(By C. E. Erdmann, U.S. Geological Survey, Great Falls, Mont.)
Mineral fuels consist of petroleum, natural gas, oil shale, and coal, and will be considered briefly in that order in this chapter. In recent years, mineral fuels, taken as a group, have accounted for over 40 percent of the value of all minerals produced in Montana. So that this may appear in proper perspective, the money value of the basic components of the raw-material economy of the State for 1961 are outlined below:
|Minerals||All other minerals||$104,843,000|
|Grand total value of raw materials||.||$650,340,000|
Note.--Information on agriculture has been supplied by the Bureau of Business and Economic Research, Montana State University, Missoula; mineral data is from "The Mineral Industry of Montana for 1960," U.S. Bureau of Mines; and information on lumber is from the U.S. Forest Service through the Montana State Planning Board, Helena.
All minerals, therefore, make about 28 percent of the total value of the raw materials, and mineral fuels alone about 12 percent.
Further analysis of the mineral contribution reveals that petroleum supplies about 40.8 percent of the total value of the mineral products; natural gas, 1.4 percent; and coal 0.7 percent for a total of about 42.8 percent of the State's mineral income. Restated in terms of energy consumption, excluding water power, during 1957 petroleum contributed 59.2 percent; natural gas, 35.7 percent; and coal, 5.1 percent (Independent Petroleum Association of America, 1959, p. 88).For more information about fuels in Montana, visit the State DEQ Energy Report.
(By Charles E. Erdmann, U.S. Geological Survey, Great Falls, Mont.)
The classical pattern of petroleum exploration in virgin territory is for the first tests to be made in the vicinity of natural surface indications of oil and gas, if any have been found. Unusual combinations of geologic conditions are required for the development of these surface features, and their occurrence is transient and infrequent. Their value, however, is that they provide tangible evidence of the local presence of hydrocarbons, thereby raising hopes that commercial accumulations may be found underground. The analogy with surface discovery of inorganic, minerals is obvious: both require bold and enterprising spirits if the usually costly and difficult adventure of development is to be undertaken. Because they are few in number, they are soon exploited, and this first stage of exploration is of short duration; but the often amateurish effort frequently clothes it with many colorful and dramatic incidents. If no significant discoveries result, and they seldom do, drilling on easily recognized geologic features such as anticlines and domes may follow with more or less delay. If drilling depths are shallow, as they are in some of the older fields in Montana, this second stage may mark the heyday of the small independent operator.
By the time the obvious surface structures have been recognized and evaluated, a more mature third stage has appeared in which search for subsurface structures and porous beds and stratigraphic traps is carried on by the sophisticated techniques of geophysical prospecting and study of formation samples or subsurface stratigraphy. Other later stages may involve deeper drilling, secondary recovery techniques and, finally, abandonment. Initially, each stage may appear in order. No firm line between them exists, however, for if the petroleum industry is to prosper, new discoveries must succeed abandonments. Review of the history of oil and gas development in Montana indicates close adherence to this pattern, which will be the outline for this chapter.
EXPLORATIONS ON SURFACE INDICATIONS, 1889-1910
(photo from MT Board of Oil & Gas Conservation)
heavy oil on a stagnant pool of water. In this instance the immediate practical application was for axle grease for the wagons. No drilling development followed, and even the report was not made for many years. Furthermore, no rediscovery seems to have been reported. The exact location, therefore, is not known, other than it was northwest of Beauvais Creek toward the East Fork of Pryor Creek Divide. A likely possibility, however, is that it was on some intermittent upper tributary of Woody Creek in T. 4 S., R. 28 E., Big Horn County, near where that drainage was crossed by the Bozeman Trail.
Roscoe seep.-The first oil seep to be drilled in Montana was the occurrence of heavy black oil or asphalt near the southeast corner NE1/4SE1/4 sec. 32, T. 6 S., R. 18 E., Carbon County, about 5.5 miles south of the old Roscoe post office. Date of discovery and name of the original locator are not known. In the late 1880's, however, the area was acquired by Thomas Cruse, who had found the famous Drumlummon lode near Marysville in 1876. The location of the first test, Thomas Cruse well No. 1, was about 650 feet northwest of the seep, and was completed and abandoned in 1889 as a dry hole in the Judith River formation at a, total depth of 1,100 feet. Insofar as known, this was the first organized attempt to discover oil by drilling in Montana. Undeterred by failure, Cruse continued operations in the vicinity of the seep during 1890 and drilled eight more dry holes that ranged in depth from 600 to 800 feet before giving up; and even then he retained ownership of the tract. Others continued to be intrigued by the possibilities, and additional tests were made in 1909, 1931, and even as late as 1947, but without success.
Kintla Lake area.--Impressive amounts of pale-yellow, high-gravity (44° API) oil issue from surficial deposits at the Sage Creek seeps in southeastern British Columbia, about 8 miles north of the international boundary at the northwest corner of Glacier National Park. The controlling structural feature appears to be a normal fault of great magnitude that can be projected into Montana where it is called the Roosevelt Fault. In 1892 active seepages of oil and gas were discovered in Montana near the northeast end of Lower Kintla Lake, not far east of where the lake crosses the trace of the fault. An organization called the Butte Oil Co. posted a location notice on August 10, 1900. Drilling began late in October 1901, the well location being NW1/4NW1/4NW1/4 sec. 18, T. 37 N., R. 20 W., Flathead County. Late in 1902 work was suspended temporarily, with the hole at a depth of 1,450 feet in very hard "black limestone and iron," probably one of the units of the Belt Series. A significant incident was the discovery of gas at a depth of 720 feet, which is said to have burned with a 4-foot flame. As will appear later however this was not the first discovery of gas in Montana by drilling.
In late June 1902 the Kintla Lake Oil Co. of Kalispell commenced operations at their No. 1 well, approximately in the center of NE1/4 sec. 12, T. 36 N., R. 22 W.; and in 1903 a second test is said to have been located toward the center of the section. Both are situated on Tertiary "lake beds" on the left bank of the North Fork of Flathead River. The No. 1 well was drilled to 1,290 feet at least, and the No. 2 to about 1,000 feet. Traces of oil and gas were reported from each, but both were abandoned as dry holes. The circumstances that led to these tests are not known, but they may have been drilled on seeps in the "lake beds" that emerged along faults cutting deep-seated Mesozoic formations.
Another old venture, for which there is good authority but no log or operational information, is the Southwest Kootenai Land Oil Co. test on Kintla Creek about 3 miles above Upper Kintla Lake. This locality is approximately in the north center of sec. 8, T. 37 N., R. 19 W., Flathead County, in a deep glaciated valley about 1.5 miles west of the Continental Divide. Drilling is reported to have commenced March 8, 1906, and continued to a depth of at least 600 feet. The bedrock formation at the surface is the Siyeh limestone of the Belt series, which is not known to contain indigenous hydrocarbons. In all probability therefore, the location was made on the basis of seeps or iridescent films of oil whose origin is more or less identical with those on Cameron Brook at Oil City, Alberta, a short distance northeast across the divide, where petroleum exploration had been going on since 1901.
Swiftcurrent Creek.--Sustained efforts to develop oil by drilling on or near obscure surface indications of petroleum and natural gas in Swiftcurrent Creek Valley throughout the 9-year period before the district became incorporated into Glacier National Park in 1910 resulted in seven tests that give it the nominal distinction of being the first oil and gas field in Montana and the only locality in the State where drilling on seepages proved successful. Credit for the recognition of these showings appears to be divided between two men: Frank M. Stevenson identified certain exposures of Upper Cretaceous marine shale as "oil shale" in the summer of 1901; and Samuel D. Somes prospecting near where Sherburne Dam is located, observed small pools of oil in irregularities on freshly broken shale and limestone on the floor of his adit in late February or early March 1902.
Within a short time, 52 oil claims were located under the placer mining law. Companies were organized and consolidated as claims were exchanged for shares, the ultimate operator being the Swift Current Oil, Land & Power Co. The first derrick was erected in November 1902, approximately at the center of SW1/4NE1/4 sec. 4, T. 36 N., R. 15 W., unsurveyed, on the Lakeside placer claim which had been located by Stevenson. Drilling began in 1903, when the hole was taken to a depth of 430 feet, with a showing of oil; but was abandoned because of inability to shut off water. The rig was then skidded 30 feet west, and work begun on location 1-A, which was completed as an oil well at a total depth of about 550 feet during the summer of 1905. Oil from this well was displayed at the State fair at Helena in the fall of 1905, where the company was awarded a diploma for "the first producing oil well in the State of Montana." Operations were terminated through lack of finances in 1907, and the properties turned over to Stevenson. In the meantime, however, one other oil well, with an initial capacity of about 20 barrels per day, by bailing, and 2 dry holes had been completed.
M. D. Cassidy, locator of a neighboring claim to the east, became aroused by this activity and organized the Cassidy-Swiftcurrent Oil Co., date of incorporation being July 15, 1905. Approximate location of the first test by this company, which may have been near a gas seep recognized by Cassidy, was in the extreme northeast corner of NW1/4NW1/4NE1/4 sec. 3, T. 35 N., R. 15 W., unsurveyed, near the center of St. Louis Placer No. 1. Drilling began in 1907, and continued at intervals into 1909 to a total depth of about 2,800 feet, where the tools were lost. Natural gas was reported from depths of 430, 1,900, and 2,800 feet, the initial shut-in pressure being about 250 p.s.i.
