COAL RESOURCES OF MONTANA

Jay A. Gunderson and John Wheaton

Montana Bureau of Mines and Geology, Billings, Montana

ABSTRACT Coal has been a valuable source of energy and vital part of Montana’s history and economy for 200 years, providing energy for heating homes and cooking, fuel for the westward expansion of railroads, raw material for smelting and other industrial applications, and since about the 1960s, a major source of fuel for generating elec- tricity. Montana ranks fi rst among the states in the size of its demonstrated reserve base, with 119 billion short tons of coal, and sixth in production at roughly 40 million short tons per year. The most economically important coal resources occur in the Paleocene Fort Union Formation in the Powder River Basin, Bull Mountain Basin, and Williston Basin in eastern Montana. These account for over 95 percent of Montana’s coal resources and all of the State’s current coal production. Since the 1970s, research on Montana coal has focused almost exclusive- ly on quantifying resource tonnages and chemical composition of Fort Union Formation coalbeds available for strip mining.

BACKGROUND ated wood is like that of the Missouri of an inferior quallity.” (Moulton, 2001). Coal has played a vital role in Montana’s history and economic development. This brief historical view Uses and Demands provides a backdrop to the discussion that follows on Heating Montana’s coal fi elds. More detailed historical reviews In 1807, Manuel Lisa, a French fur trader, built a are provided by Morgan (1966) and Chadwick (1973). trading post at the confl uence of the Bighorn and Yel- Coal-bearing formations underlie about 35 percent lowstone Rivers and utilized lignite coal from nearby of Montana (fi g. 1). Coal rank generally increases outcrops to heat buildings during the winter months from east to west—from lignite in the east, to sub- (Morgan, 1966). During the homesteading era of the bituminous and bituminous coal further west. The late 1800s and early 1900s, coal was used as fuel for coal-bearing formations range in age from Late Juras- cooking and heating homes by farmers and ranchers, sic/Early Cretaceous to Tertiary (Miocene; fi g. 2). Al- and local coal mines appeared across rural Montana. though historically, mining has been conducted in all of Montana’s coal regions, commercial mining during Coal for Metals Mining the past 50 yr has been limited to Tertiary (Fort Union With the advent of hardrock mining in Montana Formation) coalbeds in the Powder River Basin, Bull in the mid-1800s came demand for transportation and Mountain Basin, and Williston Basin (fi g. 1). smelting fuels. Steam, generated by coal, powered Discovery mine equipment (steam hoists and transportation) and provided energy to move ore from mines to smelt- On July 28, 1806, Captain William Clark of the ers. Coking coal (or metallurgical coal) from three of Corps of Discovery observed coal in the bluff s along Montana’s coal fi elds was important for use in blast the Yellowstone River of southeastern Montana: furnaces for copper smelting in the late 1800s and ear- “in the evening I passd. Straters of Coal in the ly 1900s (Averitt, 1966). Coking ovens were built and banks on either Side. those on the Stard. Bluff s was operated near Livingston, Great Falls, and Gardner. about 30 feet above the water and in 2 vanes from The demand for coking coal waned in the early 1900s 4 to 8 feet thick, in a horozontal position. the Coal due to the development of more effi cient methods of Contained in the Lard Bluff s is in Several vaines of smelting. diff erent hights and thickness. this Coal or Carbon-

1 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

FLATHEAD Havre Shelby FIELD Kalispell Glasgow BLACKFEET - NORTH-CENTRAL VALIER REGION FIELD Sidney

Great Falls WILLISTON BASIN GARFIELD Glendive GREAT FALLS - LEWISTOWN Lewistown REGION Missoula FIELD

Helena Ü TERTIARY LAKE BEDS Miles City LOMBARD BULL MOUNTAIN FIELD BASIN LEGEND Butte TRAIL CREEK - Colstrip Lignite Coal LIVINGSTON Billings Bozeman FIELD STILLWATER Hardin Subbituminous Coal POWDER RIVER TERTIARY FIELD BRIDGER - BASIN Bituminous Coal LAKE BEDS SILVERTIP FIELD Active Coal Mine Dillon ELECTRIC RED LODGE FIELD FIELD Montana PRB Outline 0 30 60 90 120 Miles County Boundary

Figure 1. Map of Montana coal regions (modifi ed from Cole and others, 1982).

Basin or Coal Period/ Formation Mid-&ontinental Field Zone Age (my) Seaway Stage

Lake beds Flathead Kishenehn? Williston Wasatch Bull Mountain Fort Union Powder River Red Lodge Blackfeet±Valier 65 Hell Creek Fox Hills Regression Bearpaw Blackfeet±Valier Transgression North Central Judith River Regression Claggett Bridger Transgression Silver Tip Eagle Stillwater Regression Trail Creek±Livingston Telegraph Creek Electric

Colorado Group

Transgression Kootenai

Great Falls± 144 Morrison Lewistown Lombard Regression Jurassic Cretaceous Tertiary

Figure 2. General stratigraphic position and age of coal-bearing formations. 2 Gunderson and Wheaton: Coal Resources of Montana Transcontinental Railroads for low-sulfur eastern Montana coal, along with use The railroads headed west through Montana to of dedicated coal trains, or “unit trains,” marked the connect major Midwestern cities to the Pacifi c coast beginning of the modern coal era in Montana. during the 1880s. To encourage expansion into the The anticipated growth in coal production and use western states, the Federal government enticed rail- was outlined in a major 1975 study that listed poten- roads to expand service by granting them land and tial for up to 52 new electrical generating plants in mineral rights in every odd-numbered, 640-acre sec- the northern Great Plains, with 9 located in Montana tion within 10 mi on each side of the track (U.S. Gov- (NGPRP, 1975). Of the 9 anticipated sites in Montana, ernment, 1862). Coal was preferred over wood as fuel only 4 were constructed and are located at Colstrip, for steam locomotives because it has higher energy Montana. The same report presented three develop- content per unit volume. In response to the demand for ment scenarios projecting growth in Montana coal coal, many small mining operations opened to supply production from 7.1 million short tons (MST) per year the railroad. By the 1950s, diesel engine locomotives in 1971 to between 58 and 393 MST per year by 2000. replaced steam engines and the early era of robust coal Even the lowest projection was never realized; instead, mining in Montana came to a close (fi g. 3). Montana’s coal production has remained level at about 30 to 40 MST per year since the late 1970s (fi g. 3). Thermal Electric Generation Coal Mining in Montana The coal industry in Montana was rejuvenated in the 1970s when the Clean Air Act of 1970 was enacted, Current Mining allowing the U.S. Environmental Protection Agency to Montana currently produces about 35 to 40 MST limit sulfur-dioxide emissions and other air pollutants per year of coal from fi ve strip mines and one under- that threatened public health. Rather than incurring ground mine (table 1, fi g. 3). Nearly all coal currently the expense of retrofi tting coal-fi red power plants with mined is used for electrical generation, with small emission controls to accommodate high-sulfur eastern amounts used for home heating. Roughly 25 percent U.S. coal, companies sought western U.S. coal with of the coal produced in Montana is used locally to lower sulfur content. As a consequence, the Powder generate electrical power; the remaining 75 percent is River Basin (PRB) emerged as one of the primary shipped overseas and to coal-fi red power plants in the coal-producing regions in the United States. Demand Midwest.

