Assessment of CoalThe Geology, Powder Resources, River and Reserve Basin Base in the Powder River Basin, Wyoming and Montana
Using a geology-based assessment methodology, the U.S. Geological Survey estimated in-place resources of 1.07 trillion short tons of coal in the Powder River Basin, Wyoming and Montana. Of that total, with a maximum stripping ratio of 10:1, recoverable coal was 162 billion tons. The estimate of economically recoverable resources was 25 billion tons.
107° 106° 105° ROSEBUD Miles City FALLON Introduction COUNTY TREASURE COUNTY Forsyth COUNTY CUSTER The U.S. Geological Survey (USGS) is responsible for COUNTY Montana Powder providing objective scientific information to support deci- River Basin sions regarding land management, environmental quality, 46° (USGS Open-File Colstrip Report 2012–1113) and economic, energy, and strategic policy. To accomplish BIG HORN this goal, the USGS periodically assesses the Nation’s COUNTY endowment of various energy resources, including coal. Lame CARTER Because the amount of economically recoverable coal Deer Ashland COUNTY Indian resource is substantially less than the total original vol- Reservations Broadus ume of coal in place, coal reserve base estimates provide Birney essential information for energy-related decisions (Luppens and others, 2009). To be classified as “reserves,” the coal POWDER RIVER Decker COUNTY must be considered economically producible at the time 45° MONTANA of classification. WYOMING Regional-scale reserve estimates are based on prefea- Northern Wyoming Sheridan Powder River Basin sibility scoping-level studies in turn based on conceptual (USGS Open-File mine designs. Prefeasibility studies have a higher degree of SHERIDAN Report 2010−1294) CROOK uncertainty than feasibility studies developed for commer- COUNTY COUNTY cialization of a specific property. Conceptual mine designs BIG Buffalo do not have sufficient detail to support a final decision by a HORN Moorcroft financial institution to support development of a coal deposit COUNTY Gillette to the production stage. WESTON East Gillette coal field COUNTY The goal of the current USGS United States Coal 44°
Resources and Reserves Assessment Project is to conduct extension Gillette coal field regional-scale, coal resource and reserve assessments of (USGS Open-File Report 2008–1202) COUNTY
the significant coal beds in all major U.S. coal basins. WASHAKIE The Powder River Basin (PRB) in northeastern Wyoming JOHNSON CAMPBELL and southeastern Montana (fig. 1) was the first basin to be COUNTY COUNTY assessed under this effort. It contains the largest deposits of low-sulfur subbituminous coal in the world. In 2011, coal MONTANA Powder River Southwestern Wyoming production from 16 mines in the basin totaled 462 million Basin Powder River Basin short tons (MST), some 42 percent of the total coal pro- (USGS Open-File 43° Report 2011–1134) duction in the United States, making the PRB the single WYOMING most important coal-producing basin in the Nation. About NATRONA Douglas COUNTY Glenrock 426 MST (92 percent of total PRB coal production) came COUNTY CONVERSE COUNTY NIOBRARA from the Gillette coal field, which is located along the 0 20 40 MILES eastern margin of the Wyoming PRB (fig. 1). The results presented here update coal resource and reserve estimates 0 20 40 KILOMETERS for the PRB and represent the first completed regional-scale Figure 1. Location of individual coalGlenrock assessment areas in the Powder assessment intended to revise the Nation’s coal reserve base. River Basin, Wyoming and Montana. From Haacke and others (2013).