No measurement of volume seems to have been taken, but, upon being ignited, the gas flow from a I-inch pipe is said to have burned to a height between 15 and 20 feet. Cassidy piped the gas into his house, where it was used for heating and lighting until 1914 when the flow ceased, due to caving in the hole. The Cassidy-Swiftcurrent well No. 1, therefore, has the distinction of being the first producing gas well in Montana, even though it had only one customer.
Boulder Creek. --Following Somes' discovery of oil in his adit on Swiftcurrent Creek in 1902, other prospects in the dark Upper Cretaceous shale (Marias River formation) were examined for traces of oil. Favorable indications were reported in an abandoned working on Boulder Creek, probably somewhere near the center NW1/4 sec. 27, T. 35 N., R. 15 W., unsurveyed, Glacier (formerly Teton) County. Recognition of this seep may have contributed to the organization of the Swift Current-Boulder Oil Co., which soon acquired substantial acreage south of Swiftcurrent Creek. Drilling commenced in July 1904, the approximate site being south of the center SE1/4SE1/4, sec. 11, T. 35 N., R. 15 W., on the left bank of Boulder Creek. A show of gas was reported in shale at a depth of about 1,750 feet, and a show of oil "of a superior quality" was found in the top of a sandstone at a depth of 2,010 feet on July 8, 1905. Operations were abandoned at this depth in the spring of 1906.
No seepage has been reported from this locality, and the reasoning that led to its selection is unknown. It may be surmised, however, that observation of oil seeping from freshly broken shale in various prospects, which had been driven in search of copper, originated the conjecture that any shale section might yield oil in commercial amounts, particularly in the subsurface. This, of course, is not generally true. The shale seeps are restricted to certain comparatively thin units of bituminous rock (as in the Cone calcareous member of the Marias River shale) that have been subjected to severe dynamic stress through being overridden by the plate of the Lewis overthrust fault, or related diastrophism in the Disturbed Belt. The associated pressure and frictional heat resulted in local destructive distillation to produce small quantities of liquid petroleum. Nevertheless, faith in the idea seems to have been responsible for several other test wells along the mountain front, whose locations are otherwise difficult to account for. Among them are those at Lubec, Midvale, Two Medicine Valley, St. Mary Valley, and Belly River Valley, all of which were dry holes.
This persistent run of failure naturally resulted in loss of interest, and by the close of 1907 such random drilling had come to an end. The Congress passed the act establishing Glacier National Park on May 11, 1910, thereby precluding new ventures. In the meantime, the Lakeside and New Era placer claims in the Swiftcurrent District had been patented, and the patent for the St. Louis No. 1 was pending but held in abeyance as Sherburne Lake project of the Bureau of Reclamation approached realization. More or less ineffectual efforts to recondition the two small oil wells and the gas well persisted for several years, but terminated in the summer of 1919 when the locations were flooded by water rising behind the Sherburne Lake Dam.
No other drilling on surface indications has been recorded in Montana.
EXPLORATION OF SURFACE STRUCTURES, 1890-1950
Many anticlines and domes are expressed in rocks at the surface on the Montana Plains. The precise number is unknown, but more than 475 areas, fields, and structures have been named. Approximately 185 fields and structures have been named on the latest edition of the "Structure Contour Map of the Montana Plains" (U.S. Geological Survey, 1955); but only about 100 areas have been proved to contain oil or gas in commercial quantities, and not all are anticlines. The producing fields are shown on figure 7, and their names are keyed by number to the list in table 1. NOTE: Links to table 1 will take you to an html version of the table. You may also wish to download an Excel spreadsheet of this table, available here. This chart, it should be noted, is not a complete list of Montana oilfields, nor does it include the major gas fields. Some repetition of names results from listing more than one pool or producing formation on the same feature, as at Cat Creek. Actually, about 20 fields produce from more than 1 formation. On the other hand, Cedar Creek anticline is not named, although 11 of the 20 pools or fields on that anticline are listed. If fields that produce both gas and oil from different formations were listed the number would be increased by 15, or from 89 to 104.
|(photo from MT Board of Oil & Gas Conservation)|
Not too many years had elapsed since the pronouncement of the anticlinal theory of oil and gas accumulation and, in the absence of an oil seep, a sharp or closely folded anticline seemed the next best feature on which to drill. Recognition of a completely exposed fold in rock requires so little imagination or interpretative skill that when they were observed they were reported, often by sheepherders, and the more evident small folds were described as "sheepherder structures." Insofar us known, the first test in Montana to be located on an anticline, presumably in accordance with the anticlinal or structural theory, was the R. O. Morse well No. 1, NE1/4 sec. 4, T. 6 S., R. 18 E., Carbon County, on the northeast flank of Roscoe dome, a "sheepherder structure" toward the west end of the Nye-Bowler lineament. It may be, however, that Morse had been attracted to the area by Cruse's exploration around the Roscoe seep a few miles south, which was then in progress. Drilling equipment in 1890 was very inadequate for a complete test of the structure and the operation was abandoned as a dry hole in the upper part of the Colorado Group at a total depth of 1,100 feet. Subsequent drilling has proved the structure to be dry into the upper part of the Cambrian series at a total depth of 5,928 feet. Chance plays an important part in exploration for oil and gas, and the first discovery of natural gas by drilling on the Montana Plains came unexpectedly in 1893 a few miles southwest of Havre at Fort Assiniboine in a water well in the upper sandstone unit of the Eagle sandstone. Although not of commercial volume, the find directed attention to the possibility of gas development out on the plains, but this did not follow for nearly 20 years.
The principal technique for the recognition and discovery of geologic structure at the surface is systematic areal and structural mapping. More than 20 oil and gas fields have been found directly by this method, which is still very useful even though most of the conspicuous surface, structures have been found. Many of the features that localized these pools were found and first described by the U.S. Geological Survey as its program of mineral classification developed following the withdrawal of the public lands for that purpose in 1006, notably the Cedar Creek anticline, Poplar dome, and the Sweet Grass arch.
The structures found by surface methods on which oil and gas have been discovered are listed here in order of that discovery, the year in parentheses following the name of the field; usually, however, the presence of structure had been known for some time: Gas City dome, Cedar Creek anticline (1913); Havre gas field (1914), abandoned; Elk Basin (1915); Boxelder gas field (1916); Bowdoin gas held (1917); Devils Basin (1919); Cat Creek (1920); Soap Creek (1921); Kevin-Sunburst (1922); Sherard (Birch Creek) gas field, 1923, shut in; Bears Den (1924);Lake Basin (1924);Bowes gas field (1926); Bannatyne (1927); Flat Coulee (1928); Frannie (1928); Dry Creek (1929) Mosser (1937); Gage (1943); Kicking Horse gas field (1943), abandoned; Plevna (1946); Ragged Point (1948); Golden dome (1953); Ivanhoe (1953).
Several of these discoveries proved the presence of commercial volumes of oil and gas, hitherto something that had been lacking, over wide areas on the plains; and the impact of others, especially in rocks of Paleozoic age, did much to sustain the hope when interest was low that many more were to come.
The first commercial show of natural gas in eastern Montana was found in Gas City dome at the north end of the Cedar Creek anticline in 1913. Drilling was initiated by the Mid-West Oil Co., in November 1912, at their No. 1 well, W 1/2 NE 1/4 NE 1/4 sec. 20, T. 14 N., R. 55 E., Dawson County; but with change of ownership the hole was completed by the Eastern Montana Oil & Gas Co., which developed the field. Drilling continued to a total depth of 2,710 feet, which was reached in April 1914. In the meantime a flow of 500,000 cubic feet of gas per day with a shut-in pressure of 220 pounds per square inch, and some water had been found between depths of 730 and 745 feet in a sand assigned arbitrarily to the Judith River formation of late Cretaceous age. Beginning in 1915 gas for domestic use was supplied to the city of Glendive 10 miles north on Yellowstone River. Peak of production was reached in the fall of 1917, when the combined flow of eight wells amounted to about 10,600,000 cubic feet of gas per month. The field was abandoned in 1925; but other gas production followed along the anticline to the south.
The year 1915 also was notable for the discovery of oil in the Elk Basin anticline in Carbon County, Mont., and Park County, Wyo., a structure which had first been noticed some 10 years previously by the U.S. Geological Survey. The discovery well, which produced from the Torchlight sand in the Frontier formation of Late Cretaceous age at depths of 1,335 to 1,402 feet, was in Wyoming; and about 87 percent of the productive acreage fell in that State. The remaining northern portion of about 120 acres became Montana's first producing oilfield. The beginning, therefore, was rather small. Four oil wells were drilled in 1915, but were not brought into production until shipping facilities became available the following summer. Two more oil wells and one dry hole were drilled in 1916, and production for the last 6 months of the year totaled 44,917 barrels, with a value of $44,019.
Excellent examples of the possible rewards of deeper drilling are furnished by the development of the Elk Basin field. Natural gas was found in the Cloverly formation of Early Cretaceous age, about 1,150 feet below the Torchlight, in 1922, but is now largely exhausted. The major discovery, however, did not come until December 1943 when oil was found in the Tensleep sandstone of Pennsylvanian age at a depth of about 4,500 feet. This reservoir proved to have about 215 feet of saturation, the thickest producing section of any field in the State. About 1,375 acres, or 27.5 percent, of this Tensleep pool are in Montana. Finally, in 1946 oil was found in the underlying Madison limestone of Mississippian age.