MONTANA COAL PRODUCTION

50 Powder River Basin 45 Other Coal Fields Total (UndiīerenƟated) 40

35

30

25

20 EPA Clean Air Act MILLION SHORT TONS 15 Diesel Locomotives

10

5

0

YEAR

Figure 3. Montana coal production (1880–2018). Data source: Montana Department of Labor and Industry; Milici, 1997. 3 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

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Future Mining is referred to the Montana Department of Environmen- Two conditions are putting downward pressure tal Quality, Coal and Uranium Program. on current coal markets in the United States. First, a OVERVIEW OF COAL IN MONTANA renewed focus on coal’s environmental impact (con-

cerns about climate change due to CO2 emissions; Coal Formation changes in local hydrologic balance; and restoration of Coal is a combustible sedimentary rock composed the soils and surface usage) has resulted in decreased of organic material [primarily carbon (C), hydrogen usage. Aging coal-fired power plants are being shut (H), and oxygen (O)] derived from the decay of plant down rather than retrofitted with additional emissions matter, plus variable amounts of moisture, volatiles, control equipment. Second, increased natural gas other elements such as sulfur and nitrogen, and inor- production from unconventional resources is replacing ganic material (mineral matter). The relative amounts coal for a larger portion of electrical power generation. of organic constituents (particularly percent carbon) Although coal will continue to be an important part of determine coal rank, an important measure of coal the U.S.’s energy portfolio, the U.S. Energy Informa- quality that is directly related to heating value or en- tion Administration (EIA) projects flat coal production ergy content (fi g. 3 and table 4 in Averitt, 1975). Coal and consumption for the next several decades (EIA, heating value is generally expressed in British thermal 2019). The future of coal may well depend on new units per pound (Btu/lb). Inorganic material within technologies for utilization—technologies that are coalbeds comes from the occasional infl ux of clastic more effi cient, cost-effective, and environmentally sediment into the coal-forming swamp or basin. This friendly than those used today. material—termed “ash”—can be dispersed within coal Environmental Impacts or occur as thin, defi nable layers, called partings. Coal mining falls under the jurisdiction of State Coal forms in wetlands (e.g., swamps, bogs, mires, and Federal bonding and reclamation rules (Montana fl oodplains, and lagoons) where the accumulation of Code Annotated, 2017; SMCRA, 1977). Reclamation decaying plant material, or peat, exceeds the rate of rules require mining companies to restore land (hy- bacterial decay and oxidation of the plant debris. As drology, topography, soils, and vegetation) that has peat is buried and compacted, heat and pressure drive been disturbed by mining activities to original con- off volatiles and moisture, to form lignite (low rank) dition and use. Impacts to and recovery of the hydro- coal. Continued metamorphism transforms low rank geologic system is discussed in detail in Meredith and coal into higher rank coal (lignite to subbituminous others (2020). Climate change, soils, air quality, and to bituminous to anthracite). It may take as much as land use fall outside the purview of the Montana Bu- 100 ft of plant material to form a 5-ft-thick bed of reau of Mines and Geology (MBMG), and the reader bituminous coal. Because heat and pressure increase 4 Gunderson and Wheaton: Coal Resources of Montana with depth, and since older coalbeds tend to have been generally higher in low-lying peat mires where fi ne- buried more deeply than younger coalbeds, there is a grained sediments can accumulate more easily than in general relationship between age and coal rank; older raised swamps. coalbeds tend to be higher rank than younger coalbeds. Coal Quality Coal Depositional Environments The term “coal” encompasses a wide range of Coal-forming environments exist in both coastal rocks with a wide range of properties and composition. (marginal marine) and terrestrial (non-marine) settings Some coal constituents are harmful to the environment (fi g. 4). Both types of depositional environments were (e.g., mercury and oxides of sulfur and nitrogen); oth- active in Montana’s past, a consequence of the Juras- ers may require special equipment and/or equipment sic and Cretaceous seas that periodically fl ooded and maintenance for combustion (e.g., sodium). The pri- drained the mid-continental region of North America mary indicator for the quality of coal as a fuel is rank, (fi g. 2). or heating value. Thus, determination of coal quality— After the Jurassic seas withdrew from North Amer- or coal chemistry—is important because it impacts ica, rivers and lakes formed on the broad low-lying coal utility and value, emissions, waste products, and fl oodplain that remained. These fl oodplain wetlands equipment design and effi ciency (e.g., turbines, boil- became sites for peat accumulations that eventually ers). Coal quality can be assessed via chemical and formed the terrestrial coalbeds of the Late Jurassic/ petrographic analyses. Early Cretaceous Morrison Formation. During the Chemistry Late Cretaceous, several cycles of marine transgres- sions and regressions led to frequent fl ooding of There are two common analytical tests performed coastal swamps and lagoons lying landward of the on coal samples to determine coal quality: proximate shoreline. Thus, Late Cretaceous coalbeds in Montana and ultimate analyses. Proximate analyses are related are primarily marginal marine in origin. Final regres- to burning characteristics and used to determine heat- sion of the sea at the end of the Cretaceous once again ing value (Btu/lb), moisture content, volatile matter, left a newly exposed land surface that was drained fi xed carbon, and percent ash. Ultimate analyses pro- by a large-scale fl uvial system(s) fl owing northward vide information about the major organic elemental into the retreating Cannonball Sea during the Tertiary composition of coal in terms of carbon, hydrogen, (Flores, 1992). Interfl uvial wetlands such as mires, sulfur, oxygen, and nitrogen. Guidelines for sample lakes, and bogs accumulated an immense amount of collection and analytical methods are given by Goli- peat, resulting in Tertiary coal deposits that are the ghtly and Simon (1989) and Speight (2005). largest in the State. Coal-quality data from Montana have been gath- The type of depositional environment—margin- ered and reported since the early 1900s. Early mappers al marine or terrestrial—can lead to very diff erent of the U.S. Geological Survey (USGS) and the U.S. coalbed distributions, geometries, and composition. Bureau of Mines (USBM) collected samples at mine Coalbeds that form in coastal wetlands tend to be thin, locations and reported results in reconnaissance map- laterally continuous, elongate parallel to the paleo- ping publications. These data were compiled by Field- shoreline, and occur in sequences of mixed marine ner and others (1932) and Gilmour and Dahl (1967). and non-marine rocks (fi g. 4). Terrestrial coal-forming A large amount of quality data were gathered on environments are typically fl uvial and lacustrine in or- coal samples from drillhole cores and from active igin and include interfl uvial mires, swamps, and other mines during the joint USGS and MBMG explorato- localized wetlands. Coalbeds formed in fl uvial envi- ry drilling program of the 1960s, 1970s, and 1980s. ronments are lenticular and discontinuous; they often Results of proximate and ultimate analyses were pub- merge and split over relatively short distances. lished along with drillhole stratigraphic data in several Coal quality, or chemistry, is also infl uenced by USGS reports: for example, Aff olter and Hatch (1980) depositional environment (Fort Union Coal Assess- and Tewalt and others (1989). Other coal-quality data ment Team, 1999). For example, sulfur content tends are found in many of the coal publications cited in this to be higher in marginal marine coal than terrestrial review, such as Matson and others (1973). coal. Ash content from the infl ux of clastic material is