U.S. Department of the Interior Fact Sheet 2012–3143 U.S. Geological Survey Printed on recycled paper February 2013 The Powder River Basin, 107° 106° 105° an elongate, north-northwest- ROSEBUD CUSTER COUNTY EXPLANATION trending basin, covers about TREASURE COUNTY 19,500 mi2 (see fig. 1), exclusive COUNTY FALLON Qal Quaternary alluvium and COUNTY terrace deposits of the part of the basin within the Twr Tertiary White River Formation Crow and Northern Cheyenne Tw Tertiary Wasatch Formation Indian Reservations in Montana. 46° Tfu Tertiary Fort Union Formation The PRB forms a broad asymmet- (undifferentiated) ric syncline with gentle westward Tftr Tongue River Member dips along the eastern flank and Tftrl Tongue River and Lebo Qal Shale Members steep eastward dips along the Tfl Lebo Shale Member BIG HORN COUNTY western flank. The synclinal axis Tft Tft Tullock Member lies nearer the basin’s western POWDER RIVER COUNTY Basin axis margin, trending north-northwest CARTER COUNTY in the Wyoming part of the PRB Tftr and gradually turning north- northeasterly in the Montana part MONTANA MONTANA (fig. 2). Because the coal beds are 45° WYOMING thick, shallow, and gently dipping along the eastern margin of the CROOK SHERIDAN COUNTY Wyoming part of the basin, large COUNTY open-pit mines have been devel- Powder River oped there to extract near-surface Basin coal resources (fig. 3). BIG Tftrl WYOMING Four assessment subdivi- HORN COUNTY sions in the Powder River Basin Tw are based on geography, geology, WESTON mining conditions, and topog- COUNTY 44° raphy, which resulted in differ- Tfu ences in the sizes of databases Twr Figure 2. Generalized COUNTY among these areas and facilitated WASHAKIE geology of the Powder a more timely release of assess- CAMPBELL River Basin, Wyoming and JOHNSON COUNTY ment results. The four assessed COUNTY Montana. Modified from areas (fig. 1) were the Gillette Flores and Bader (1999). coal field, Wyoming (Luppens Tfl and others, 2008); the Northern Wyoming Powder River Basin NATRONA (Scott and others, 2010); the COUNTY Southwestern Wyoming Powder 43° River Basin (Osmonson and others, 2011); and the Montana CONVERSE
COUNTY COUNTY Powder River Basin (Haacke and NIOBRARA others, 2013). 0 20 40 MILES 0 20 40 KILOMETERS
The primary objectives of the PRB assessment were to: • Improve geological assurance by updating the stratigraphic database using information obtained from recently completed coal bed methane (CBM) and oil and gas wells; • Develop more-comprehensive in-place coal resource computer models that also support regional reserve base estimates; and • Complete a surface-mining economic evaluation customized to the environmental and technological restrictions of each assessed area and thereby derive regional estimates of the coal reserve base. 107° 106° 105° Study Methodology R. 35 W. R. 37 W. R. 39 W. R. 41 W. R. 43 W. R. 45 W. R. 47 W. R. 49 W. R. 51 W. R. 53 W. R. 55 W. R. 57 W. TREASURE ROSEBUD CUSTER T. 7 N. Earlier assessments of the Powder COUNTY COUNTY COUNTY FALLON EXPLANATION River Basin were based on correlations of COUNTY Thickness (feet) T. 5 N. 300 and > coal zones, which included stratigraphic 250 intervals that were as much as 800 ft thick 46° T. 3 N. 200 and included as many as six or more beds 150 125 with combined coal thickness of 100 ft or T. 1 N. 100 more (Flores, 1999). To evaluate mining T. 1 S. BIG HORN 75 economics, assessment of economically COUNTY 50 recoverable resources must be based on the T. 3 S. 25 thickness and extent of individual coal beds. CARTER 5 COUNTY The tremendous growth of CBM well devel- T. 5 S. Mine area opment in the PRB has facilitated a unique POWDER RIVER COUNTY view of subsurface geology. With comple- T. 7 S. tion of more than 25,000 drill holes, includ- MONTANA MONTANA ing corresponding geophysical logs which T. 9 S. 45° WYOMING facilitate coal bed correlations, coal beds T. 57 N. have been more accurately defined and cor- Powder River Basin related. For the Gillette coal field assessment T. 55 N. SHERIDAN WYOMING alone, approximately 8,000 new drill holes COUNTY CROOK CAMPBELL COUNTY were added to the geologic database for a T. 53N. COUNTY total of 10,210 data points. Interpretation of
T. 51 N. these new data provided an unprecedented BIG assessment of the coal resources and the HORN COUNTY reserve base in the PRB. A total of 29,928 T. 49 N. drill holes (of which 21,393 holes were Figure 3. Isopach T. 47 N. non-proprietary) were used in the overall 44° map of the combined PRB assessment (Haacke and Scott, 2013). Smith, Anderson, The assessment consisted of three T. 45 N. and Canyon coal beds (almost 40 COUNTY phases, with the first being data collection, COUNTY