The immediate effect of the original Elk Basin discovery was to direct attention to the Montana extension of the Bighorn Basin and the country to the north where interest still centered on sharp-dip structures; but prospecting from 1916 until late in 1910 resulted only in dry holes. One of the more prophetic of these efforts was the first test on the large, Woman's Pocket anticline, which was spudded May 6, 1916, by the Foster Oil Co., in C SW1/4NE1/4 sec. 15, T. 8 N., R. 20 E., Golden Valley County, and completed in June 1918 by the Tri City Oil Co., at a total depth of 2,215 feet. The trace of oil from 1,550 to 1,565 feet, and two other minor shows at greater depth, were the first evidence in Montana of the occurrence of petroleum in the Kootenai formation of Early Cretaceous age, and provided the incentive for further exploration of that unit. Although still far short of commercial production, more tangible encouragement soon came from the Devil's Basin anticline to the northeast where the Van Duzen Oil Co. well No. 1 spudded in the Kootenai formation in NE1/4SW1/4NW1/4 sec. 24, T. 11 N., R. 24 E., Musselshell County, on August 10, 1919. Drilling continued to a total depth of 2,031 feet on November 6, 1919. In the meantime, 10 or 12 barrels of oil had been found in a 6-foot limestone at a depth of 1,167 feet in what was then called the Quadrant formation. Later stratigraphic work has shown the producing horizon to be in the Heath Formation of the Big Snowy Group, which is late Mississippian in age. The trend of exploration continued toward the northeast, and the next structure to be drilled was Mosby dome on the elongate Cat Creek anticline. Here the Franz Oil Corp. well No. 1 (now Continental Oil Co., Charles 1-A) was started December 18, 1919, and was completed at a total depth of 1,014 feet as a 30-barrel oil well on February 20, 1920. Production was obtained from the second Cat Creek sand of the Kootenai formation between the depths of 998 to 1,014 feet. This famous discovery well, which in itself never produced more than 700 barrels of oil, resulted in the development of the Cat Creek field, the first important field in the State and, for its size, still one of the most productive and most profitable that has been found. Exploitation was rapid. A peak production of 2,080,826 barrels per year was reached at the close of 1923; and cumulative production to the close of 1961 has amounted to nearly 20 million barrels of oil.
The discovery of the Cat Creek field definitely carried the struggling Montana petroleum industry beyond the nascent stage; but national significance was not achieved until the oil discovery on the Kevin-Sunburst dome in Toole County 2 years later. Actually, there were two oil discoveries; and the short interval between them compounded the excitement they raised. Oil was found first on April 14, 1922, when the Gordon Campbell, Kevin Syndicate-A. Goeddertz well No. 1, NE1/4NE1/4NE1/4 sec. 16, T. 35 N., R. 3 W., was completed as a 20-barrel producer between depths of 1,770 to 1,790 feet in the basal sandstone unit of the Sawtooth formation of Middle Jurassic age and the eroded, weathered upper surface of the Mission Canyon formation of the Madison group of Mississippian age, which are separated by an unconformity. In the second, oil was found June 5, 1922, when the Ohio-Sunburst Oil Cos.'-R. Davey well No. 1, SE1/4SE1/4SW1/4 sec. 24, T. 36 N., R. 2 W., was completed as a 150-barrel producer between depths of 1,535 to 1,564 feet in a sand at the base of the Kootenai formation, which later was named the Sunburst sand. These discoveries, together with the great size of the dome, first production from rocks of Paleozoic age in the Rocky Mountain region, shallow drilling, and demonstrated production from a low-dip structure, attracted immediate attention from major oil companies and numerous small operators. The result was remarkable and, by the close of 1922, out of a total of 42 completed tests the field could show 22 producing oil wells, 4 wells producing both oil and gas, 3 Sunburst sand gas wells, 3 dry holes with shows of oil and gas, and 8 dry holes, with 11 tests drilling. Peak oil production of 6,457,217 barrels of oil was reached rapidly in 1926, since when it has been declining; and cumulative production to the close of 1961 has amounted to 66,189,439 barrels, which is not far from its estimated ultimate production of 70 million barrels of oil. Peak production of natural gas of 4,950 million cubic feet was reached in 1928, with cumulative production of about 78 billion cubic feet through 1961. With the development of the Kevin-Sunburst field the petroleum industry became firmly established.
Partial indications of structure at perhaps as many more fields or pools as were found by areal or structural geology also were found by surface mapping but development did not follow until the geologic structure of the concealed part or the deeper structure had been worked out by geophysical methods; usually detailed seismograph surveys. In some districts, for example, the Poplar or Cedar Creek anticline, the interval between recognition of surface structure and the seismic surveys that led to deep drilling has been as long as 30 to 45 years. Much of this delay occurred before the development of prospecting with the reflection seismograph, and these poorly exposed structures were natural testing grounds. Among the fields that have been found by this combination of methods are the following, and the list may not be complete: West Utopia (1943); North Reagan (1947); Big Wall (1948); Bowes, oil (1949); Glendive, Cedar Creek anticline (1951); Ash Creek (1952); Little Beaver, Cedar Creek anticline (1952, 1954); Marcus Snyder (1952); Poplar, East (1952); Cabin Creek, oil, Cedar Creek anticline (1953, 1956); Ash Creek, South (1954); Big Coulee (1954); Clarks Fork (1954); Little Beaver, East, Cedar Creek anticline (1954); Gas City, oil, Cedar Creek anticline (1955); Pennel, Cedar Creek anticline (1955); Pine, Cedar Creek anticline (1955); Clarks Fork, North (1956); Belfry (1958); Monarch, Cedar Creek anticline (1958). Beginning with the discovery in 1951 of oil in rocks of Ordovician age in the Glendive field at the north end of Cedar Creek anticline, the impact of this combination method on rate of discovery and oil production in eastern Montana (Williston Basin) was literally explosive, about 20 deep pools being found on Cedar Creek anticline alone. According to the records of the Montana Oil and Gas Conservation Commission, oil production in eastern Montana jumped from nothing to 56.4 percent of the total for the State for 1961, or by more than 17 million barrels (see figure 8).
SUBSURFACE EXPLORATION, 1940-60
Subsurface stratigraphy.- By the close of 1961 approximately 12,840 wells had been drilled for oil and gas in Montana. Of this number about 10,500 are field wells; for example, at that time the total number of wells in the Kevin-Sunburst field was 4,019. The remaining 2,342 tests were "wildcats," or wells drilled in search of oil and gas in unproved areas. Most of the wildcat wells were dry, those that made discoveries being included with the field wells drilled around them. All of these tests, especially the widely scattered wildcat tests, have contributed more or less information to subsurface stratigraphy such as what formations are present, the rate and direction in which they thicken or thin, lithologic changes, presence of unconformities, and data on porosity and permeability. These are the basic data for petroleum exploration. Subsurface stratigraphy seldom has been used alone in Montana, but the tendency to do so is increasing. At least seven discoveries have been credited to it during the past 10 years, and as the data increase in amount and quality so will its capability for discovery. Fields whose discovery has resulted primarily from applications of subsurface stratigraphy include the following (with year of discovery): Reagan (1941); Cut Bank, north (1945); Pondera Coulee (1951); Grandview (1952); Brady, west (1958); Stensvad (1958); Cut Bank, southwest (1959); Pondera Coulee, west (1959); Keg Coulee (1960). Discovery of the Pumpkin Creek field in 1954 is credited to both surface and subsurface mapping.
Geophysical methods.--Fields whose discovery is credited to detailed seismograph surveys, with little or no assistance from other methods, are as follows: Reagan, north (1942); Sumatra (1949); Richey (1951); Deer Creek, Cedar Creek anticline (1952); Richey, southwest (1952); Wolf Creek (1952); Brorson (1954); Fertile Prairie, east flank Cedar Creek anticline (1954); Bredette (1955); Cupton, east flank Cedar Creek anticline (1955); Wolf Springs (1955); Bredette, north (1956); Outlook (1956); Delphia (1957); Line Coulee (1957); Outlook, south (1957); Blackleaf (1958); Redstone (1958); Dwyer (1959); Tule Creek (1960). The Wolf Springs field (1955) was found by a combination of gravity and seismic methods. The increasing use of the seismic method in the last decade will be noted, and this trend is expected to continue.
Subsurface and seismic.--Fields whose discovery is credited to a combination of subsurface and seismic methods are few: Blackfoot (1955); Red Creek (1958); Gold Butte (1959); Middle Butte (1950); Sand Creek, north end Cedar Creek anticline (1959). However, their number is expected to increase as subsurface data from northeast Montana become more abundant.
Random drilling is not a disciplined or technical method of oil finding. Usually the values involved are subjective, and may include simple faith, curiosity, an intuitive impression, or a hunch that a certain tract will be underlain by oil; that the trend of discovery will be in a certain direction; a layman's misinterpretation of geologic data; or flagrant wildcat promotion. On the other hand, it may be eminently practical, such as an opportunity to test a large block of acreage for the cost of a single well. Whatever the reasons, and no matter how great the odds may be, random drilling sometimes results in discoveries that would not have been made by conventional thinking. Where drilling depths are shallow and costs relatively low this type of exploration will continue but its use is becoming more and more infrequent. Among the discoveries that have been made by random drilling are the following fields: Hardin (1913); Whitlash (1918); Berthelote (1926); Cut Bank, gas field (1926); Devon (1926); Pondera (1927); Border-Red Coulee (1929); Brady (1943); Gypsy Basin (1958).