5 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

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ŚĂŶŶĞů^ĂŶĚƐƚŽŶĞŚĂŶŶĞů^ĂŶĚƐƚŽŶĞ Figure 4. Simplifi ed sketches of coal-forming environments in marginal marine (coastal) and terrestrial (fl uvial) settings. (A) Marginal marine setting (regression). (B) Marginal marine setting (transgression). (C) Fluvial setting. Some data are publicly available from the USGS Sulfur Content coal quality (COALQUAL) database (Palmer and oth- The modern mining surge in Montana was the di- ers, 2015) and from the National Coal Quality Inven- rect result of a national concern with acid rain during tory (NaCQI) database (Hatch and others, 2006). the early 1970s. Burning coal releases sulfur in the

Petrography form of SO2 that can react with water and oxygen in the atmosphere to form sulfuric acid (H SO ), which At the microscopic level, coal is made up of or- 2 4 can then be incorporated into rain water. The need to ganic particles called macerals (e.g., liptinite, vitrinite, mitigate acidic rain led to demand for the low-sulfur inertinite), similar to the way an igneous rock is made (<1 percent sulfur) coal found in the Powder River up of minerals. The petrographic approach to the study Basin (PRB) of Montana and Wyoming. Nationally, of coal composition employs the idea that coal macer- sulfur content in coal ranges from about 0.5 to 5.0 als have distinct physical and chemical properties that percent (Chou, 2012). Sulfur content in Tertiary coal control the overall composition and behavior of coal of eastern Montana is typically less than 1.0 percent. (Stach and others, 1982). Few petrographic studies have been conducted on Montana coalbeds; one ex- In coal, sulfur is present primarily in mineral form such as pyrite (FeS ) and associated sulfi de minerals, ample is a study by Sholes and Daniel (1992) on the 2 Knobloch coalbed. secondarily as organic sulfur, and lastly in sulfate or 6 Gunderson and Wheaton: Coal Resources of Montana elemental forms. The sulfur originates from two pre- on distance from the drillhole or outcrop, and depths dominant sources: parent plant material and sulfate up to 2,000 ft. in seawater that fl oods coastal peat swamps (Chou, Strippable Deposits and Reserves 2012). Most freshwater streams and rivers are low in sulfate concentrations, and contribute little sulfur Beginning in the 1960s, attention shifted to iden- to coal. Generally, low-sulfur coal such as that in the tifying and quantifying coal reserves rather than total PRB and Williston Basin was deposited in freshwater, resources, with greater emphasis on strippable depos- fl uvial systems. its in eastern Montana (i.e., coalbeds in the Fort Union Formation). Averitt (1965) provided an estimate of Resource Assessments what he termed strippable resources of 5.1 BST for The quantity of coal in Montana has been estimat- several fi elds with thick coalbeds located near existing ed several times, and these estimates vary depending infrastructure in eastern Montana. During the next 5 on the data available and the economics of foresee- yr, several reports quickly increased strippable reserve able demands. Estimates are described as resources estimates to nearly 25 BST based on additional map- and reserves, which have diff erent, yet very specifi c ping and drillhole information (e.g., Ayler and others, meanings. Coal resources include tonnage estimates of 1969; Matson, 1969). identifi ed and hypothetical resources for coal zones of In a comprehensive report on eastern Montana a minimum thickness and within certain depth limits strippable coal, Matson and others (1973) provided de- (commonly 0–2,000 ft deep). Coal reserves, a subset tailed coalbed and overburden thickness maps, litho- of coal resources, are considered economically pro- logic data, and coal-quality data for 32 individual coal ducible at the time of classifi cation. A review of these deposits in the PRB. Their compilation of coal depos- and other terms related to coal tonnage estimates is its in the Montana portion of the PRB gave an indicat- provided by Wood and others (1983) and Pierce and ed strippable coal reserve (as they defi ned the term) of Dennen (2009). The reader should be aware that many 32 BST based on coal thickness, overburden thickness, authors (past and present) use criteria similar to Wood and a 2- to 3-mi radius around data points. Matson and others (1983), but with their own modifi cations. (1975) later included several more strippable deposits For coal tonnage estimates cited in this paper, we re- in the Williston Basin, Bull Mountain Basin, and on tain the original authors’ use of the terms resource and Indian lands in the PRB to get a total of 42.6 BST of reserve. strippable coal reserves (as he defi ned the term). Total identifi ed coal resources in Montana were In 1975, the USBM published a report summariz- estimated in 1975 to be 291.6 billion short tons (BST; ing the strippable and underground coal reserve base Averitt, 1975). This includes all bituminous coal more of the United States by state and county (Hamilton than 14 in thick and all subbituminous coal and lig- and others, 1975). This report estimated Montana’s nite beds more than 30 in thick to a depth of 3,000 ft. demonstrated reserve base to be 108.4 BST: 65.8 BST The EIA (2018) currently estimates the demonstrat- underground and 42.6 BST strippable. ed reserve base for Montana to be 118.6 BST, with 74.4 BST deemed to be recoverable (38.5 BST sur- Availability and Recoverability face-minable and 35.9 BST underground-minable). The proportion of coal resources that are recover- Early Estimates of Total Resources able from undisturbed deposits varies from less than 40 percent in some underground mines to more than Coal tonnage estimates were a critical component 90 percent at some surface mines. The USGS also of early 1900s mapping and formed the foundation for recognized that better resource and reserve estimates some of the fi rst statewide coal resource and reserve could be obtained by considering and excluding cer- estimates for Montana. Combo and others (1949) tain land-use and technological restrictions such as summarized coal in Montana by county, rank, reliabil- cemeteries, roads, other infrastructure, alluvial valleys, ity category, and thickness. Their estimate of 222 BST etc.—issues that could signifi cantly reduce the amount of coal included 2.4 BST of bituminous, 132.1 BST of coal considered to be recoverable (Fort Union Coal of subbituminous, and 87.5 BST of lignite coal. They Assessment Team, 1999; Luppens and others, 2009). specifi ed reliability categories of measured and indi- Thus, new coal tonnage estimates by the USGS and cated, inferred, and unclassifi ed as to thickness based state geological surveys were needed to place more 7 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics emphasis on “available” and “recoverable” coal. (1966) and Cole and others (1982). Matson (1975) In 1995, the USGS began the U.S. National Coal provides a summary of fi eld mapping completed on Resources Assessment (NCRA) project, in cooperation strippable coal deposits in eastern Montana. A partic- with state geological surveys, to conduct systematic, ularly useful publication by Pinchock (1975) gives geology-based, regional assessments of the Nation’s a location map and references for coal fi eld studies remaining recoverable coal resources for signifi cant completed prior to the mid-1970s. Rather than cite all coalbeds in major coal provinces and regions (Pierce of the early studies in this review, we refer the reader and Dennen, 2009). to Combo and others (1949) and Pinchock (1975). The Powder River Basin was the fi rst of four We present here a brief summary of Montana’s Northern Rocky Mountains and Great Plains coal coal regions and individual coal fi elds (fi g. 1), orga- regions to be assessed in the NCRA program. Coal nized by the three major periods of coal formation in availability studies were produced for signifi cant Montana: the Late Jurassic/Early Cretaceous, Late individual coalbeds (Fort Union Assessment Team, Cretaceous, and Tertiary. Table 2 provides basic char- 1999), fi rst over specifi c 7.5-minute quadrangles in the acteristics of each fi eld or region for comparison. All Montana PRB (e.g., Gruber, 1990; Wilde and Sandau, coal-quality information in table 2 and in the text that 2004), and then for the entire Montana portion of follows is reported on an “as-received” basis unless the PRB (Haacke and others, 2013). These local and stated otherwise. regional studies formed the underpinnings for a com- Late Jurassic/Early Cretaceous (Morrison Forma- plete reassessment of coal reserves in the Montana– tion) Wyoming PRB (Luppens and others, 2015). Of the Following the last of the Jurassic marine regres- estimated 1,162 BST of original coal resources in the sions (~155 Ma), fi ne-grained distal sediments of the PRB, only 25 BST—or about 2 percent—are deemed Morrison Formation were deposited on the emerging to be reserves. These results demonstrate the impact of surface of marine sediments during Late Jurassic and various technical constraints, land-use restrictions, and Early Cretaceous. The coal-bearing upper part of the mining economics on computing coal reserves. Morrison Formation, considered to be Early Creta- DESCRIPTION OF MONTANA COAL FIELDS ceous (Engelhardt, 1999; Vuke, 2000), includes mud- stones, siltstones, and fi ne-grained sandstones formed Many studies and reports that describe and inven- in a mixed fl uvial-lacustrine environment (Harris, tory coal in Montana have been generated. Territorial 1966, 1968; Walker, 1974). geologists, railroad geologists, and USGS workers Coalbeds deposited in the Early Cretaceous Mor- began mapping coal ahead of the westward expansion rison Formation occur in the Great Falls–Lewistown of the railroad during the late 1800s and early 1900s. fi eld and the Lombard fi eld. Although continental, Working solely from surface information, they de- nearshore, and lacustrine deposition continued in the scribed and mapped Montana’s coal-bearing regions Early Cretaceous, no other Lower Cretaceous coalbeds and specifi c coal fi elds or deposits. One example of have been recorded in Montana. original mapping is by Woolsey and others (1917), who describe the Bull Mountains Coal Field. Anoth- Great Falls–Lewistown Field er example is work published by Baker (1929), who Coalbeds in the Great Falls–Lewistown fi eld occur mapped the northward extension of the Sheridan Coal within the upper part of the 180- to 200-ft-thick Juras- Field in Wyoming into Montana. These early publica- sic–Cretaceous Morrison Formation along the north tions provide reconnaissance geologic maps and town- slopes of the Little Belt and Snowy Mountains in ship-by-township, or in some cases section-by-section, central Montana (fi g. 1). The primary coal zone varies descriptions of near-surface coal deposits in Montana. in thickness from 3 to 18 ft with coalbeds in 2 to 3 An excellent compilation of these early inventories benches averaging 4 to 5 ft thick each, separated by 1- is provided by Combo and others (1949). They includ- to 10-in-thick shale partings. ed descriptive fi eld summaries, a list of references for Coal in the Great Falls–Lewistown fi eld is bitu- the early 1900s publications, and estimates of total minous with heating values ranging from 8,700 to coal reserves (as they defi ned the term) for Montana. 12,900 Btu/lb and averaging 10,200 Btu/lb. Sulfur Other statewide summaries are given by Bateman content varies from 1.7 to 4 percent and ash content 8 Gunderson and Wheaton: Coal Resources of Montana 



