Petroleum County, Montana: History
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 (built 1900) 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 (built in western Fergus County in 1880) 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 320 acres (130 ha); 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 Oil Field, producing high-grade crude. By 1922 it was producing 2.2 million barrels annually. In 1925, 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, 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.