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Petroleum County
Petroleum County Courthouse in Winnett
Petroleum County Courthouse in Winnett
Map of Montana highlighting Petroleum County
Location within the U.S. state of Montana
Map of the United States highlighting Montana
Montana's location within the U.S.
Country  United States
State  Montana
Founded February 25, 1925
Named for Petroleum fields
Seat Winnett
Largest town Winnett
 • Total 1,674 sq mi (4,340 km2)
 • Land 1,655 sq mi (4,290 km2)
 • Water 19 sq mi (50 km2)  1.1%
 • Total 496
 • Density 0.2963/sq mi (0.11440/km2)
Time zone UTC−7 (Mountain)
 • Summer (DST) UTC−6 (MDT)
Congressional district At-large
  • Montana county number 55

Petroleum County is a county in the U.S. state of Montana. As of the 2020 census, the population was 496, making it the least populous county in Montana and the eighth-least populous in the United States. Its county seat is Winnett. The county's area was partitioned from Fergus County to become the last of Montana's 56 counties to be organized.


The area was home to Native American hunter gatherer tribes of the Crow, Blackfoot, Nez Perce, and Sioux. In 1868, a trading post was established at the mouth of this river and was named Musselshell. The county seat, Winnett,was named for Walter John Winnett. He was a Canadian rancher who was captured by Sioux and later adopted into the tribe; he was given the name Eagle Eyes because of his remarkable shooting skills. Winnett established a ranch in Montana Territory in 1879 near an active trading post and the Hangman's Tree used by vigilantes in the area. The massive ranch house, which he built in 1900, housed his own family and served as a gathering place for the community. Winnett built up a freight line business to haul supplies from Billings. In 1910, he built a store and petitioned for a post office—and with that, Winnett became an official town. Fort Magginis was built in 1880 and thereafter, live stock farming proliferated. At that time, gold was also found in Fergus County, which was the original county from which Petroleum County was carved out. During the period from 1911 to 1915, stakes in the county were claimed by an influx of settlers with claims limited to 320 acres (130 ha) of land to each settler and squatter. However, in 1930 many of these lands reverted to the Government as settlers deserted the town.

Cat Creek oil field 1921
Cat Creek Oil Field, Montana

Oil was discovered at the Cat Creek Oil Field on February 18, 1920, at the southeast corner of the county. Petroleum found in the Cat Creek was of high grade and extraction of crude oil started soon. In 1922, the crude oil extracted was 2.2 million barrels.

The county got its present name, "Petroleum County", from its status as the site of the first major oil discovery in Montana. When the state government established Petroleum County in 1925, splitting it from Fergus County, Winnett became the county seat and the county was named after the emerging petroleum industry of Cat Creek.

The county was always one of the most sparsely populated areas of the US, and the population has continued to decline as rural folk move out to more populated areas to find work. According to the 1930 census, there were 2,045 people living in the county, with 408 people living in the county seat of Winnett. The county was brought under administrative format of the "county manager" in 1944. By the time of the 1980 census, there were just 685 people living in the county.


Winnett Rims from the Highway
Winnett Rims from the Highway

The county’s eastern boundary is formed by the Musselshell River. According to the U.S. Census Bureau, the county has a total area of 1,674 square miles (4,340 km2), of which 1,655 square miles (4,290 km2) is land and 19 square miles (49 km2) (1.1%) is water. Its land topography lies in an elevation range of 2,250–4,000 feet (690–1,220 m). The land lies in the Missouri Plateau section of the Great Plains. The uplands are generally of fairly level land and valleys while the general topography is of rolling hills and valleys except for sharp gullies on the side slopes of Missouri and Musselshell Rivers in certain stretches. The southern part of the county also consists of moderate hill slopes and gentle valleys, interspersed with steep cliffs in some stretches.

Petroleum County is bordered to the north by Phillips County, Garfield County to the east, Rosebud County to the southeast, Musselshell County to the south, and Fergus County to the west. Notable towns in the county include Winnett, Cat Creek, Flatwillow, Kelley, and Teigen. State Highway 200 passes through the middle of county, State Highway 244 from the east end is from Winnet to the county, U.S. Route 87 passes through the southwest corner of the county. There is also a network of internal roads that connect to various places in the county. A Tyrannosauridae fossil skeleton was discovered in the Judith River Formation, while Alamosaurus was discovered in the Hell Creek Formation.

Left: Missouri River, Charles M. Russell National Wildlife Refuge. Right: Fort Peck Dam on Missouri River in Fort Peck, Montana Left: Missouri River, Charles M. Russell National Wildlife Refuge. Right: Fort Peck Dam on Missouri River in Fort Peck, Montana
Left: Missouri River, Charles M. Russell National Wildlife Refuge. Right: Fort Peck Dam on Missouri River in Fort Peck, Montana

Rivers running through the county include the Missouri River and Musselshell River. The Musselshell River is drained mostly by its tributaries within the county. However, the northern tributaries drain into the Missouri River and eventually into the Fort Peck Reservoir. The creeks which flow through the county are all ephemeral in nature, flowing only during the rainy season, while the larger creeks have formed flood plains which are composed mainly of silt, clay and to some extent sand and pebbles. There are several creeks, including Bender Creek, Bear Creek, and the other creeks are the Flat Willow Creek, Bender Creek, Box Elder Creek, Brickyard Creek, Buffalo Creek, Cat Creek, Cottonwood Creek, Sacagawea River, and Dovetail Creek. Wild Horse Lake is located in the county formed in a natural depression. Some of the important lakes and reservoirs in the county are: Little Bear Lake, Wild Horse Lake, War Horse Lake, Petrolia Lake, War Horse Lake, Yellow Water Reservoir and Headman-Field Reservoir. Only the northern part of the county is rich in surface water resources, but there is hardly any agricultural land to use it. In the rest of the county, the reliance for agricultural use is on ground water.