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9 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics ranges from 8 to 30 percent. Generally, there is lower ingston–Trail Creek fi elds and the North-Central ash content and higher sulfur content in coal from the coal region. Coalbeds in the Judith River Formation Lewistown portion of the fi eld. Silverman and Harris (mixed terrestrial and marine deposits) occur in the (1967) estimated reserves (as they defi ned the term) to North-Central coal region. The coal in the Eagle and be 822 MST. Judith River Formations is almost all accessible only Workable coal thicknesses are concentrated in via underground mining. several separate “depositional basins” within the Great In northwestern Montana, continental deposits Falls and Lewistown fi elds. Silverman and Harris of the Two Medicine and overlying St. Mary River (1967) provide a stratigraphic, petrographic, and Formations contain coalbeds in the Blackfoot–Valier economic review of the individual coal basins. Some fi eld. The Two Medicine Formation is, at least in part, underground (room and pillar) mining at Sand Coulee correlative to the marginal marine and coastal deposits (Great Falls portion of the fi eld) in the early 1900s of the Eagle, Claggett, Judith River, and Bearpaw For- produced coking coal. The Anaconda Mining Com- mations. The St. Mary River Formation is equivalent pany operated coke ovens at Belt, but the ovens and to the Hell Creek Formation of eastern Montana. Both mines were abandoned in the early 1900s because only are primarily terrestrial in origin. a portion of the coal had coking properties and it was Bridger–Silvertip–Stillwater Fields too diffi cult to separate coking from non-coking coal (Averitt, 1966). These three fi elds are located along the margins of the northern Bighorn Basin (fi g. 1), where strata Lombard Field are tilted, exposing the coal-bearing Eagle Formation. The Lombard fi eld is a small (6 mi2) area of north- One or two minable coalbeds up to 6 ft thick occur in ern Gallatin and southern Broadwater Counties (fi g. each of these fi elds. A typical coal sample has heating 1). The coal-bearing Morrison Formation in this area values of just over 10,000 Btu/lb, sulfur content from has been folded and faulted. Coalbeds up to 6 ft thick 0.3 to 0.5 percent, and ash content of 13 to 19 percent. occur in lenses and have variable rank due to the de- Livingston–Trail Creek Field formation (Combo and others, 1949). Coal is bitumi- nous, with a typical heating value of about 10,000 Btu/ In the Livingston–Trail Creek fi eld (fi g. 1), coal is lb, but tectonism may have altered coal to essentially present in an abnormally thick section of sandstone in graphite in some places (Stebinger, 1914). Sulfur con- the Eagle Formation. One to four coalbeds are present tent is about 8 percent and ash content near 30 percent. at any one location, and they vary from 2 to 5 ft thick. The area is structurally complex, with faulting, fold- Late Cretaceous (Eagle, Judith River, Two Medi- ing, and steeply dipping coalbeds, most in excess of cine, Hell Creek, and St. Mary River Formations) 30°. Heating values range from 9,900 to 11,500 Btu/lb During the Late Cretaceous (~100–65 Ma) in and average 10,950 Btu/lb. Sulfur content averages 0.6 eastern Montana, several cycles of marine transgres- percent and ash content averages 8.5 percent. sions and regressions created conditions favorable for Some of the coal from this fi eld was mined and peat deposition in coastal wetlands that rimmed the used to produce coke during the late 1800s and early Western Interior seaway. Major delta systems existed 1900s, but it is no longer being commercially mined. along the shoreline, between which, lagoons, estuaries, The best coking coal occurs in the least deformed ar- and swamps were available for peat accumulation. eas along the fl anks of anticlines and synclines. Rob- Swamps of Eagle time were elongate and parallel erts (1966) estimated remaining coal reserves (as he to the shoreline, evidenced by the fact that coalbeds defi ned the term) of 301 MST in beds 14 in or more in interfi nger with clastic shoreline deposits seaward thickness and within 3,000 ft of the surface. Of these, (eastward) and interfi nger with terrestrial deposits 70 MST occur in the Cokedale bed, the coalbed with landward (westward). Late Cretaceous coalbeds are the best coking properties. present in the Eagle, Judith River, and Two Medicine Formations. Electric Field Coalbeds in the Eagle Formation lie within a Coal of the Electric fi eld (fi g. 1) has heating values north–south corridor of central Montana that includes averaging 11,400 Btu/lb. Beds are up to 5 ft thick, but the Bridger, Silvertip, Stillwater, Electric, and Liv- average about 3 ft thick. Sulfur content averages about 10 Gunderson and Wheaton: Coal Resources of Montana 1.3 percent and ash content averages about 19.5 percent. al swamps. The delta-plain sands and shales of the Coal mined from the No. 1 coalbed in this region Hell Creek Formation grade upward into fl uvial-dom- was used to produce coke in the late 1800s and early inated sediments of the Fort Union Formation that 1900s. Mining was diffi cult because of complex struc- include interbedded shale, siltstone, sandstone, and ture; beds are folded, faulted, and steeply dipping. abundant deposits of coal. The overlying continental Most of the coke was shipped to copper smelters in sediments of the Eocene Wasatch Formation contain Anaconda and Butte, but high ash content was prob- thick coalbeds in Wyoming, but are almost completely lematic and smelters abandoned using coking coal eroded from Montana other than a small area of south- from this fi eld (Averitt, 1966). ern Montana where a few minor coalbeds remain. The Fort Union Formation covers much of eastern North-Central Region Montana (fi g. 1) and is the most prolifi c coal-bearing Coalbeds in the North-Central region are present formation in the State. Thousands of feet of clastic in both the Eagle and Judith River Formations (fi g. sediments and associated thick coalbeds are preserved 1). Coalbeds tend to occur as thin, lenticular bodies in several structural depressions, namely the Bighorn, that generally thicken westward. Most beds are 2 to Powder River, Bull Mountain, and Williston Basins. 