More systematic categories of drilling exploration include successively deeper drilling, such as that which led to the Tensleep discovery (1942) and the Madison discovery (1946) at Elk Basin. Now however, unless oil is found at shallow depth, the major operators tend to make complete tests of the available stratigraphic section with the first exploratory well. As a method, therefore, deeper drilling is largely restricted to the older fields whose original discoveries were shallow. Stepout drilling, or simply following the trend of oil occurrence, is credited with the discovery of the Darling pool in the north part of the Cut Bank district in 1939, and the Melstone field in the central part of the State in 1948. Lastly, fields or pools have been discovered by revaluation and testing of apparently noncommercial tests, as happened in the very prolific Lander pool (1935), in the Cut Bank field, and in the Redstone field in the northeast part of the State in 1958.
(By Charles E. Erdmann, U.S. Geological Survey, Great Falls, Mont.)(See also online information from the Montana Board of Oil & Gas Conservation.)
The petroleum resources of Montana occur east of the Continental Divide beneath the Great Plains. This region, outlined on figure 7, is essentially the area of the "Structure Contour Map of the Montana Plains" of the U.S. Geological Survey (1955). That map covers 112,060 square miles, or about 76 percent of the State, yet does not include some 9,900 square miles of mountainous country also east of the Continental Divide southwest of Bozeman and south of Butte. The area of Montana, east of the Continental Divide, therefore, is about 121,960 square miles, or nearly 83 percent of the total area of the State.
Not all of this vast country is suitable for the occurrence of hydrocarbons. Approximately 3,100 square miles are underlain by pre-Belt crystalline and metamorphic rocks, including 60 square miles of the Stillwater igneous complex, which, along with younger igneous rocks, have never contained any oil or natural hydrocarbon gases. Another 3,300 square miles are occupied by the sedimentary rocks of the Belt series of Precambrian age, which include about 6 square miles of Purcell lava sills. Thin films of carbon in haphazard arrangement on the bedding surfaces of some of those ancient sediments have been interpreted quite properly as evidence of Precambrian life; but no inference that these rocks once contained oil and gas has ever been made, and the Belt terrane is generally regarded as nonproductive. Where the Belt rocks are involved in the Lewis overthrust fault on the east side of Glacier National Park they are underlain by Upper Cretaceous strata that have yielded shows of oil and gas, as has been pointed out, but since this locality is now proscribed from prospecting by park regulations it is included with the Belt series as nonproductive territory. East of the Continental Divide, therefore, the total area of the Precambrian outcrop is about 6,400 square miles. Extensive areas of Cretaceous and Tertiary igneous rocks also are present. The Cretaceous igneous terrane totals about 1,560 square miles, and can be separated roughly into 1,180 square miles of extrusive rocks, including the agglomerate member of the Livingston formation, 176 square miles of intermediate (andesitic) intrusives, and about 83 square miles of diorite and gabbro. Tertiary igneous rocks are even more abundant, totaling about 4,120 square miles. Extrusive volcanics cover 2,310 square miles, considerable areas being in the Bearpaw and Highwood Mountains out on the Plains; the Boulder batholith and broadly related stocks appear over 1,300 square miles, and the aggregate area of the smaller intrusives is about 500 square miles. It is also necessary to exclude about 1,080 square miles of Cambrian sedimentary rocks and about 530 square miles of Devonian rocks on the grounds that where these older Paleozoic strata are at the surface any hydrocarbons they may once have contained have been dissipated, and all the younger overlying beds that may once have contained oil or gas have been removed by erosion. The total area cast of the Continental Divide in which prospects for oil or gas are negative is thus about 13,930 square miles.
Simple subtraction, therefore, gives about 108,000 square miles east of the Continental Divide as the region in which the stratigraphic column is generally favorable and of sufficient thickness to offer some prospect for the discovery of oil and gas. Two small districts west of the divide, 180 square miles in the valley of the North Fork of the Flathead River, and 140 square miles southwest of Marias Pass also fall in this category. By no means should they be ignored indefinitely; but they are omitted now because together they are equivalent to only about 0.3 percent of the prospective area east of the divide, which is the country that contains Montana's petroleum resources. Very little of it, however, is actually producing or has produced oil or gas, and the crux of the petroleum resource problem is the whereabouts of whatever undiscovered pools there may be. Addition of the productive areas given in table 1 gives the total area producing oil on January 1, 1962, as about 228,650 acres, or 357 square miles, which is only 0.33 percent of the favorable part of the Plains region. Estimates of the aggregate area producing natural gas come only to about 456,990 acres (710 square miles), or 0.68 percent of the Plains region. The total acreage producing oil and gas thus has a productivity index, if it may be called that, of about 1 percent.
For purposes of this summary it will be convenient to consider the petroleum resources of Montana briefly from the standpoint of past discoveries, which may be subdivided further into cumulative production--which is now history, and proved reserves--or present reality, and the ultimate potential reserve or the amount of oil and gas that remains to be found and produced in the future. Many of the basic data for an appraisal of past discoveries are given in table 1. The data particularly show the very small number of major fields, the large number of small fields, the large number of fields that produce from only one horizon, and the comparative thinness of the pay zones. This is a record of past experience, however, and should not be assumed to indicate future performance, even though it may be suggestive.
Petroleum occurs in Montana in 21 geologic formations which range stratigraphically from the Red River formation of Late Ordovician age upward to the Shannon sandstone member of the Cody shale of Late Cretaceous age. The extensive range of this occurrence is brought out in the generalized stratigraphic correlation chart (fig. 6) in which the producing zones are indicated by standard oil well symbols (small black circles) together with the name of the field or fields at that horizon. The same information also is included in table 1, but the stratigraphic position and frequency of occurrence are not evident in that tabulation. (Note that the stratigraphic columns of figure 6 may display in Internet Explorer in small, low-resolution mode. To see the full image, move the cursor over the image and click the expander button that appears in the lower right.)
Cumulative oil production dating from the first commercial wells at Elk Basin in 1916 to the close of December 31, 1960, was about 364,400,000 barrels and had an aggregate value of about $734,200,000 (Montana Oil and Gas Conservation Commission, "Statement of Crude Oil Production 1960"). In 1960 Montana ranked 12th in petroleum production by States, producing about 30,240,000 barrels of oil, or 1.18 percent of the U.S. total, and, at the close of that year, the State ranked 13th in proved reserves with about 267,690,000 barrels, or approximately 0.84 percent of the national total (American Petroleum Institute, 1961, p. 35). Cumulative oil production for Montana through 1996 was estimated (Oil & Gas Journal) at 1,419,958,000 barrels, ranking Montana 13th among the States. Total Montana production in the late 1990s ranged from 15,200,000 to 16,500,000 barrels per year, according to the Energy Information Administration. The reserve estimate for 1961 was somewhat greater, 334 million barrels (table 1), indicating extension of known fields and new field discoveries during that year. At the 1960 rate of production, which is shown graphically in figure 8, subdivided into the provinces from which it came, reserves of this magnitude would be 10 or 11 years' supply, which has been the approximate status of the national reserve for some years; and it will also be augmented substantially as time goes on by oil from secondary recovery projects of which 15 or more are now in operation. The proved petroleum reserve in Montana, therefore, probably will continue to remain at about this level for a number of years, and there need be no concern about the immediate future.
It is evident, however, that production from the older northern fields, which for more than 20 years supported the petroleum industry in the State, is falling off rapidly and unless a new major discovery is made, at the rate of decline indicated in figure 8, in 20 years the province virtually may cease to produce. This, of course, is the ultimate fate of all extractive industries, and the reason it is so necessary that new discoveries continue to be made. Rut, in view of the thorough exploration, it is questionable if sufficient area remains on the Sweetgrass arch in which another extensive field could occur. In the meantime the loss has been more than made up by the marked increase in production from central and south-central Montana, where the fields are numerous but small. On the other hand, the probable life expectancy of the great new fields in the eastern part of the State may be as long as 20 to 30 years, and during that period many extensions and new discoveries doubtless will be made.
Estimation of the ultimate potential petroleum reserve is a very difficult matter, and no completely satisfactory method has ever been devised. According to Weeks (1958, p. 433), "The only determinants that have any general application, and which can be classified as basic today are (1) geology, and (2) experience on the broadest possible scale in what geology means in terms of oil occurrence." Some of these factors are involved in the productivity index referred to above, which is an approximate measure of probability of occurrence, and will now be considered at greater length. The real or ultimate value of this index is uncertain and can only be determined by the results of future exploration. Obviously, it does not suggest that 1 percent of any specific tract of land will yield both oil and gas; the larger the area to which it is applied and the more favorable structures that area contains, the more accurate it is likely to be. Nor does it mean that gas is more apt to be found than oil, much of the natural gas in Montana is not associated with petroleum; and it offers no clue to where a discovery will be made. Even the value of the index is subject to change and should improve (increase) with future discovery, an increment of 0.18 for oil having occurred since 1951 in consequence of the discoveries in eastern Montana. If this oil increment should continue to increase to the 1.7 percent (Pratt, 1944, pp. 67, 68) which has been reported for some of the more thoroughly explored States, the future for petroleum in Montana would indeed be bright.
Either the productivity of the sedimentary rocks in the Plains region of Montana is less than equivalent sedimentary sections in other parts of the United States that produce oil, as Is suggested by the comparatively low success ratio for new field wildcats, or exploration is inadequate, which is suggested by the obviously low density of exploratory tests. Some data favor the latter interpretation, and certainly many cubic miles of rock remain undrilled. In line with this is the fact that the most thoroughly explored structural province in Montana--the Sweetgrass arch--through 1961 produced 45.6 percent of the cumulative production of the State, or about 180,732,000 barrels of crude oil. Further confirmation of the more or less direct relationship between exploration and production is now available from the country east of R. 46 E., broadly the western part of the Williston Basin province, where deep systematic drilling to Paleozoic terranes has been underway only since 1950. During the first 4 years production raised so sharply (fig. 8) that it almost equaled that from the Sweetgrass arch, and exceeded it in 1955. By the close of 1961 cumulative production for eastern Montana amounted to 108,178,000 barrels, or about 27.4 percent of the total cumulative production of the State at that date, and if the current annual rate can be sustained or increased the cumulative record for the Sweetgrass arch will be exceeded in 4 or 5 years.