The mean annual precipitation recorded is 13 inches (330 mm). The mean annual temperature is in the range of 42–47 °F (6–8 °C). Frost is recorded during the season for 105–135 days.

While rainfall is 13 inches (330 mm) in Flatwillow, it is 16 inches (410 mm) at Grass Range, and 13 inches (330 mm) at Mosby; at these locations, the mean winter temperatures are 24 °F (−4 °C), 25 °F (−4 °C) and 23 °F (−5 °C) respectively. The lowest temperature recorded at Mosby was on January 24, 1969, of −43 °F (−42 °C). The maximum temperature recorded was 108 °F (42 °C) on July 19, 1960, at Flatwillow. Nearly 70% of rainfall occurs from April to September, also the growing season for many crops in the county. The average seasonal snowfall is 40 in (100 cm) at Flatwillow, 63 in (160 cm) at Grass Range and 36 in (91 cm) at Mosby. Average wind speed is about 17 miles per hour (7.6 m/s) which is highest during the winter.

Protected areas

Dung Beetles on War Horse NWR (7345230728)
Dung beetles represent some of the fauna on the War Horse NWR

The War Horse National Wildlife Refuge is a protected area in the county. The Charles M. Russell National Wildlife Refuge is located in the northern region of the county. It provides opportunities for hunting, fishing, boating, backpacking and other recreational activities.


Metengonoceras teigenense SR EA 07-04-01
A ~11.5 cm diameter fossil ammonite Metengonoceras teigenense from the Cretaceous Mowry Shale, Petroleum County

A sandstone ridge with steep dips passes through the county starting from the Musselshell River in the northern end running up to the boundary on the west end of the county. This geological feature, an anticline, is petroleum-bearing. The anticline divides the county into two geological regions - one is to the north of the anticline with geological formations of Bearpaw Shale or the Hill Creek Formation while the other region, to the south of the anticline, is of formations containing older Cretaceous rocks. Marine shales (generally in dark gray turning to black color), classified under the geological formations of the Colorado Group, are found as outcrops throughout the county and these are of Cretaceous age. Sandstones are also noted in alternate sequences and these are identified at deeper depths in Cat Creek. Of the anticlines noted in the area, the Cat Creek Anticline, which runs in an east–west direction is dominant.

Flora and fauna

Rangeland contains wheatgrass, forbs, shrubs, green needlegrass, blue grama, big sagebrush, plains pricklypear, wooly indianwheat, weedlike forbs, broom snakeweed, Nuttal saltbush, prairie sandreed, horizontal juniper, plains reedgrass, golden pea, and prairie rose. Forest land covers 64,296 acres of which 6,500 acres are characterized as commercial forest land; Ponderosa pine, Douglas fir, and Plains cottonwood are noted. The forest understory features obtuse sedge, creeping juniper, Rocky Mountain juniper, bluebunch wheatgrass, Little lbuestern, and hawksbeard.

The wild life found in the country are elk, white-tailed deer, mule deer and pronghorn antelope. These provide for good hunting opportunities. The pheasant population has been increasing in the county, eagles frequent War Horse Lake, and Gray, or Hungarian, partridge are an introduced species.


Historical population
Census Pop.
1930 2,045
1940 1,083 −47.0%
1950 1,026 −5.3%
1960 894 −12.9%
1970 675 −24.5%
1980 655 −3.0%
1990 519 −20.8%
2000 493 −5.0%
2010 494 0.2%
2020 496 0.4%
U.S. Decennial Census
1790-1960 1900-1990
1990-2000 2010-2020

As of the 2010 census, there were 494 people, 225 households, and 143 families residing in the county. The population density was 0.3 inhabitants per square mile (0.12/km2). There were 324 housing units at an average density of 0.2 per square mile (0.077/km2). The racial makeup of the county was 98.8% white, 0.0% from other races, and 1.2% from two or more races. Those of Hispanic or Latino origin made up 1.0% of the population. In terms of ancestry, 35.1% were German, 15.9% were Norwegian, 15.7% were English, 13.2% were Irish, and 3.5% were American.

Of the 225 households, 28.0% had children under the age of 18 living with them, 55.1% were married couples living together, 4.0% had a female householder with no husband present, 36.4% were non-families, and 35.1% of all households were made up of individuals. The average household size was 2.20 and the average family size was 2.83. The median age was 47.3 years.

The median income for a household in the county was $36,875 and the median income for a family was $39,107. Males had a median income of $25,991 versus $21,705 for females. The per capita income for the county was $21,008. About 18.1% of families and 16.7% of the population were below the poverty line, including 26.3% of those under age 18 and 12.7% of those age 65 or over.



Unincorporated communities


As of March 2012, the cost of living index in the county was 82.255 below the national average of 100. Petroleum and cattle raising are the principal economic activities; livestock farming accounts for 89.7% of the farm income. Crude oil from the Cat Creek and Rattlesnake Butte fields is piped to refineries in Billings. Agriculture, forestry, fishing and hunting are attributed to 66% of the population. 58.2% of people in the county are employed in mining. Another 15.5% are employed in educational, health and social services. Some of the notable ranches in the county are: McArthur Ranch, Maxwell Ranch, Crooked Creek Campground, Fail Ranch, and Novak Homestead. As of 2012, the average size of farms was 6,045 acres and the area under all harvested wheat grain was 14,720 acres (5,960 ha). The major crops grown in the county are wheat (both winter wheat and spring wheat) and barley. Alfalfa and grass hay are grown as cattle feed under irrigated conditions along the main river course and also on the banks of creeks such as Flat Willow, Box Elder, and Macdonald.

See also

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