3 ft thick, although they can be up to 8 ft thick in the The Powder River Basin is particularly important be- Judith River Formation. Heating values average 9,000 cause it contains some of the thickest and most exten- Btu/lb and ash content ranges between 10 and 12 sive deposits of low-sulfur, subbituminous coal in the percent. world (Molnia and Pierce, 1992). Only one or two coalbeds occur in the Eagle For- Studies of the Fort Union Formation Coal mation—typically just above the massive sandstone known as the Virgelle Member. The Judith River For- Because of their economic importance, the Fort mation can have up to 8 or 10 coalbeds occurring in Union coalbeds have been studied in considerable two distinct zones, one each near the top and base of detail over the past 50 yr, with an overall goal to the formation. Individual beds are not laterally contin- inventory coal resources available for use in power uous, but the two coal zones are persistent in the area generation. Over time, these studies included detailed north and northwest of the Bearpaw Mountains (Gun- fi eld mapping, exploration drilling, subsurface coalbed derson, 2018). The Judith River Formation coalbeds correlation, sedimentary facies descriptions to under- extend northward into Saskatchewan, Canada, where stand the depositional processes that control coalbed equivalent Belly River Formation coalbeds may be distribution, and the use of computers and geograph- viable coalbed methane targets (Frank, 2006). ical information systems (GIS) to improve resource and reserve estimates. Many facies studies and dep- Blackfeet–Valier Field ositional models have been presented in attempts to Coalbeds of the Cretaceous Two Medicine and St. understand the depositional settings that infl uence Mary River Formations in the Blackfeet–Valier fi eld coalbed geometries and coal quality. Results of these (fi g. 1) are described as bituminous, but no detailed studies allow construction of sedimentologic models quality information is available. Beds are generally that provide a basis for the prediction of coal resources only 20 to 36 in thick. in areas where data are sparse. The models also help explain coal quality and thickness variations, and aid Late Cretaceous–Tertiary (Hell Creek, Fort Union, in exploration and development of the resource. Wasatch Formations) A joint USGS–MBMG–Bureau of Land Manage- Stratigraphic units of the Late Cretaceous (Fox ment (BLM) program to drill exploratory coal holes Hills and Hell Creek Formations) and Early Tertiary in eastern Montana was active from 1969 to 1982. (Fort Union and Wasatch Formations) form a re- Roughly 1,700 coal exploration holes were drilled and gressive sequence that records the fi nal retreat of the logged with lithologic sample descriptions and various Cretaceous seaway from the mid-continent region of geophysical logs. Annual drilling reports were joint- North America. The Fox Hill Formation is a shoreface ly published; a few examples are USGS and MBMG sandstone with no appreciable coal; the overlying Hell (1977, 1980) and Kirschbaum and others (1982). A Creek Formation is composed of fl uvial-deltaic depos- complete list of drilling reports can be found in Fine its with thin coalbeds, probably from short-lived coast- (1981) and Heald (1983). Coal stratigraphic data are 11 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics archived in USGS and MBMG databases, both avail- environments, correlating individual coalbeds and coal able to the public (USGS COALSTRAT; http://www. zones across the PRB has been diffi cult (Culbertson mbmg.mtech.edu/datacenter/datacenter.asp). Raw and Saperstone, 1987a,b; Fort Union Coal Assessment records and original fi eld notes from the drilling pro- Team, 1999; Flores and others, 2010). In some cases, gram are stored at the MBMG in Billings. the same name has been applied to diff erent coalbeds The program also produced a series of over 20 in diff erent coal fi elds. In others (e.g., CRO/CDP reports for Montana and other western states, referred mapping), a single coalbed has been assigned diff erent to as the Energy Mineral Rehabilitation Inventory names in adjacent quadrangles. and Analysis (EMRIA) reports. One example is BLM Coal correlations, basin stratigraphy, and various (1977). Archived copies of EMRIA reports are avail- names for individual coalbeds and coal zones have able at the Billings offi ce of the MBMG. been published for the PRB for the past 30 yr. A report Another USGS program begun in 1977 to compile by Kent and others (1980) describes the northern part information on unleased Federal coal resources in the of the Gillette coal fi eld in Wyoming and established PRB of Wyoming and Montana was the Coal Re- a coalbed nomenclature system that formed the foun- source Occurrence/Coal Development Potential (CRO/ dation for much of the PRB in Wyoming and Mon- CDP) program. This, and others, were initiated and tana. Molnia and Pierce (1992) also described coalbed maintained to update national data and information stratigraphy in the central PRB in Wyoming and Mon- with the most current coal resource knowledge. The tana; their nomenclature follows the usage of Culbert- USGS published coal stratigraphic data, coal outcrop son and others (1979), Law and others (1979), Flores maps, and coal resource estimates for 108 7.5-minute (1979), Derkey (1986), Culbertson and Saperstone quadrangles in Montana. Trent (1985, 1986) provides (1987a,b), Culbertson (1987), McLellan and Biewick a summary of the CRO/CDP program and a list of the (1988), and McLellan and others (1990). A west-to- open-fi le reports that were published as a result. The east cross section from Decker 80 mi eastward to Bear open-fi le reports are available online from the USGS Skull Mountain further connects the framework and Publications Warehouse (http://pubs.er.usgs.gov/). illustrates the basin structure and sedimentation (Mc- Digital coal outcrop patterns (shape fi les) for Montana Lellan, 1991). The stratigraphic framework extends PRB coalbeds from these reports are available from southward into the PRB of Wyoming. The resulting the MBMG (Gunderson, 2015). interlocking network reveals not only the extent of the coalbeds, but also furnishes information on the deposi- Many others of the MBMG, BLM, USBM, and tional history of the Tullock, Lebo, and Tongue River USGS were involved in mapping and reporting on Members of the Fort Union Formation in the Powder individual strippable coal deposits in the Fort Union River Basin. Recent work has refi ned these interpre- Formation of eastern Montana. In 1973, the MBMG tations, but made few structural changes (Flores and published a signifi cant work that included fi eld maps others, 2010). of minable areas for 32 strippable coal deposits, most- ly within the Powder River Basin in Montana (Matson In the Williston Basin region of eastern Montana, and others, 1973). Other studies are cited in the Re- geologic mapping, coal correlations, and stratigraph- source Assessments section of this report. Published ic studies were conducted in the Sidney, Glendive, estimates of strippable coal reserves in eastern Mon- Baker, and Wibaux 30´ x 60´ quadrangles of eastern tana based on work in the 1970s are presented in table Montana by the MBMG under cooperative agreements 3. Coal fi eld locations are shown in fi gure 5. with the USGS (Sholes, 1988; Sholes and others, 1989; Mathews and Wilde, 1989a,b,c,d). Additional Stratigraphic Framework and Correlation research (Belt and others, 1984; Flores, 1992; Hardie Coalbeds formed in fl uvial environments are lens and Arndt, 1990) has helped to clarify the stratigra- or disk shaped—thick in the middle, tapered in all phy, structure, and sedimentology in some areas, but directions. They generally form elongated bodies, correlation of specifi c bed names and zones across the roughly parallel to channel fl ow direction; they merge, region has not been completed. Coal nomenclature thin, and split over short distances when traced per- and coal correlations in the Williston basin continue to pendicular to depositional dip. Because there is an evolve. overall lack of continuity of coalbeds formed in fl uvial