WASHAKIE T. 43N. editing, and correlation of individual coal WESTON percent of the JOHNSON COUNTY in-place resources) beds. Both the Wyoming State Geological COUNTY T. 41 N. Survey and the Montana Bureau of Mines showing extent of resources in beds at and Geology facilitated data acquisition. The Powder River Basin T. 39 N. least 5 ft thick within second phase involved modeling the coal the Powder River beds using a digital multibed geologic model- T. 37 N. Basin, Wyoming and ing program to generate coal isopach, struc- 43° Montana. ture, and overburden maps for each assessed T. 35 N. coal bed. Separate models were generated CONVERSE for the resource and reserve base evaluations NATRONA T. 33 N. COUNTY COUNTY COUNTY using minimum coal thicknesses of 2.5 ft and COUNTY NIOBRARA 5.0 ft, respectively. To calculate unrestricted R. 87 W. R. 85 W. R. 83 W. R. 81 W. R. 79 W. R. 77 W. R. 75 W. R. 73 W. R. 71 W. R. 69 W. R. 67 W. R. 65 W. coal resources potentially available for min- 0 20 40 MILES ing, the coal data were imported into a Geo- 0 20 40 KILOMETERS graphic Information System (GIS) in which coal resources affected by mining restrictions models were created, one for each stripping Once the available resource volumes and were subtracted from the resource base. ratio (thickness of overburden to total thick- estimated cost to mine by stripping-ratio inter- Mining restrictions included towns, railroads, ness of coal) from 1:1 to 10:1. Geologic and val were completed, a cost curve was generated environmentally sensitive areas, and coal too mining conditions for each assessment area relating sales price to resources (fig. 4). The thin or too deep to mine economically. were used to customize the mine size and amount of coal at or below the current sales The last assessment phase was a costing, and a discounted cash flow (DCF) price was designated as “reserve base.” The prefeasibility-level economic mining analysis at a given rate of return (ROR) was cost curve for the Gillette coal field demon- evaluation (± 25 percent) to determine used. Finally, GIS programs were used to strates that a reserve base estimate is not a economically recoverable coal resources calculate the volume of available coal for each single value and that the amount of reserve base (the reserve base). Ten hypothetical mine of the 10 stripping ratios. fluctuates as changes occur in the market price. $70 economically recoverable resources (the reserve base). 77 BST Most of the beds assessed for recoverable resources were $60 those with in-place resources of 50 BST or more (fig. 5). That 25 BST of coal resources classified as the $50 reserve base, however, does not mean that the total $40 amount of coal left in the Powder River Basin could be produced by surface mining technologies. Mining
(dollars) $30 costs and coal sales prices are not static, as both tend to DCF cost/ton increase over time. If market prices exceed mining costs, $20 18.5 BST the reserve base will grow (the converse is also true). For $14.00 $10.47 example, coal being mined today would not have been $10 classified as reserve base in 1990. The cost curve from 10.1 BST the Gillette coal field assessment (fig. 4) illustrates the 0 10 20 30 40 50 60 70 80 dynamic nature of reserve assessments. For example, a Billions of tons sales price increase of only $3.43 per ton nearly doubled (BST) the estimate of the reserve base. Figure 4. Cost curve showing reserve estimates at $10.47/ton (as of January 2007) and For longer-term energy planning, total estimated $14.00/ton (as of March 2008) for the Gillette coal field, Powder River Basin, Wyoming recoverable resources of 162 BST (table 1) represent a (Luppens and others, 2008). [Abbreviation: DCF, discounted cash flow] more important value than current reserve base estimates. In 1990, for example, resources with a stripping ratio greater than 2:1 were not classified as reserve base. Yet Resource Assessment Results today, coal being mined at stripping ratios of 3:1 to 4:1 Based on methodology outlined above, the USGS assessed coal resources obviously now would be classified as reserve base. for 47 coal beds in the Powder River Basin. The most significant coal resources There are no current underground mining opera- lie within the Roland (Baker), Smith, Anderson, Dietz 3, Canyon, Lower tions in the Powder River Basin, nor are any anticipated Canyon, Werner/Cook, Otter, Gates/Wall, and Rosebud/Knobloch coal beds of in the foreseeable future. There is a significant deep coal the Tongue River Member of the Paleocene Fort Union Formation. Although resource base in the basin, however, and preliminary there are seven coal beds in the Eocene Wasatch Formation, they are of minor assessments of deep coal with thicknesses amenable for importance in terms of recoverable coal. These beds generally are thinner and longwall mining were completed for the Southwestern of poorer quality than beds in the Fort Union Formation. Wyoming Powder River Basin (Osmonson and others, The USGS calculated an original in-place coal resource of 1.07 trillion 2011), the northern Wyoming PRB (Scott and others, short tons (TST) for the 47 coal beds in the PRB, with just ten individual 2010), and the Montana PRB (Haacke and others, 2013). coal beds representing more than 75 percent (fig. 5) of the total resources The combined underground resource for those three (816 billion short tons, BST). Three cross sections (figs. 6–8) and one com- areas, in beds 10–20 ft thick (the optimal mining height bined isopach map of the Canyon, Anderson, and Smith beds from the Wyodak for longwall mining systems), was estimated at 302 BST. coal zone (fig. 3) display coal bed geometry in the PRB. Fig- ure 6 illustrates the structural asymmetry of the basin whereas Roland (Baker): 49 BST 4.6% figure 7 shows that nearly all of the thick, shallow coal beds on the eastern side of the basin are thinner toward the southwestern Smith: side of the basin. Figure 8 illustrates the thinning and splitting 126 BST 11.8% of beds within the Wyodak-Anderson coal from the Gillette Remaining 37 coal beds: 255 BST 23.8% Anderson: coal field northward into Montana. The combined isopach of Rosebud/Knobloch: 130 BST 12.1% the three most prominent coal beds (that is, Smith, Anderson, 52 BST 4.9% and Canyon; fig. 3) shows how these beds have influenced Canyon: coal development in the PRB. Essentially all of the significant 150 BST 14% mining to date in the PRB is limited to shallow depths of these Dietz 3: thick coal beds, with the exception of the Rosebud-Knobloch Gates/Wall: 50 BST 4.7% 62 BST 5.8% and Flowers-Goodale coal beds, which are mined in the north- Otter: ern part of the Montana PRB (fig. 3 and fig. 8). 65 BST 6.1% Werner/Cook: Lower Canyon: Not all coal beds were included in evaluating mining eco- 73 BST 6.8% nomics for each assessment area. Only those beds with substan- 59 BST 5.5% tial areal extent, exceeding 5 ft in thickness, and with stripping ratios less than 10:1 were included. A summary of reserve base Figure 5. Pie diagram showing percentages and tonnages of individual coal beds in relation to the total 1.1 trillion short tons of coal resources in the Powder River assessments for the PRB is shown in table 1. An estimated total Basin, Wyoming and Montana. [Abbreviation: BST, billion short tons; %, percent] of 25 billion short tons of coal resources met the definition of A A’ EXPLANATION Coal beds Approximate land surface 4,000 ft Wyodak-Anderson coal zone Anderson (Flores and Bader, 1999) Canyon Dietz 3 3,000 ft Flowers-Goodale Gates/Wall MONTANA Lower Canyon Pawnee 2,000 ft Roberts/Terret Roland (Baker) A A’ Rosebud/Knobloch 1,000 ft Smith Powder
Elevation above sea level Werner/Cook River Basin 0 ft WYOMING 0 10 20 MILES
0 10 20 KILOMETERS
Figure 6. West—east cross section A—A’ showing subsurface distribution of coal beds through the central part of the Powder River Basin, Wyoming and Montana. (Beds dip steeply along the western margin of the basin in contrast to gently dipping beds along the eastern margin.)
B B’
Wyodak-Anderson coal zone EXPLANATION Coal beds 5,000 ft (Flores and Bader, 1999) Approximate land surface Wyodak-Anderson coal zone Anderson (Flores and Bader, 1999) Canyon 4,000 ft Dietz 3 Felix MONTANA Flowers-Goodale Gates/Wall 3,000 ft Lower Canyon Pawnee Roberts/Terret 2,000 ft B Roland (Baker)
Elevation above sea level B Powder B’ Rosebud/Knobloch River Basin Smith 1,000 ft WYOMING Werner/Cook 0 10 20 MILES
0 10 20 KILOMETERS
Figure 7. West—east cross section B—B’ showing subsurface distribution of coal beds through the southern part of the Powder River Basin, Wyoming and Montana. (Beds dip more steeply along the western margin of the basin in contrast to gently dipping beds along the eastern margin.)
