the west. Structurally, the coal beds of the Overland Member dip to the west from their outcrops along the eastern rim of the Greater Green River Basin.

Cross section B–B’ (fig. 8) is a north to south section showing the individual benches of the Cherokee coal bed in the central and western parts of the study area. The Cherokee bench #2 is combined with the Cherokee main bench in most of the section. Where the benches are combined, the bed is referred to as the Cherokee main bench. The lateral persistence of the Horse Butte coal interval in this part of the study area should also be noted. The coal beds in this cross section ride over the anticlinal Wamsutter arch.

Cross section C–C’ (fig. 9) is another north to south section along the eastern margin of the study area, passing through the area of the thickest Cherokee coal. Cherokee coal bench #4 splits off the thickest part of the Cherokee coal bed in the south-central part of the section. The Cherokee leader coal bed is found mainly in the north-central part of the section. The Cow Butte coal bed is thick and laterally persistent in this cross section. The coal beds in the central part of cross section C-C’ are draped over the Wamsutter arch.

Coal Density Calculations

The density value assigned to the Cherokee coal bed that was used to calculate coal resource tonnage was derived from 32 coal quality analyses reported in USGS Professional Paper 1836 (Scott and others, 2019). Analytical results for the Upper Cherokee and Lower Cherokee coal beds were not used, because they were not whole bed samples from the Cherokee coal interval.

Coal density can be calculated in terms of specific gravity using equation 1, as derived from linear regression analysis (Peter J. Bethell, Marshall Miller & Associates, Inc., unpub. data¹ ):

The coal-quality data for ash provided in Scott and others (2019) is on an as-received basis. To use the equation shown above, the ash content must be converted to a dry basis. That can be accomplished using equation 2 (American Society for Testing and Materials, 2015):

The average raw ash content of the 32 samples of the Cherokee coal bed is 13.53 percent. The average total moisture for those 32 samples is 22.38 percent. This yields an average raw, dry ash value for the Cherokee of 17.43 percent.

Using equation 1, an approximate density can be estimated from the calculated raw, dry ash value, yielding a specific gravity for the Cherokee coal bed of 1.424. Multiplying 1.424 by 62.4 (density of water in pounds per cubic foot) yields a density of 88.87 pounds per cubic foot of coal for the Cherokee coal bed in the study area. Although the density estimated for the Cherokee coal bed is higher than the values typically used for a subbituminous coal, as referenced in Wood and others (1983), the Cherokee coal bed has a relatively high average ash content that accounts for the high-density value.

Because of the relatively small number of samples (n=32) analyzed, it should be noted that the estimated average density value may have an impact on the calculation of available resources in the study. If the estimated average density was skewed by anomalous samples, then the calculated quantity (in short tons) of available resources would be similarly skewed.

In-Place or Remaining Resources

The in-place or remaining coal resources for the Cherokee coal bed within the study area were calculated using stochastic geostatistical methods. One hundred different realizations, all compatible with what is known about the coal bed, were generated by sequential Gaussian simulation, with each realization being an equally likely possibility of the actual tonnage (Olea and others, 2021). Four different realizations of Cherokee coal bed in-place resources are illustrated by maps in figure 11. The first three maps (fig. 11A–C) shows three different realizations, with figure 11A showing the realization with the fifth smallest tonnage calculation, figure 11B showing the realization with the median tonnage, and figure 11C showing the realization with the fifth largest tonnage. Figure 11D shows the average in-place tonnage (E-type realization) for each cell for all 100 realizations.

If the extent and geology of a coal deposit were precisely known, which is rarely the case, it could be described by a single tonnage value. More commonly, the nature of the coal deposit is imperfectly known, which introduces uncertainty about the true tonnage, because numerous values can be calculated from the different iterations of the data. In stochastic geostatistical methodology, the difference between the 5th percentile and the 95th percentile of all possible tonnage values, called the 5–95 spread, defines the 90-percent confidence interval for the calculated tonnages. Figure 12A summarizes all the possible in-place tonnage values generated from the 100 realizations. Several facts can be discerned from figure 12A, including:

• • There is a 75-percent probability (upper quartile) that the actual in-place tonnage is not going to exceed 15.434 billion short tons (bst).

• • There is a 50-percent probability (median) that the actual in-place tonnage could either exceed or be less than 15.223 bst.

• • There is a 5-percent probability (5th percentile) that the actual in-place tonnage will be less than or equal to 14.797 bst and a 95-percent probability (95th percentile) that it will be less than or equal to 15.724 bst. This implies that there is a 90-percent probability that the actual in-place tonnage is going to lie between those two values (14.797 bst and 15.724 bst).