It should be borne in mind, however, that most of this exploration in the Williston Basin province, both in Montana and North Dakota, has been on well-defined surface structures that had been known for many years. Now that their development is established, new field discoveries must be searched for with seismic and subsurface techniques on much more obscure features and the rate of discovery may be expected to level off or even decrease. This endeavor has already begun. Brorson, Dwyer, Redstone, and Sidney, to mention only a few, are examples of fields not on obvious surface structures that have been found by seismic surveys. The comparatively recent Dwyer field, however, which has already produced about 1 million barrels of oil, seems in some way to have been localized by a major transcurrent fault. An intersection of another element of this trend, which is northeast, with the complementary Cedar Creek axis is illustrated in the Richey field. Lineaments similar to these and the many irregularities of stratigraphy, porosity, and other inherent variables that they afford, should provide many different kinds of traps for oil and gas in eastern Montana. That province, therefore, is considered to have the most favorable prospects for future discoveries of petroleum in Montana, and to contain the greater part of the ultimate potential reserve. Any estimate in specific units of production, however, would be hazardous and very subjective.
Reference has been made to the impending expenditure of the ultimate potential of the Sweetgrass arch. Ultimate potentials for central and south-central Montana, which together during 1961 contributed about 30 percent of the total production of the State, are not regarded as impressive and it seems probable that they may not equal the original recoverable reserves of the known fields. Reasons for this opinion are based on many factors that include the comparatively small area involved, its rather complete structural compartmentation, relatively high density of dry holes and the number of structures proved to be dry, time of migration and accumulation, possibility of hydrostatic flushing, productive age of known fields, absence of some of the Paleozoic formations that produce in eastern Montana, and the failure of others that are present to produce.
Some parts of the Plains region that have received little or no drilling also may have latent potentials for oil and gas. Among the larger of such areas are: the north end of the Powder River basin; the upper or outer flanks of the Crazy Mountain basin; the lower flanks of the Bearpaw arch, which are concealed by volcanics; and, on more than a statistical basis, the Disturbed Belt, whose exploration illustrates how elusive and discouraging realization of a prospect may be.
The narrow, structurally complex Disturbed Belt has an area of about 2,000 square miles in Montana, where it separates the mountains from the Plains (Sweetgrass arch), extending from the Missouri River northwest to the International Boundary, and continues on into Alberta, Canada, where it is known as the Foothills Belt. Surface indications of petroleum have been known from it since 1902. Most of the 30 tests made before 1940 were too shallow to be conclusive or even informative and all were abandoned as dry holes without shows of oil or gas except for the few on Swift Current Creek and the test on the Boulder Creek. More competent and determined exploration beginning in 1946, since when approximately the same number of tests have been made, all of them deep and to objectives known to contain oil and gas on the plains, also has been without material return. This effort has been all the more frustrating because the extension of the Belt into southern Alberta includes among other fields: Turner Valley, which to the close of 1956 had produced about 124 million barrels of high-grade (39° API) paraffin base oil and about 1,760 million cubic feet of gas; Pincher Creek, with its literally enormous gas-condensate reserve; the more recent Waterton gas field; and, since 1960 a gas well discovery within 6 miles of the boundary. The consequences of finding an extension of this productivity in Montana would be momentous, and might add greatly to the oil and gas reserve. Exploration in the Disturbed Belt may therefore be expected to persist until a significant find has been made or accumulated geologic data indicate the probability that commercial prospects are not present. In this connection, however, it should be remembered that before success was obtained in the Alberta Foothills the exploration phase of some of its great fields was long, difficult, and expensive.
(By Charles E. Erdmann, U.S. Geological Survey, Great Falls, Mont.)
Natural gas has been found in Montana in 40 or more fields, some of them one- or two-well pools, and perhaps 150 wildcat tests have been abandoned as dry holes with shows of gas and oil or shows of gas alone. This rather large number of occurrences, many of which will be mentioned by name later, has given rise to the belief that natural gas is abundant in Montana. Weeks (1958, p. 46) has remarked that "experience in this country (i.e., the United States) has shown that about 5,000 cubic feet of natural gas have been discovered for each barrel of liquid petroleum." At the close of 1962, in terms of this ratio, the cumulative oil production and proved petroleum reserve of Montana, respectively, should have been associated with 2.1 and 1.7 trillion cubic feet of natural gas, for a total of 3.8 trillion cubic feet, without consideration of the amount that may be contributed in the future by the development of the ultimate potential oil reserve. Although not particularly impressive as a reserve figure for so vast a country as the Plains region, 3.8 trillion cubic feet of gas provides a convenient frame of reference against which the natural gas resources of Montana may be viewed.
Information on natural gas in Montana is not so complete or systematic as the data for petroleum. Much of this shortcoming results from the early exclusive emphasis on petroleum exploration, from which gas was an incidental, unwanted byproduct with little or no market that seldom brought a price of more than 3 cents per thousand cubic feet at the casing head, if it could be sold at all. Such circumstances offered operators no encouragement to prospect for gas; and to some extent this tradition of low field prices for natural gas still prevails even though the value of the commodity is now much greater. Hence, other than the pioneer commercial gas development on Gas City Dome at the north end of Cedar Creek anticline in 1915, and at Havre in 1916, little interest was shown for 8 or 10 years. With increasing evidence of marketable reserves, however, some short lines were laid from several fields to nearby towns as that from the Kevin-Sunburst gas area to Shelby in 1923, and from Bowes field to Chinook and Havre in 1926. The first long line was built from the Kevin-Sunburst field to the city of Great Falls in 1928; and the large-diameter line from the Cut Bank district to the copper smelter at Anaconda followed in 1931. Descriptions of gas development at this stage have been given by Bartram and Erdmann (1935) and by Perry (1937).
In 1953 the American Gas Association estimated the proved recoverable natural gas reserve for the United States at about 211.5 trillion cubic feet. During that same year Casper (1953 p. 209) provided a somewhat more conservative estimate of the national reserve as about 197 trillion cubic feet at 600 F., and 14.65 pounds per square inch absolute. Only 800 billion cubic feet, or about 0.41 percent of Casper's total, were credited to Montana, with no breakdown by individual fields. This estimate has just been proved low, for at the close of 1961 it was exceeded by measured cumulative production totaling about 880 billion cubic feet which, of course, is less than the amount of gas actually withdrawn. Inclusion of the proved reserve of about 325 billion cubic feet, exclusive of the ultimate gas potential, brings the total original reserve estimate for Montana to about 1.2 trillion cubic feet, or about 2.6 trillion cubic feet less than the amount indicated on the basis of the national ratio between oil and gas production.
Reflecting new discoveries and field extensions during the past decade, current estimates of the national gas reserve are of the order of 267.7 trillion cubic feet. An approach to the probable Montana fraction of this figure from the viewpoint of Casper's ratio, which may still be valid even though the estimates on which it was determined seem to be low, gives about 1.1 trillion cubic feet, which is remarkably near the original reserve estimate of 1.2 trillion cubic feet based on cumulative production and proved reserves. Probably this is a fortuitous circumstance, for they are not strictly comparable because about 73 percent of the original reserve estimate consists of gas that has been produced and, therefore, not in the reserve. However, if that volume of gas, about 0.9 trillion cubic feet, is added to the 1.1 trillion cubic feet derived by taking 0.41 percent of the current national reserve the resulting sum Is 2 trillion feet. This is about as large an ultimate recovery as can be rationalized front the incomplete data available, but it is still only about half of the 3.8 trillion cubic feet estimate that might be anticipated from national experience. This critical condition has been appreciated in the local natural gas industry for some time, and potential shortages fortunately have been averted by imports from both Wyoming and some of the fields with large reserves in southern Alberta. However, before considering these imports in greater detail, reference should be made to the geologic and geographic occurrence of natural gas in Montana.