C C’ EXPLANATION Coal beds Wyodak-Anderson coal zone Approximate land surface (Flores and Bader, 1999) Anderson 5,000 ft Wyodak-Anderson coal zone Canyon (Flores and Bader, 1999) Dietz 3 Felix Flowers-Goodale 4,000 ft Gates/Wall Lower Canyon MONTANA C Pawnee 3,000 ft Roberts/Terret Roland (Baker) Rosebud/Knobloch Smith 2,000 ft Elevation above sea level Powder Werner/Cook River C’ Basin 1,000 ft WYOMING 0 20 40 MILES
0 20 40 KILOMETERS
Figure 8. North—south cross section C—C’ showing subsurface distribution of coal beds through the central part of the Powder River Basin, Wyoming and Montana. Table 1. Powder River Basin coal resources and reserve base, Wyoming and Montana, reported by assessment area, including Gillette coal field, Wyoming; NWPRB (Northern Wyoming Powder River Basin); SWPRB (Southwestern Wyoming Powder River Basin); MTPRB (Montana Powder River Basin); and PRB (Powder River Basin). (Amounts reported in billions of short tons.)
Gillette coal field NWPRB SWPRB MTPRB PRB Roland (Baker), Smith, Roland (Baker), Smith, Roland (Baker), Roland (Baker), Anderson Rider, Anderson, Coal beds assessed Anderson, Dietz 2, Smith, Anderson Smith, Anderson, Dietz 3, Dietz 2, Dietz 3, Canyon, for reserve base Dietz 3, Canyon, Rider, Anderson, Canyon, Lower Canyon, Lower Canyon, Werner/Cook, Rosebud/Knobloch, Dietz 3, Canyon Werner/Cook Rosebud/Knobloch, Flowers-Goodale Flowers-Goodale Total resources 201 285 369 215 1,070 Total Economic Evaluation 165 158 165 488 Recoverable 77 50 35 162 Economic 10 2 13 25
References Cited Flores, R.M., 1999, Database creation and resource evaluation methodology, chap. DB of Fort Union Coal Assessment Team, 1999 Resource assessment of selected Tertiary coal beds and zones in the Northern Rocky Mountains and Great Plains Region: U.S. Geological Survey Professional Paper 1625–A, 2 CD-ROMs. Flores, R.M., and Bader, L.R., 1999, Fort Union coal in the Powder River Basin, Wyoming and Montana—A synthesis, chap. PS of Fort Union Coal Assessment Team, 1999 Resource assessment of selected Tertiary coal beds and zones in the Northern Rocky Mountains and Great Plains Region: U.S. Geological Survey Professional Paper 1625–A, 2 CD-ROMs. Haacke, J.E., Scott, D.C., Osmonson, L.M., Luppens, J.A., Gunderson, J.A., and Pierce, P.E., 2013, Coal geology and assessment of coal resources and reserves in the Montana Powder River Basin: U.S. Geological Survey Open-File Report 2012–1113, 133 p. Haacke, J.E., and Scott, D.C., 2013, Drillhole data for coal beds in the Powder River Basin, Montana and Wyoming: U.S. Geological Survey Data Series DS 713, 15 p. Luppens, J.A., Scott, D.C., Haacke, J.E., Osmonson, L.M., Rohrbacher, T.J., and Ellis, M.S., 2008, Assessment of coal geology, resources, and reserve base in the Gillette coal field, Powder River Basin, Wyoming: U.S. Geological Survey Open-File Report 2008–1202, 127 p. Luppens, J.A., Rohrbacher, T.J., Osmonson, L.M., and Carter, M.D., 2009, Coal resource availability, recoverability, and economic evaluations in the United States—A summary, chap. D of Pierce, B.S., and Dennen, K.O., eds., The national coal resource assessment overview: U.S. Geological Survey Professional Paper 1625–F, 17 p. Osmonson, L.M., Scott, D.C., Haacke, J.E., Luppens, J.A., and Pierce, P.E., 2011, Assessment of coal geology, resources, and reserve base in the Southwestern Powder River Basin, Wyoming: U.S. Geological Survey Open-File Report 2011–1134, 135 p. Scott, D.C., Haacke, J.E., Osmonson, L.M., Luppens, J.A., Pierce, P.E., and Rohrbacher, T.J., 2010, Assessment of coal geology, resources, and reserve base in the Northern Powder River Basin, Wyoming: U.S. Geological Survey Open-File Report 2010–1294, 136 p.
View of a surface coal mine in the Powder River Basin near the town of Gillette, northeastern Wyoming. Photograph by J.A. Luppens, U.S. Geological Survey.
Supporting assessment reports are For Further Information available at the USGS Energy program website, at http://energy.usgs.gov.
James A. Luppens, David C. Scott, Coal Assessment Team Lee M. Osmonson, Jon E. Haacke, and Paul E. Pierce.
James A. Luppens ([email protected]) Contact Information David C. Scott ([email protected])