Petroleum County is a county in Montana, United States, on the state’s central plains where the Musselshell River meets the Missouri River country. It had a population of 496 at the 2020 census and covers 1,655 square miles (4,290 km2) of land centred near 47°07′N 108°16′W, making it Montana’s least populous county.
Winnett is the county seat, and its postal code is 59087. The county has one incorporated town, a scatter of unincorporated places, and no civil townships. It sits in the Mountain time zone.
Towns & Villages in Petroleum County
Petroleum County has one incorporated town and a handful of unincorporated communities, several of them now largely abandoned. The table below uses Town for the sole incorporated place and Unincorp. for everything else.
| Place | Type | Pop. | ZIP |
|---|---|---|---|
| Winnett | Town · seat | 136 | 59087 |
| Teigen | Unincorp. | — | 59084 |
| Cat Creek | Unincorp. | — | — |
| Flatwillow | Unincorp. | — | — |
| Kelley | Unincorp. | — | — |
| Mosby | Unincorp. | — | — |
| Valentine | Unincorp. | — | — |
Mosby and Valentine both straddle the Petroleum–Garfield county line, with only their western halves inside Petroleum County. Flatwillow and Kelley are largely inactive today, remembered mainly as rural post-office names and ranch-community markers rather than settled places with a resident population.
ZIP Codes
- 59087 — Winnett (also serves the rest of the county)
- 59084 — Teigen
Cat Creek, Flatwillow, Kelley, Mosby and Valentine use the Winnett or Teigen ZIP depending on which side of the county they fall on; none carries its own postal code.
Geography
Petroleum County sits on Montana’s central plains, a landscape of shortgrass prairie cut by badland breaks along its river corridors. The Musselshell River forms the county’s eastern boundary, and the Missouri River defines part of its northern reach through the rugged Missouri Breaks country. Elevations range from river-bottom lowlands to open range tableland used almost entirely for cattle grazing and, in the Cat Creek and Rattlesnake Butte fields, oil extraction.
Petroleum County borders Phillips County to the north, Garfield County to the east, Rosebud County to the southeast, Musselshell County to the south, and Fergus County to the west.
Demographics
The 2020 census counted 496 residents, split mostly between Winnett (136) and the surrounding ranch country. Winnett and Teigen are the county’s only named population centers with any recorded residents. Median household income runs below the Montana median of $70,804, reflecting a local economy dominated by ranching and a small oil workforce. At 496 people across 1,655 square miles, the county’s density is roughly 0.3 people per square mile, among the lowest of any county in the contiguous United States.
Economy & Local Life
Cattle ranching is the backbone of Petroleum County’s economy, with agriculture accounting for a majority of local employment. The county’s oil industry, centered on the Cat Creek and Rattlesnake Butte fields discovered in the early 20th century, still supports a share of local jobs and gave the county its name — the first place in Montana where petroleum was found. Winnett hosts the county’s school, courthouse, and the small businesses that serve the surrounding ranches.
History
Petroleum County was created on February 25, 1925, when the Montana legislature split it off from the eastern portion of Fergus County — the last of Montana’s 56 counties to be organized. It took its name from the Cat Creek oil discovery nearby, one of the earliest petroleum strikes on record in Montana. Homesteaders had settled the plains around Winnett only a decade or two earlier, drawn by dryland wheat farming before drought and thin soils pushed most of them out in favor of large cattle ranches. The county’s population peaked decades ago and has declined steadily since, a pattern common across Montana’s sparsely settled plains counties.
Getting There
Montana Highway 244 runs through Winnett, connecting to U.S. Route 191 and the larger town of Lewistown to the west.
- Lewistown — 50 miles west
- Roundup — 55 miles south
- Malta — 90 miles north
- Billings — 140 miles southeast
The nearest commercial airport is Billings Logan International Airport, roughly 140 miles southeast of Winnett.
Frequently Asked Questions
Where is Petroleum County, Montana?
It sits on the central Montana plains, bordered by the Musselshell and Missouri Rivers, about 50 miles east of Lewistown.
What is the population of Petroleum County?
496 people at the 2020 census, making it Montana’s least populous county.
How big is Petroleum County?
1,655 square miles (4,290 km2) of land, plus 19 square miles (49 km2) of water.
What is the county seat of Petroleum County?
Winnett is the county seat and the county’s only incorporated town.
How many towns are in Petroleum County?
One incorporated town, Winnett, plus a handful of small unincorporated places including Teigen, Cat Creek, Flatwillow, Kelley, Mosby, and Valentine.
What ZIP codes are in Petroleum County?
59087 (Winnett) and 59084 (Teigen).
Why is it called Petroleum County?
It was named for the Cat Creek oil field nearby, the first place in Montana where petroleum was discovered.
More counties in this state: Towns and villages in Montana.