12 Gunderson and Wheaton: Coal Resources of Montana

Plentywood DANIELS 23 24 22 Ü SHERIDAN

BLAINE Malta VALLEY ROOSEVELT 21 PHILLIPS 20 Glasgow Wolf Point

16

RICHLAND 17 Sidney 15 MCCONE 12 19 18 13 11

14 10

DAWSON 9 7 GARFIELD 26 PETROLEUM Glendive 6

8 5 25 PRAIRIE WIBAUX 4

MUSSELSHELL 1 27 Miles City 2 FALLON ROSEBUD 3 28 TREASURE CUSTER

YELLOWSTONE 50 59 44 56 42 57 Billings 58 Hardin 55 61 41 54 60 43 CARTER 62 BIG HORN 45 40 52 POWDER RIVER Legend 39 51 53 Subbituminous Coal 34 48 49 35 36 33 47 Lignite Coal 46 38 63 37 Strippable coal deposit 29 30 32 31 County boundary Montana PRB outline 030609012015 Active Coal Mine Miles

Figure 5. Map of strippable coal deposits in the Fort Union Formation of Montana (modifi ed from Cole and others, 1982). Numbers correspond to table 3.

13 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

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14 Gunderson and Wheaton: Coal Resources of Montana

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15 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics Depositional Models for the Fort Union Formation Coal Fields Multiple depositional models for Fort Union sed- Powder River Basin iments have been proposed over the decades of re- The PRB is an elongate, north–northwest-trending search that has occurred in the Tertiary coal fi elds of sedimentary basin that covers about 22,000 mi2 of eastern Montana. Ayers (1986) suggested the coarse northeastern Wyoming and southeastern Montana (fi g. sediments of the Tongue River Member represented 1). It forms a broad asymmetric syncline, bounded by deltas formed of material from the Black Hills, build- structural uplifts: the Bighorn Mountains to the west ing westward into Lake Lebo, thought to occupy the and the Black Hills Uplift to the east. The synclinal basin axis. Additional sediments came from the south- axis lies near the basin’s western margin and trends west and northwest. The coalbeds in this model were generally north–northeasterly in the Montana portion considered to be formed in low-lying swamps along of the basin. The basin shallows to the north against the lake margins between deltas. McLellan (1992) pro- the southern fl ank of the Miles City Arch. Thickness posed that uplift surrounding the Powder River Basin of the Fort Union Formation in the Montana portion of by the Cedar Creek Anticline, Black Hills, and Big- the PRB ranges from a few hundred feet in the north- horn Mountains, along with associated subsidence of ern PRB to approximately 4,000 ft at the Montana– the basin, created stable swamps stretching hundreds Wyoming border. of miles. The Fort Union Formation is divided into three The model of raised swamps was fi rst proposed by members: from oldest to youngest, they are the Tull- Jackson (1979, cited in Seeland, 1993). These raised ock Member, Lebo Member (informally referred to as swamps were postulated to be fed either by ground- the Lebo Shale), and Tongue River Member. Together, water or by a combination of groundwater and surface they form a thick sequence of interbedded and later- water. The topographic position of raised swamps ally discontinuous sandstones, pebble conglomerates, would naturally restrict clastic input and lead to a low- siltstones, shales, and coalbeds. Coalbeds are present ash coal, as is the case with most Fort Union coalbeds. in all three members of the Fort Union Formation and Flores (1981) considered the basin to be drained in the overlying Eocene Wasatch Formation, but the by a large, northward-fl owing river system with peat important coalbeds in the Montana PRB are confi ned accumulating in the raised swamps between the trib- to the Tongue River Member of the Fort Union Forma- utaries. The lenticular coalbeds of the Wyodak–An- tion (fi g. 6). As many as 26 persistent coalbeds occur derson coal zone (Smith, Anderson, Dietz 2, Dietz 3, within this unit and some attain thicknesses of up to 80 Canyon, Lower Canyon, Ferry, and Werner/Cook) are ft. The basin has been subdivided into many coal fi elds laterally split by, and pinch out into, strata that were where individual strippable deposits have been delin- deposited in the adjacent fl uvial channels (Ellis and eated and inventoried by Matson and others (1973; fi g. others, 1999). In outcrop, sandstone channel geome- 5, table 3). tries and paleo-current indicators refl ect the dominant Coalbeds within the Tongue River Member in the paleo-drainage to the east–northeast toward the Paleo- PRB are classifi ed as lignite to subbituminous based cene Cannonball Sea (Flores, 1992; McLellan, 1992; on heat of combustion values. There is a general de- Seeland, 1993). crease in coal rank from west to east, but the transition These models continued to be refi ned and were is gradual from subbituminous to lignite coal (fi g. 5). most recently presented in detail after the interpreta- Heating values for the subbituminous coal ranges from tion of thousands of coalbed-methane well logs (Flores 6,770 to 10,900 Btu/lb and average 8,140 Btu/lb. Sul- and others, 2010). Based on their model, Fort Union fur content ranges from 0.11 percent to 1.63 percent, deposition was controlled by tectonic factors (uplift and ash content from 3.8 percent to 10.6 percent. and subsidence) and internal factors including chan- There are four active strip mines in the PRB of nel geometry, migration, and abandonment. Channel Montana; they are located in Big Horn and Rosebud migration leads to coalbed geometries that split and Counties (fi g. 5). Mined beds range from 25 to 80 ft merge; the raised swamps explained the low ash, and thick. Beds currently being mined are the Anderson, the freshwater fl uvial system would create a low-sul- Dietz 1 and 2, Rosebud, and McKay (table 1, fi g. 6). fur coal. Although not currently mined, the Knobloch bed along Otter Creek southeast of Ashland, Montana has been 16 Gunderson and Wheaton: Coal Resources of Montana