Commercial volumes are present in 16 or 18 geologic formations which range from Late Ordovician to Late Cretaceous in age-- essentially the same as that for petroleum. Most of the formations yielding large amounts are indicated in the Generalized Stratigraphic Correlation Chart (fig. 6) by standard gas well symbols (small open circles with short perpendiculars on the circumference) together with the name of the field producing from that horizon (table 2).
|Upper Cretaceous||Montana||Judith River|
|Upper Cretaceous||Montana||Eagle sandstone||Upper part|
|Lower Cretaceous||Colorado|| Bowdoin
|Lower Cretaceous||Colorado||Blackleaf|| Bootlegger member
Taft Hill Glauconite mbr
|Lower Cretaceous||Kootenai|| Sunburst
Third Cat Creek
|Upper Jurassic||Ellis|| Swift
|Upper Permian||Embar (of former usage)|
|Upper and Lower Mississippian||Madison||Castle Reef dolomite||Sun River member|
|Upper and Lower Mississippian||Madison||Mission Canyon Limestone|
|Lower Mississippian||Madison||Lodgepole limestone|
|Upper Devonian||Potlach anhydrite|
|Middle Silurian||Interlake (of Canada)|
|Upper Ordovician||Bighorn||Red River|
|Upper Ordovician||Bighorn||Stony Mountain|
In general, the gas from the Upper Cretaceous sands is a "dry," sweet (no hydrogen sulfide) gas high in methane (CH4) or "marsh gas," with little or no ethane (C2H6). Often a few percent of nitrogen (N2) and sometimes carbon dioxide (C02) are present as impurities. Gross B.t.u. content may average about 970 B.t.u. These gases do not seem to be associated with petroleum and, in some units such as the upper part of the Eagle sandstone, appear to have been derived by decomposition of carbonaceous debris. The gas from rocks of Early Cretaceous and Late Jurassic age is "wet" sweet solution gas from oil. While predominantly methane, these gases contain ethane (sometimes as much as 13 percent), propane (C3H8) and butane (C4H10), with percentages of nitrogen and carbon dioxide somewhat larger than in the Upper Cretaceous gas. Heat values usually run over 1,000 B.t.u. Gas from the Cut Bank sand falls in this group, and where of average composition may contain about 215 gallons of gasoline per million cubic feet, which is removed before the gas is distributed for industrial or domestic use. The gas from the older (Paleozoic) formation usually is characterized by the presence of hydrogen sulfide, which may vary from a trace to concentrations of more than 16 percent as in the Embar-Tensleep reservoir at Elk Basin, which is mentioned briefly in the chapter on Sulfur. Most of the gas in rocks of Paleozoic age is exsolution gas from petroleum. Good examples of its occurrence may be found in the Sun River member of the Castle Reef dolomite in the Reagan field, and in the Interlake, Red River, and Stony Mountain formations on Cedar Creek anticline. Gases rich in carbon dioxide and nitrogen have been found in some Devonian and Cambrian units on the Sweetgrass arch, from where they have been described by Dobbin (1935, p. 1057, 1065), and along the wedge-edge of the Ordovician system to the northeast. Small percentages of helium have been reported from the nitrogen fraction of some of these gases, but there is no evidence that the volume of the nitrogen reserve is large enough to warrant processing for helium. In contrast to these occurrences, the richest helium gas in Montana, 3.91 percent associated with 81.6 percent nitrogen (Anderson and Hinson, 1951, p. 74, 75. Index No. 678) was found in the base of the Swift Formation on the Kootenai Dome of the Cat Creek anticline by the J. C. Neudigate Oil Co., Root well No. 1, sec. 28, T. 16N., R. 26E., Petroleum County. The volume of gas was small and was soon drowned out by water which caused the hole to be abandoned. This occurrence is believed to be related to deep-seated faulting associated with the development of the structure, as no helium has been reported from later penetrations of the Swift formation on other parts of the anticline.
Both gas and oil are produced in Montana in 15 fields. Gas occurs alone in 29 or 30; but 6 small pools have been exhausted, 11 are of minor importance, and only 12 actually produce gas alone. The fields that produce both gas and oil, but which do not have large separate gas areas are shown simply as oilfields on figure 7. Oil and gas fields of Montana, the index numbers in circles (also in parentheses after the names that follow) correspond to those listed in table 1. These fields are: Bears Den (3), Belfry (4), Bowes (11), Clark's Fork, north (20), Dry Creek (27, 28), Elk Basin (30 to 34), Flat Coulee (35), Gypsy Basin (41), Lake Basin (not producing), Reagan (68), Red Creek (69), Whitlash (86). A new gas well with a reported initial open-flow capacity of about 52 million cubic feet per day from the upper part of the Madison group (Sun River formation) was found during October 1962 at Gypsy Basin, improving the potential of that district. Bowes and Whitlash of this group may be considered as local major gas fields, each having production capability of a little more than 1 billion cubic feet of gas per year, but their reserves are considerably depleted and this rate may be expected to decline in the near future. Recently, however, a west extension to the Whitlash Field gaged 58 million cubic feet, improving the outlook there but the area of the extension probably will be small. In some of the other fields where the gas and oil are in the same reservoir, for example, Red Creek, the gas wells have been shut in to maintain pressure for production of oil.
The exhausted and minor gas fields either are not numbered on the map, or are not shown. The old field at Havre ceased production in 1925, and unsuccessful efforts were made to revive it between 1946 and 1950. Other small exhausted fields are Bow and Arrow, Haystack Butte, Kicking Horse, and Marias River (Nadeau). The field at Box Elder ceased producing in 1959, and is now being used for underground gas storage. The Brown's Coulee, Cassidy, Kremlin, Sherard, Signal Butte, and Winifred districts, which hardly merit the distinction of being called fields, are either abandoned or shut in. All are in the folded and faulted country surrounding the Bearpaw Mountains where they mark small accumulations of methane in the upper sandstone unit of the Eagle Sandstone or in the Judith River formation. At Sherard and Winifred it can be shown that the gas does not occur in the anticlinal (allochthonous) structure but in porosity traps in the upper sandstone bed of the Eagle in place in the underlying autochthon where local structural relief is considerably less. With the exception of Bowes field, therefore, the potential of these districts for Upper Cretaceous gas should be discounted substantially.
In the northern part of the State several small fields that have been shut in without having produced, except possibly for local use are Armells, Blackleaf, Gildford, North Gold Butte, and Rudyard. The little field at South Gold Butte has only recently received a pipeline connection. Small shut-in fields in the southern part of the State include Liscom Creek, McKay Dome, Mosser Dome, Pumpkin Creek, Rapelje, and Sixshooter. Lack of pipeline outlet at these localities suggests either low productive potential or no local market.
Fields that produce gas only, or have gas areas large enough to be shown separately, are indicated on the map, figure 7, by numbers in open squares which are keyed to the following list (table 3):
|2 Big Coulee||Lakota-Morrison||890,509|
|3 Bowdoin||Colorado: Bodowin-Phillips||4,013,919|
|4 Cedar Creek||Judith River-Eagle||5,492,270|
|4 Cedar Creek||Siluro-Ordovician||1,919,801|
|5 Cut Bank||Kootenai; Sun River||12,275,935|
|7 Fred and George Creek||Blackleaf||No Record|
|8 Golden Dome||Eagle||12,876|
|9 Grandview||Marias River shale-cone||146,776|
|11 Keith Block||Sawtooth-Madison||2,126,038|
|13 Plevna||Judith River||168,546|
This list includes the more important gas fields in the State, except for Bowes and Whitlash which are shown on the map as oil fields. Gas production from the fields that produce both oil and gas amounted to 5,418,394 Mcf in 1961 to bring the total amount withdrawn in the State to 34,519,342 Mcf. Seven fields: Bowdoin, Bowes, Cedar Creek, Cut Bank, Keith, Kevin-Sunburst, and Whitlash, produced about 85.5 percent of the gas. At the present overall rate of withdrawal their proved reserves may soon be depleted, perhaps within 8 or 10 years, and some of the smaller older fields before then.
This possibility, of course, had been recognized in Montana's natural gas industry for some time, and first became critical during World War II when large amounts of gas were required for defense production operations of the Anaconda Copper Mining Co. Beginning in 1952, the reserves depleted by defense mineral production were to some extent replenished by the grant of a license from the Canadian Government and the Federal Power Commission to the Montana Power Co. to import 43.8 billion cubic feet of gas over a period of 5 years, which has since been extended, from the Lake Pakowki-Manyberries area in southern Alberta. Gas imports from this area in 1961 amounted to about 15.7 billion cubic feet. Further relief also came in 1961 through a contract between the Montana Power Co. and the Alberta and Southern Gas Co. to import 11 billion cubic feet of gas annually on a "take it or pay for it" basis from the large reserves in southwestern Alberta. Any excess or unused gas will be placed in temporary underground storage in the Cobb field in the north part of the Cut Bank district. Through this arrangement it is expected that the annual import from the Lake Pakowki area may be reduced to about 5 billion cubic feet.
Natural gas also is imported into Montana from Wyoming by the Montana Power Co., and from both Wyoming and North Dakota by the Montana-Dakota Utilities Co. In 1961 the Montana Power Co. import consisted of about 4.9 billion cubic feet from the Heart Mountain field, Wyoming, which was brought to Billings for operation of their gas-steam plant at that place; and about 3.7 billion cubic feet purchased from the Montana-Dakota Utilities Co., at Warren, Mont., where it had been brought from Worland, Wyo. The total gas import of the Montana Power Co. in 1961 was thus of the order of 24.3 billion cubic feet. Excess gas in the Montana Power Co. System in the southern part of the State is stored underground in their Madison River gas bubble in T. 1 N., R. 2 E., west of Bozeman. Imports of gas into Montana by the Montana-Dakota Utilities Co. during 1961 consisted of about 20.7 billion cubic feet from Wyoming, and 5.7 billion cubic feet from North Dakota. The company also stores considerable volumes of gas in Cedar Creek anticline in some depleted parts of the Judith River formation as follows:
From Wyoming________ 3,500 million cubic ft
From North Dakota___ 1,665 million cu ft
From Montana________ 92 million cu ft
Exports to South Dakota (including Colony, Wyo.) amounted to about 9 billion cubic feet, and about 5 billion cubic feet were returned to North Dakota. However, virtually all of the exported gas is of Wyoming and North Dakota origin. The net gain to Montana was thus shout 12.4 billion cubic feet. On balance, therefore, total imports of natural gas into Montana during 1961 came to about 36.7 billion cubic feet, or 2.2 billion cubic feet more than was produced within the State. Total gas consumption in Montana during 1961 was therefore of the order of 71.2 billion cubic feet.