• • There is a 90-percent probability that the actual in-place tonnage will be 15.261 ± 0.464 bst. The value of 15.261 bst is the average of the values from the 5th percentile and the 95th percentile. The value ± 0.464 is half of the interval between the 5th percentile and 95th percentile. The formulas used to calculate the values are:

  • ▪ Center of interval for in-place tons = (14.797 bst (5th percentile) + 15.724 bst (95th percentile))/2

  • ▪ 90-percent probability range = (15.724 bst (95th percentile) – 14.797 bst (5th percentile))/2

In mining-related assessments, there is a need to know the uncertainty not only in the total in-place tonnages, but also in the cell-by-cell uncertainty. In the distance-based approach to determining uncertainty in tonnage calculations, as described in Wood and others (1983), the magnitude of the uncertainty in the calculations was intended to be expressed within a range of distances from a data point or drill hole to the next nearest data point or drill hole. An improved method is to utilize the data generated by the 100 realizations, in which 100 values are generated for each cell. This method constitutes an improvement over the distance-based methodology of Wood and others (1983) because the uncertainty in the tonnage calculations is based on values calculated from the realizations on a cell-by-cell basis, rather than determined arbitrarily based on the distance between data points. An individual histogram could be prepared to show the tonnage fluctuations within each cell, which would result in thousands of histograms to be processed and analyzed. A more efficient approach is to prepare a map that plots the 5–95 spread for each cell. The 5–95 spread has proven to be a fairly accurate predictor of calculated tonnage, as validated with additional drilling data (Olea, 2018). The spread of 5-percent to 95-percent probability is a commonly used spread in resource assessments that provides a 90-percent certainty that the calculated tonnage will fall within that range. The stochastic geostatistical methodology used in this study allows resources to be calculated to any degree of certainty desired by the modeler. For example, a 10–90 spread may be desired, which would provide an 80-percent certainty that the calculated resources would fall within that specified range.

Figure 12B shows the spatial fluctuations in uncertainty in tonnage calculations, according to the 5–95 spread, for the total in-place tons at each cell in the 100 realizations shown in figure 11. Figure 12C shows the cumulative frequency distribution fluctuations in uncertainty, according to the 5–95 spread for the total in-place tons at each cell in the 100 realizations.

Methods Parameters

The following parameters are used in this study to determine available resources that meet the parameters for surface mining methods:

The stripping ratio is the ratio of cubic feet of overburden to the tons of coal underlying the same area of overburden. For example, 75 feet of overburden overlying a coal bed, assuming a 1 yard by 1 yard area, converts to 675 cubic feet or 25 bank cubic yards of overburden. The term “bank” indicates that the overburden is intact, and no bulking factor has been applied. If the coal bed is 10 feet thick, with a density of 80 pounds per cubic feet, then there would be 7,200 pounds or 3.6 short tons of coal beneath the overburden. The stripping ratio would be 25 cubic yards of overburden to 3.6 short tons of coal, or 6.94:1.

Available Surface Mining Resources

Available coal resources for the Cherokee coal bed that meet the parameters for surface mining methods within the study area were estimated using stochastic geostatistical methods that generated 100 different realizations, as previously discussed. Four different realizations for available Cherokee coal bed resources that meet the parameters for the use of surface mining methods are illustrated in figure 13. The first three maps (fig. 13A–C) shows different realizations, with figure 13A showing the realization with the fifth smallest tonnage calculation, figure 13B showing the realization with the median tonnage, and figure 13C showing the realization with the fifth largest tonnage. The fourth map, figure 13D, shows the average available coal resources tonnage that meet the parameters for surface mining methods at each cell for all 100 realizations (E-type realization). Figure 14A shows the uncertainty in the available resources that meet the parameters for surface mining methods from values generated from the 100 realizations shown in figure 13. The minimum tonnage calculated from those realizations is 0.738 billion short tons (bst). The maximum tonnage calculated is 0.871 bst. The mean or average tonnage calculated from those 100 realizations is 0.811 bst. The available resources that meet the parameters for surface mining methods for the Cherokee coal bed, as calculated from values within the 5–95 spread, or within the 90-percent confidence interval, is 0.813 ± 0.038 bst.

Figure 14B shows the spatial fluctuations in uncertainty, according to the 5–95 spread, for the available coal resources that meet the parameters for the use of surface mining methods at each cell in the 100 realizations shown in figure 13. Figure 14C shows the cumulative frequency distribution fluctuations in uncertainty, according to the 5–95 spread, for the available coal resources that meet the parameters for surface mining methods at each cell in the 100 realizations.

Underground Mining Methods Parameters

The following parameters are used in this assessment study to determine available resources that could be extracted using underground mining methods:

The minimum and maximum mining heights are based on the projected size and reach of a longwall shearing machine. Using 7.5 ft. shearer drums (the round drums mounted on a longwall shearer that are used to cut the coal) would limit the minimum mining height to 8 feet to insure clearance of the shearer and drums under the protective canopy of the longwall shields. The same size shearer drums would limit the maximum mining height to approximately 15 feet.