CoalEed Wasatch Eocene Roland

Smith

Anderson (Garfield) Dietz (Dietz1, Dietz2)

Canyon (Monarch) Ferry Carney Cook Otter Wall Elk Pawnee (Poker Jim, E, Dunning) Brewster-Arnold Tertiary Odell Paleocene Cache (T) X Fort Union Formation Tongue River Member Tongue C - D King Sawyer A Knobloch Nance Rosebud McKay Flowers-Goodale Broadus Kendrick (Terret) Lebo Shale Member

Figure 6. Commonly used nomenclature for coalbeds in the Powder River Basin, Montana (modifi ed from Wheaton and Donato, 2004; Meredith and others, 2012). Not all coalbeds exist at any single location. This chart shows a hypothetical sequence based on studies in multiple coal fi elds.

17 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics the focus of recent mine permit application. ranking coal commercially mined in Montana is pro- In many parts of southeastern Montana, exposed duced from the Bull Mountain Basin. Coal in the re- coalbeds have burned from natural causes such as gion has heating values ranging from 9,270 to 11,000 lightning, wildfi res, and spontaneous combustion. Btu/lb and averaging 9,730 Btu/lb. Coal sulfur content Heat from burning coalbeds melts, bakes, fuses, and ranges from 0.4 to 1.3 percent, and ash content ranges otherwise alters adjacent sedimentary rocks, creating from 3.3 to 7.7 percent. various brightly colored yellow, orange, and red rocks Minable coalbeds in the Bull Mountain region known as clinker. Clinker covers approximately 1,000 include the Roundup (4 to 6 ft thick), McCleary (up mi2 of the surface area in the Montana portion of the to 9 ft thick), Rehder (5 to 12 ft thick), and Mammoth PRB (Heff ern and others, 1993). Clinker beds are a (5 to 15 ft thick) beds (fi g. 8). The Mammoth and direct indicator that coal is—or was—present, and Rehder beds have been mined periodically, by both clinker thickness is generally two to three times the underground and some surface strip mining. Connor original thickness of the coal that has burned (Matson (1988, 1989) published structure, outcrop, and isopach and others, 1973). maps of the Mammoth coalbed. Other beds that may Williston Basin have minable potential include the Dougherty and the Carpenter. The Carpenter bed has been mined in the The Paleocene Fort Union Formation in the Willis- northeast portion of the basin. ton Basin region (including the small area in western Garfi eld County) blankets much of eastern Montana Near the town of Roundup, and mainly south of and western North Dakota (fi g. 1). Although the coal- the Mussellshell River, the Roundup bed was mined beds are laterally discontinuous, major coalbeds ap- from commercial-scale underground mines from 1907 pear to be confi ned to specifi c stratigraphic horizons to the 1960s. The coal was primarily used as fuel for within the mapped Fort Union Formation members the railroads. A typical heating value for the Roundup and can be correlated as coal zones (Sholes, 1988; bed was about 11,000 Btu/lb. Based on volumes of the Sholes and others, 1989; Flores, 1992). Lower Fort mine voids estimated from mine maps, about 20 MST Union coalbeds are mostly lenticular, while upper Fort of coal was extracted during the life of these mines Union coalbeds tend to be thicker and more continu- (Wheaton, 1992). ous. Coalbeds generally thin and become less continu- Montana’s only active underground coal mine is ous to the north. located in the Bull Mountain Basin. Coal from the Lignite beds locally reach thicknesses of more than Mammoth bed was intermittently produced at this 40 ft, but are generally less than 15 ft thick. Heating mine during the 1990s, and the mine has been continu- values range from 5,800 to 7,525 Btu/lb, and average ously active since it was reopened in 2009. Key devel- approximately 6,670 Btu/lb. The average sulfur con- opmental milestones include installation of longwall tent varies from 0.2 to 1.1 percent, and ash content mining equipment and construction of a rail link con- varies from 3.5 to 10.7 percent. nection to regional railroad lines. Major coalbeds that have, at one time or another, Red Lodge fi eld been considered minable in the central to southern In the Red Lodge fi eld, the Fort Union Formation portion of the region are the Pust bed in Richland is up to 8,500 ft thick along the western edge of the County, the Harmon bed (C bed?) in Wibaux and Bighorn Basin and along the Beartooth Mountain Dawson Counties, the S bed in McCone County, and Front (fi g. 1). Stratigraphically from top down, the the Dominy bed in Fallon and Custer Counties (fi g. 7). Fort Union Formation is composed of 2,000 ft of The only lignite mine in the State produces coal from barren clastic sediments, an 800-ft interval containing the 15- to 25-ft-thick Pust bed near Savage, Montana. up to nine coalbeds, and a lower 5,000 ft of non-coal- Bull Mountain Basin bearing rock. The Bull Mountain Basin is an oval-shaped syn- Coalbeds are 3.5 to 11 ft thick and designated sim- clinal basin in southern Musselshell County and north- ply by numbers 1 through 9. Heating values are 10,000 ern Yellowstone County (fi g. 5). Here, the Fort Union to 12,000 Btu/lb and average 10,330 Btu/lb. Sulfur Formation is approximately 2,000 ft thick, with as content is 1 to 1.3 percent, and ash content varies from many as 26 persistent coalbeds. Some of the highest 5 to 13 percent. 18 Gunderson and Wheaton: Coal Resources of Montana

1 2 3 CoalEed CoalEed CoalEed

Prittegurl Prittegurl

Pust Pust Pust 1,2 (Harmon 1,2) Elvirio Hansen P

Sears Kolberg Ranch Q Sears Tertiary

Paleocene Budka R

Fort Union Formation Lane Peuse (Dominy?)

S Carroll Poverty Flats

Newton Ranch (T-Cross)

Contact

Local Cretaceous Hell Creek Formation

1 Nomenclature used in McCone County (Collier and Knechtel, 1939); correlations from Wicenentsen (1979). 2 Nomenclature used in northern Dawson and Richland Counties (Mathews and Wilde, 1989a) 3 Nomenclature used in southern Dawson and Wibaux Counties (Sholes, 1988; Sholes and others, 1989) Figure 7. Commonly used nomenclature for coalbeds in the Williston Basin, Montana. Not all coalbeds exist at any single location. This chart shows a hypothetical sequence based on studies in multiple coal fi elds. Coalbeds have not been correlated across fi elds except where indicated by a dotted line.

