The area was home to Native American tribes of the Crow, Blackfoot, Nez Perce, and Sioux. In 1868 a trading post was established at the mouth of Musselshell River; it was named "Musselshell". Walter John Winnett, a rancher who had been adopted into the Sioux tribe, started a ranch in Montana Territory in 1879. His ranch house became a gathering place for the area. In 1910, he built a store and petitioned for a post office; thus Winnett became an official town. Fort Magginis subdued Indian raids in the area, allowing cattle raising to prosper. Gold was also discovered in the Fergus County mountains; from 1911 to 1915, stakes in the county were claimed by prospectors. Area claims were limited to ; in 1930 many of these lands reverted to the federal government as settlers deserted the town. In February 1920, oil was discovered in the SE part of the county. This area developed into the Cat Creek Field, producing high-grade crude. By 1922 it was producing 2.2 million barrels annually. The state legislature split the eastern area of Fergus County into a separate entity, named Petroleum County to denote its status as the first place in Montana where petroleum was discovered, in 1925, and designated Winnett as its seat. The county was always one of the most sparsely populated areas of the US, and the population has continued to decline. The 1930 census listed 2,045 residents. The county was brought under administrative format of the "county manager" in 1944. By the time of the 1980 census, the population was reduced to 685.
Geography
The county’s eastern boundary is formed by the Musselshell River. According to the US Census Bureau, the county has a total area of, land and water. Its average elevation is. 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 consists of moderate hill slopes and gentle valleys, interspersed with steep cliffs.
The northern part of the county has abundant surface water resources, with little agricultural land to use it. In other parts of the county, agriculture is reliant on underground water resources.
The mean annual precipitation is. The mean annual temperature is in the range of. Frost is recorded during the season for 105–135 days. Rainfall is in Flatwillow, it is at Grass Range, and at Mosby; at these locations, the mean winter temperatures are, and respectively. The lowest temperature recorded at Mosby was on January 24, 1969, of. The maximum temperature recorded was 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 at Flatwillow, at Grass Range and at Mosby. Average wind speed is about and is higher in winter months than summer months.
A sandstone ridge with steep dips runs along the northern end of the county. This anticline is petroleum-bearing, and divides the county into two areas: to the north, geological formations of Bearpaw Shale or the Hill Creek Formation; to the south, formations containing older Cretaceous rocks. Marine shales of Cretaceous Age are found as outcrops throughout the county. Sandstones are noted in alternate sequences and are identified at deeper depths in Cat Creek. A Tyrannosauridaefossilskeleton was discovered in the Judith River Formation, while Alamosaurus was discovered in the Hell Creek Formation.
Flora and fauna
Rangeland contains wheatgrass, forbs, shrubs, green needlegrass, blue grama, big sagebrush, plains prickly pear, wooly Indian wheat, 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 county has elk, white-tailed deer, mule deer and pronghorn antelope. There are pheasant, eagles and Gray, or Hungarian, partridge.
Economy
As of March 2012, the cost of living index in the county was 82.2 below the national average of 100. Petroleum and cattle raising are the principal economic activities; livestock farming accounts for 89% 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. Mining accounts for 58.2% of people in the county while in educational, health and social services 15.5% of people are engaged. 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. The major crops grown in the county are 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.
Demographics
As of the 2010 United States Census, there were 494 people, 225 households, and 143 families residing in the county. The population density was. There were 324 housing units at an average density of. 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.
Politics
Voters in Petroleum County generally vote Republican in national elections. Since 1928 they selected the Republican candidate in 78% of the elections.