The purpose for leaving a minimum of 4.0 feet of coal in the mine roof is to insure the stability of the mine roof in the projected underground mine workings. The strata in the roof of a hypothetical Cherokee coal bed underground mine are projected to be unconsolidated and weak. This is based on an underground mining analog in the Hanna Basin, to the east of the study area, in similarly aged strata, where at least four feet of coal had to be left in the roof to maintain its stability due to the overlying strata being fractured and unconsolidated (Charles N. Thompson, Thompson Geo Service, Inc., oral commun., 2020).

Hypothetical Underground Mining Horizons

A series of geologic cross sections were developed to show hypothetical mining horizons in the benches of the Cherokee coal bed that meet the parameters for underground mining methods (fig. 15). The hypothetical mining horizon within the Cherokee coal bed differs depending on the thickness of the individual benches and the rock partings that separate those benches. The horizons may also vary based on compliance to the underground mining methods parameters described in the “Underground Mining Methods Parameters” section. The correlation datum for all the cross sections described in this section is the base of the Cherokee main bench.

Cross section M1–M1’ (fig. 16) is a west to east cross section that anticipates where underground mining in the Cherokee coal bed may be initiated – in the eastern part of the section, where depths of cover are the lowest. Underground operations could commence by driving drift entries into the Cherokee coal bed at the base of the highwall after initial surface mining operations. Initial underground mining operations could be conducted in a thick part of the bed, which comprises the main bench, bench #2, and bench #3. Mining operations could continue westward until a parting is encountered that splits the combined main bench and bench #2 away from bench #3. That parting quickly thickens to greater than 5 feet in the vicinity of drill hole API 4903723594, which exceeds the maximum parting thickness parameter.

Cross section M2–M2’ (fig. 17) is a north-south section that shows the transition of mining operations from bench to bench in the Cherokee coal interval. On the north (left) side of cross section M2–M2’, mining would be projected to occur in a thick part of the Cherokee, as identified in cross section M1–M1’. As mining operations proceed to the south (to the right on cross section M2–M2’), a parting is encountered that splits the main bench and bench #2. As this parting thickens south of drill hole API 4903723001, the mining horizon could migrate upward, ramping up on the parting to mine in bench #2 and bench #3. Another parting is encountered in the vicinity of drill hole API 4903723001 that splits bench #2 and bench #3. Mining operations could then continue to the south in the cross section, extracting bench #2 and bench #3. Near drill hole API 4900720379, the parting splitting bench #2 and bench #3 thickens rapidly, forcing mining operations again to ramp upwards on top of this parting. Projected mining operations could continue to the south, mining in bench #3 and bench #4. Mining operations could continue to the south until the parting separating bench #3 and bench #4 reaches the constraining parameter of a minimum thickness of 5 feet.

Cross section M3–M3’ (fig. 18) is another west to east section that intersects the southern part of cross section M2–M2’. Cross section M3–M3’ shows continued mining operations in a horizon comprising bench #3, bench #4, and a parting separating those benches. The projected limit of potential mining operations is reached on both edges of the cross section where the thickness of the parting separating bench #3 from bench #4 exceeds 5 feet. Because of the rapid thickening and thinning of the partings in the Cherokee coal interval below bench #3, floor rolls, faults, and other differential compaction features may be encountered during mining operations.

Available Underground Mining Resources

Available coal resources for the Cherokee coal bed that meet the parameters for underground mining methods within the study area were estimated using stochastic geostatistical methods to generate 100 different realizations, as previously discussed. Four different realizations for available Cherokee coal bed resources that meet the parameters for underground mining methods are illustrated by maps in figure 19. The first three maps (fig. 19A–C) shows three different realizations, with figure 19A showing the realization with the fifth smallest tonnage calculation, figure 19B showing the realization with the median tonnage, and figure 19C showing the realization with the fifth largest tonnage. The fourth map (fig. 19D) shows the average available coal resources that meet the parameters for underground mining methods at each cell for all 100 realizations (E-type realization). Figure 20A shows the uncertainty in the available resources that meet the parameters for underground mining methods generated from the 100 realizations. The minimum tonnage calculated from those realizations is 2.381 billion short tons (bst). The maximum tonnage calculated is 2.469 bst. The mean tonnage calculated from those 100 realizations is 2.418 bst. The available resources that meet the parameters for underground mining methods for the Cherokee coal bed, as calculated from values within the 5–95 spread, or within the 90-percent confidence interval, is 2.393 ± 0.055 bst.

Figure 20B shows the spatial fluctuations in uncertainty, according to the 5–95 spread, for the available coal resources that meet the parameters for underground mining methods at each cell in the 100 realizations. Figure 20C shows the cumulative frequency distribution fluctuations in uncertainty, according to the 5–95 spread, for the available coal resources that meet the parameters for underground mining methods at each cell in the 100 realizations shown in figure 19.