Between the late 1800s and mid-1960s, the Red with an average of about 6,700 Btu/lb, 20 percent ash Lodge fi eld (including the Bear Creek area to the east) content, and 1.0 percent sulfur content. supported at least eight separate underground mines. Tertiary coalbeds in southwestern Montana are up The Smith Mine at Bear Creek was the site of the to 7 ft thick and vary in rank from lignite to bitumi- worst coal mine disaster in Montana. Seventy-four nous, with heating values ranging from 6,000 to 9,700 miners died as a result of a gas and coal dust explosion Btu/lb and averaging 8,000 Btu/lb (Dyni and Schell, on February 27, 1943. 1982). Sulfur content ranges from 1 to 6 percent, and Tertiary (Other than Fort Union Formation) ash content ranges from 21 to 30 percent. Tertiary Lake Beds Flathead Field Several coalbeds are associated with Tertiary (Eo- In northwestern Montana, Tertiary coal in the Flat- cene–Miocene) lake beds that formed in the intermon- head fi eld (fi g. 1) ranks subbituminous with a typical tane basins of western Montana, primarily between heating value of 8,120 Btu/lb. Beds are only up to 3 ft Helena, Butte, and Missoula (fi g. 1). These coalbeds thick and sometimes occur in multiple benches. Sulfur are 5 to 25 ft thick and tend to occur in small lentic- content of a typical sample is about 3 percent with 15 ular bodies. Quality varies, but most beds are lignite percent ash. 19 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics OTHER COAL-RELATED TOPICS

Coalbed Methane Hydrocarbon gases are generated during all phases CoalEed of coal maturation through a variety of chemical and biological actions. Anaerobic microbes produce meth- ane from organic compounds through a multistep pro- cess of anaerobic respiration (methanogenesis). Under Summit Fattig certain chemical and physical settings, large volumes of these gases (mainly methane) can remain in coal- beds or migrate and become trapped in adjacent strata. Gases that remain in the coalbed are adsorbed on coal cleat (fractures) and micro-pore surfaces and held in place by weak Van der Waals bonds and hydrostatic pressure. Gas is released when the production of water Bull Mountain reduces hydrostatic pressure, allowing the adsorbed gases (primarily methane) to migrate to the well bores. In some cases, wells must produce water for years before gas production volumes surpass water volumes Rock Mesa and the wells become commercially viable. Produced Rehder split water is an environmental consideration and must be Mammoth managed at the surface (Meredith and others, 2012). The production life of typical coalbed-methane (CBM) wells is estimated to be about 10 to 12 yr, although production from multiple coalbeds may extend the life Dougherty of CBM wells. Coalbed-methane production in Montana began in 1999 and fi eld development proceeded at a much Tertiary Paleocene Fort Union Formation River Member Tongue slower pace than in neighboring Wyoming. Just over 1,100 CBM wells have been drilled in the southern part of the Montana PRB near the Montana–Wyoming Roundup state line, while over 30,000 CBM wells have been McCleary drilled in the Wyoming portion of the PRB. Carpenter Coalbeds in the Montana PRB are not as well suit- ed to CBM development as those in Wyoming because they are not as deep or as thick. In the Montana por- tion of the PRB, coalbeds become shallower and even- tually crop out to the north in response to structural and topographic control. While this may be advanta- geous for surface mining, it is not benefi cial for CBM development. Coalbeds at shallow depths or near outcrops are likely to have lower hydrostatic pressures

Lebo Shale Member (the pressures necessary to hold the adsorbed methane in place). Under reduced pressures, methane gas in Figure 8. Commonly used nomenclature for coalbeds in the Bull Mountain Basin, Montana (modifi ed from Woolsey and others, coalbeds can migrate toward the outcrop and eventu- 1917; Connor, 1989). ally escape to the atmosphere. Van Voast and Thale (2001) show, qualitatively, areas of low, moderate, and high CBM potential for the Knobloch and Ander- son–Dietz coalbeds based on coal thickness and depth, proximity to outcrop, and groundwater chemistry. 20 Gunderson and Wheaton: Coal Resources of Montana Coalbed-methane drilling and production activity and precipitated in carbon-rich rocks such as coalbeds, was short-lived in Montana. The number of producing carbonaceous shales, and carbonaceous sandstones. wells active at one time peaked at 670 in 2008. By Hail and Gill (1953) examined beds of coal and 2017, only 37 wells were producing methane in Mon- carbonaceous shale at 22 areas throughout western tana. One CBM well was drilled in 2018, the most Montana. Other than one 0.5-ft-thick bed of carbona- recent drilling at the time of this writing. The emer- ceous shale in Lewis and Clark County, none of the gence of gas produced from shales has displaced CBM samples collected and analyzed contained uranium gas production. concentrations greater than 0.01 percent. Groundwater Aquifers Rare Earth Elements As much as 70 percent of all water usage in the Trace elemental analyses indicate that some coal- PRB comes from groundwater aquifers (Van Voast and beds, lignites in particular, attract unusual elements Reiten, 1988). Coalbeds are more laterally continuous including rare earth elements (lanthanide series plus and generally have higher hydraulic conductivity than scandium and yttrium). In 2015, the U.S. Department channel sandstones in the same area. For these rea- of Energy created the Rare Earth Elements from Coal sons, fractured coalbeds are aquifers and furnish much and Coal By-Products Research and Development of the local livestock and domestic water supply in Program. Lin and others (2018) analyzed data from many areas of eastern Montana. the USGS COALQUAL database and found about 9 to The resources in coalbeds can involve as many 13 percent of the coal samples would be classifi ed as as three separate mineral estates (oil and gas, coal, “promising” for rare earth elements. North Dakota re- and water as part of the surface rights). Conflicts can cently tested lignite beds in western North Dakota and occur between mineral owners, so sound water man- found rare earth elemental concentrations higher than agement requires third-party science. The MBMG the 300 parts per million threshold for economic de- has, therefore, been publishing research results on velopment (Murphy, 2019). Elevated concentrations of coal hydrogeology in the PRB since the early 1970s. rare earth elements may also exist in eastern Montana Issues being considered include: the effect of mining lignite beds and a similar study should be conducted in coalbeds on water quality; the degree and extent of Montana. hydrostatic-pressure declines as coalbed aquifers are REFERENCES mined or depressurized as a result of coalbed-methane development; the effect of mine spoils as groundwa- Aff olter, R.H., and Hatch, J.R., 1980, Chemical anal- ter barriers; and management and disposal of coalbed yses of lignite from the Fort Union Formation, methane production water (Meredith and others, 2020; McCone, Richland, Dawson, and Wibaux Coun- Wheaton and Brown, 2005; Brinck and others, 2005). ties, Montana, and Golden Valley County, North Uraniferous Lignites Dakota: U.S. Geological Survey Open-File Re- port 80-179, 40 p. 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