Through commendable foresight of the public service companies it is evident that Montana will suffer no shortage of natural gas within the foreseeable future. However, the prospect of developing additional supplies of gas within Montana at the present time is not promising, and probably will not improve until higher field prices stimulate sufficient drilling exploration to locate some of the ultimate potential reserve. This problem is analogous to and part of the petroleum resource problem; and because of the association of gas with oil in the older rocks quite probably there will be a joint solution. Whether that will mean discovery of a high-pressure gas-condensate field in Paleozoic rocks in the Disturbed Belt with trillions of cubic feet of gas in reserve, which is really what is needed, or large fields out on the Plains remains for future determination.
(By C. B. Bentley, U.S. Geological Survey, Great Falls, Mont.)
Oil shales are organic shales which contain kerogen, a mineraloid consisting of a complex of macerated organic debris and forming the hydrocarbon content of oil shale (Levorsen, 1954, p. 656). Kerogen consists chiefly of algae, pollen, spores, and spore coats, and may contain the remains of insects. Chemically it is a mixture of large molecules containing hydrogen, carbon, oxygen, nitrogen, and sulfur. Oil shale, as defined by Winchester (1923, p. 14), refers to any shale that contains material (kerogen) which yields oil by distillation and is distinguished from shale that contains free oil which can be extracted with a solvent or by mechanical means. The organic debris from which kerogen originates may accumulate either in a marine or a nonmarine environment. Both types of accumulation are represented by the deposits in Montana.
Oil shale of marine origin occurs as thin beds and laminae in the Phosphoria formation of Permian age in many of the mountain ranges in southwestern Montana (Bowen, 1918, p. 319; McKelvey and others, 1959, p. 29) (fig. 9, locality No. 1). Yields vary from less than 10 to 21 gallons of oil per ton of shale (Condit, 1920, pp. 24-26). Other occurrences have been noted by R. N. Miller (1953, "Oil Shales in Montana," unpublished thesis, Montana School of Mines, pp. 17, 26-28, 59-78) in the Lodgepole and Heath formations of Mississippian age in the Big Snowy Mountains south of Lewistown (Nos. 2, 3), and in the Heath formation from depths of 5,065-5,094 feet and 5,259-5,279 feet in the Farmers Union Oil Co. Nason No. 1 well (No. 4). Condit (1920, pp. 17-18) collected several samples from the Heath formation in T. 14 N., Rs. 1 and 2 E. (No. 5). Stebinger (1919, pp. 157-164) noted oil shale in the Blackleaf and Marias river formations of Cretaceous age along the mountain front west of Choteau (No. 6). Condit (1920, pp. 18-19) reported an occurrence of oil shale in the Three Forks formation of Late Devonian age in the Jefferson River Valley (No. 7), but the only samples which yielded more than slight quantities of oil were predominantly coal.
Oil shale of nonmarine origin occurs as beds of light brown shale alternating with beds of sandstone, shale, and lignite in Tertiary lake-beds up to 1,000 feet thick in intermontane basins west and southwest of Dillon (Pardee, 1913, pp. 230-235). Richer looking layers as much as 5 feet thick were reported by Condit (1920, pp. 27-23) to yield about 24 gallons of oil per ton. Unweathered beds below the surface may yield even larger quantities. The principal belt of oil shale-bearing lakebeds extends for about 28 miles from a point near Bannack in T. 8 S., R. 11 W., south to Horse Prairie, near Grant, in T. 9 S., R. 12 W., thence up Medicine Lodge Creek (No. 8). The other occurrence is in Muddy Creek Basin in Tps. 12, 13, and 14 S., Rs. 10 and 11 W. (No. 9). The basin is about 12 miles long and 3 miles wide.
Oil shale has also been reported in Tertiary beds in the valley of the Middle Fork of the Flathead River and in the Flathead Valley above the mouth of the Middle Pork (C. E. Erdmann, personal communication) (No. 10). The oil shale resembles in appearance some of the oil shale of the Green River formation of Colorado and Wyoming.
None of the oil shale in Montana is as rich or as extensive as that in Colorado, Utah, and Wyoming. The oil shale in the Phosphoria formation and in the Tertiary lakebeds near Dillon may become commercial in the future, but for the present it possesses at best a latent resource potential. The oil shale reported to occur in the Three Forks, Lodgepole, Heath, Blackleaf, and Marias River formations, with the exception of a few thin beds, is only slightly petroliferous. Not enough information about the oil shale in the Flathead Valley is known to determine its resource potential.
No development has been undertaken, and available data do not permit tonnage estimates. The petroliferous portion of the Phosphoria formation is present over at least 600 square miles at the surface or in the subsurface of southwestern Montana. The Tertiary lakebeds in which oil shale is known to occur occupy about 280 square miles.
(By Paul Averitt, U.S. Geological Survey, Denver, Cole.) Montana contains about 13 percent of the coal reserves of the United States and ranks second among the States in the total quantity originally present in the ground. Of the 222 billion tons of original reserves currently estimated for the State, 2 billion tons is bituminous coal, 132 billion tons is subbituminous coal, and 88 billion tons is lignite (table 4). The estimate includes reserves to a maximum depth of 2.000 feet below the surface, but 75 percent of the total is less than 1,000 feet below the surface.
|Lignite||Total||Percent of total
|* incomplete||* incomplete|
The Montana coal fields cover 51,000 square miles, or about 35 percent of the total area of the State (fig. 10). Reserves are present in 35 out of 56 counties, but are concentrated in the eastern part of the State. Big Horn, Powder River, and Rosebud Counties alone account for more than half of the total in the State. Parts of the coal-bearing area totaling nearly 5,000 square miles and representing about 10 percent of the area of coal-bearing rocks have been omitted from consideration in preparing the reserve estimates because of the lack of detailed information. The areas omitted are concentrated for the most part in the northeastern corner of the State where the coal-bearing rocks are concealed by glacial drift. In many of the areas included in the estimates, beds remote from outcrops or at depth were also omitted. As mapping and exploration are continued in Montana it is certain the additional coal will be discovered and that the total estimated tonnage will be increased accordingly.
The Montana coal fields can be divided into two major regions, the Fort Union, or eastern region, and the north-central region; and into many subordinate fields and areas as shown on figure 10. NOTE: Figure 10 is an oversized image which Internet Explorer may display in a low-resolution mode. To view the full image, move the cursor over the image and click the enlarger button which appears in the lower right corner. The more important of these are discussed briefly in the following paragraphs:
FORT UNION REGION
|Colstrip, Montana. Coal strip mining near Colstrip, one of the largest operations in Montana, is apparent in this southwest-looking, low-oblique photograph. Rosebud Creek lies east of the strip mining operations where subbituminous coal is produced. Strip mining for coal, which flourished here during the 1920s, faded because of competition from cheap oil and natural gas. With new laws for land reclamation increasing the demand for power and diminishing reserves of oil and gas, a revival of strip mining occurred during the late 1960s. (NASA photo)|
Most of the present and past mining in the region has been concentrated in Custer, Powder River, Richland, Rosebud, and Sheridan Counties. Because the coal is flat lying and locally near the surface, most of the mining is by stripping methods. Two large strip mines in Richland County account for most of the present production in the region. Much of the lignite produced in Richland County is consumed in the steam-electric generating plant of the Montana-Dakota Utilities Co. located at Sidney. During 1962 the Montana Power Co. acquired the coal property of the Northwest Improvement Co. (Northern Pacific Ry.) at Colstrip, in Rosebud County, about 25 miles south of Forsyth. At this locality the 28- to 30-foot Rosebud bed is underlain at an interval of 6 to 8 feet by the 7- to l0-foot McKay bed. Drilling exploration of these beds has been carried out in anticipation of resuming mining coal for a new steam-electric plant now in the planning stage.
The rank of the coal in the region increases progressively westward from lignite at the North Dakota line to subbituminous C west of Miles City and to subbituminous B farther west in southern Rosebud and eastern Big Horn Counties. A line on the accompanying map (figure 10) shows the approximate boundary between lignite and sub-bituminous coal. The change is very gradual, however, and the difference between coal on opposite sides of the line is negligible.
BULL MOUNTAIN FIELD
The Bull Mountain field is mainly in Musselshell and Yellowstone Counties. The coal is concentrated in the Tongue River member of the Fort Union formation, but thin and impure beds occur in the underlying Lebo and Tulloch members. The main central part of the field is a broad, shallow, west-trending synclinal basin in which the dips are nearly horizontal. At the west end of the basin, however, the coal-bearing rocks extend to the west and northwest in two sharply accentuated anticlines in which the dips steepen to a maximum of 360 Most of the mining in the field is in Musselshell County, where underground methods are generally employed. In 1961 there were seven operating mines in the county, which accounted for about 80 percent of bituminous and subbituminous coal production in the State. Mining is concentrated in the Roundup and Carpenter Creek beds, which are about 50 feet apart stratigraphically. The Roundup bed, which is 4 to 6 feet thick near Roundup, is mined in the northern and northwestern part of the field. The Carpenter Creek bed is mined in the northeast part of the field, where it is 4 to 8 feet thick. The coal is of subbituminous A and B ranks.
The north-central region (figure 10) comprises four areas which differ in one or more aspects because of structure, age of coal-bearing rock, or rank of coal. These four areas are described below under separate headings as follows: Great Falls field, Lewistown field, area surrounding the Bearpaw Mountains, and the Blackfeet-Valier area:
Great Falls Field.--The Great Falls field is in Cascade and Judith Basin Counties. It extends in a generally eastward direction for a distance of 60 miles from a point 25 miles southwest of Great Falls to a point southeast of Stanford. The coal-bearing rocks in the field dip gently northward away from the north flank of the Little Belt Mountains. The coal occurs at a single stratigraphic horizon in the upper part of the Morrison formation of Late Jurassic age and is geologically the oldest coal in the State. Because of downwarp and erosion in post-Jurassic time the coal horizon is discontinuous, but is preserved in three basins, which define three producing districts. These are known from west to east, as the Sand Coulee, Otter Creek, and Sage Creek districts.
In the Sand Coulee district, which is the most important of the three, the coal bed is 4.6 feet thick at Belt, 8.0 feet at Sand Coulee, 7.5 feet at Smith River, and averages about 4 feet at Hound Creek. In the Otter Creek district the coal is 3 to 6 feet thick and is separated into two benches by a bone parting. In the Sage Creek district south of Stanford the coal commonly occurs in three benches, which in total contain 2.5 to 7.0 feet of coal. The lower bench is typically 2 feet thick and contains the best coal.
The coal in the Great Falls field is of high volatile B and C bituminous ranks and is moderately high in heat value. In the Sand Coulee district, certain benches, some as thin as 7 to 9 inches, have coking properties and have been used in the manufacture of coke. The Anaconda Mining Co. formerly operated a battery of beehive coke ovens at Belt but these were abandoned many years ago, in part because of the difficulty and expense of separating coking from noncoking coal by handpicking, and in part because of the decline in use of the blast furnace in nonferrous smelting.
Lewistown Field.--The Lewistown field is in Judith Basin and Fergus Counties, centering around Lewistown, and is an eastward extension of the Great Falls field. As in the Great Falls field, the coal occurs in several different basins at or near a single stratigraphic horizon in the Morrison formation. Where mined the coal beds range in thickness from 2.5 to 8.0 feet and commonly occur in two or more benches separated by bone partings.
Area surrounding the Bearpaw Mountains.--North of the Great Falls and Lewistown fields, in Hill, Toole, Liberty, Choteau, Fergus, and Blaine Counties, is an area of 10,500 square miles underlain by flat-lying coal-bearing rocks. These rocks are largely concealed by a cover of glacial drift. The coal in this area occurs primarily in the Judith River formation and to a minor extent in the Eagle sandstone, both of late Cretaceous age. At one locality coal is mined from a bed in the Fort Union formation of Paleocene age.
Most of the coal in the Judith River formation occurs in two thin discontinuous beds in the upper 75 feet of the formation. In the Milk River field in Blaine and Hill Counties these beds are being mined for local use near Chinook and formerly were mined north of Havre. At these localities the coal ranges in thickness from 2.5 to 6.7 feet, but is impure and contains bone partings.
The coal in the Eagle sandstone occurs as thin lentils in the basal part of a 75-foot unit of carbonaceous shale, but is too thin and too low grade to be minable, except locally. Insofar as known, none has been mined since about 1925.
A bed of coal in the Fort Union Formation is mined at the Rocky Boy mine in sec. 31 T. 29 N., R. 15 E. to supply local demand at the Rocky Boy Indian Reservation.
The coal in the area surrounding the Bearpaw Mountains is typically of subbituminous A and B ranks, but in northwest Liberty and northeast Toole Counties is of high volatile B bituminous rank as a consequence of proximity to the Sweetgrass Hills intrusive uplift.
Blackfeet-Valier area.-The Blackfeet-Valier area extends in a narrow belt from the Canadian border south through Glacier, Pondera, and Teton Counties to a point about 30 miles south of Choteau. The coal in this belt occurs at three or four stratigraphic horizons in the Two Medicine formation and at two horizons in the St. Mary River formation, both of late Cretaceous age, but the beds are generally thin and discontinuous. All the coal is of high volatile B and C bituminous ranks.
In the Valier field in Pondera County coal in the lower part of the Two Medicine formation is 20 to 24 inches thick, including a 2-inch clay parting, and the beds are gently dipping. This area has been dormant since the early 1930's.
In the Blackfeet Indian Reservation in Glacier County coal has been mined in the past from beds in both the Two Medicine and St. Mary River formations. The coal in this area is locally as much as 3.5 feet thick, but the beds are steeply dipping and are broken by faults.
BRIDGER AND SILVERTIP FIELDS
The Bridger and Silvertip fields are part of a northern extension of the Bighorn Basin of Wyoming into eastern Carbon County, Mont. The coal in these fields is of high volatile C bituminous rank. Coal beds occur at three stratigraphic horizons in the Eagle Sandstone of Late Cretaceous age. The beds are discontinuous, and coal of usable thickness is present at only one horizon in any one locality. Mining has been carried on near Joliet and Fromberg, and also at Bridger.
RED LODGE FIELD
The Red Lodge field is also in Carbon County about 20 miles west of the Bridger and Silvertip fields. The coal in the Red Lodge field is also of high volatile C bituminous rank. Eight beds in the Port Union formation have been mined in the vicinity of Red Lodge and Bear Creek.
The Electric field covers an area of less than 20 square miles in central Park County. The held is part of a folded and downfaulted block in which coal-bearing rocks of Late Cretaceous age have been preserved. Over most of the area of the field the coal-bearing rocks are deeply buried but they are exposed locally in two small synclines covering an area of about 3 square miles. Where exposed the coal-bearing sequence is about 300 feet thick and includes three coal beds, each ranging in thickness from 3 to 5 feet, including partings. The uppermost bed, the only one mined, is of high volatile A bituminous rank. In the past coal from this bed was used to manufacture coke for smelters at Anaconda and at Butte. The use of this coke was discontinued because the operators were unable to meet smelter specifications for a coke containing less than 18 percent ash, but otherwise the coke was reported to be of good quality.
LIVINGSTON-TRAIL CREEK FIELD
The Livingston-Trail Creek field extends in a narrow strip across three townships in Gallatin and Park Counties. The coal-bearing rocks in this field are probably stratigraphically equivalent to rocks in the Electric field, 30 miles to the east, and are strongly folded, faulted, and locally overturned. The coal-bearing sequence contains several coal beds, each ranging in thickness from 2 to 5 feet, including partings. The coal ranges in rank from high volatile C to A bituminous, depending in part on the amount of deformation. Prior to 1908 coal near Cokedale was mined and used for the manufacture of coke; at one time 100 beehive ovens were in operation.
As noted in previous paragraphs, three areas in Montana--the Great Falls, Livingston-Trail Creek, and Electric field--yield coal with coking properties. In the past coal from each of these fields was used to manufacture coke, but the product was inferior to or more expensive than coke manufactured elsewhere and the operations were abandoned.
The cumulative recorded production of coal in Montana from the date of earliest record to January 1,1962, totals 171 million tons, and actual production has probably been somewhat larger. In common with most other Rocky Mountain States, annual production in Montana has declined drastically in recent years. Between 1944 and 1958, for example, production declined from a maximum of 4,844,000 tons to a minimum of 305,000 tons (fig. 11, below). Since 1958, production has increased slightly but in 1961 the production was only 371,000 tons. The decline in production is attributed to the replacement of steam by diesel-driven locomotives, to the increased use of oil and natural gas for the generation of electric power, to the increased use of hydroelectric power, and to the increased use of natural gas and liquefied petroleum gases for residential and commercial heating. Coal production since 1980 has been in the range of 30,000,000 to 40,000,000 tons per year, as shown in the table below Fig. 11.
PETROLEUM AND NATURAL GAS
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Gasper, John C., 1953, Oil and Gas Jour.: Tulsa, Okla., Oil and Gas Jour., Jan. 26, 1953, p. 209.
Dobbin, C. E., 1935, Geology of natural gases rich in helium, nitrogen, carbon dioxide, and hydrogen sulphide in Geology of natural gas: Tulsa, Okla., Am. Assoc. Petroleum Geologists, p. 1053-1072.
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---- 1955, Structure contour map of the Montana plains: U.S. Geol. Survey Oil and Gas Inv. Map OM-17&A.
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Pardee, J. T., 1913, Coal in the Tertiary lakebeds of southwestern Montana: U.S. Geol. Survey Bull. 531-G, p. 229-244.
Reeves, Frank, 1931, Geology of the Big Snowy Mountains, Montana: U.S. Geol. Survey Prof. Paper 165-D, p. 135-149.
Stebinger, Eugene, 1919, Oil and gas geology of the Birch Creek-Sun River area, northwestern Montana: U.S. Geol. Survey Bull. 691-E, p. 149-184.
Winchester, D. E., 1923, Oil shale of the Rocky Mountain region: U.S. Geol. Survey Bull. 729, 204 p.
--- 1928, The oil possibilities of the oil shales of the United States, Report 2 of the Federal Oil Conservation Board to the President of the United States.
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Berryhill, L. R., and Averitt, Paul, 1951, Coking-coal deposits of the western United States: U.S. Geol. Survey Circ. 90, 19 p.
Combo, J. X., Brown, D. M., Pulver, H. F., and Taylor, D. A., 1949, Coal resources of Montana: U.S. Geol. Survey Circ. 53, 28 p.
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Fulkerson, F. B., Kauffman, A. J., Jr., and Knostman, R. W., 1962, The mineral industry of Montana: U.S. Bur. Mines Minerals Yearbook 1961, v. 3, p. 615-640.
Kepferle, R. C., 1954, Selected deposits of strippable coal in central Rosebud County, Montana: U.S. Geol. Survey Bull. 995-I, p. 333-381.
Warren, W. C., 1959, Reconnaissance geology of the Birney-Broadus coal field, Rosebud and Powder River Counties, Montana: U.S. Geol. Survey Bull. 1072-5, p. 561-585.