Petroleum County /

Petroleum County is a county located in the U.S. state of Montana. In 2010, the population was 494, making it Montana's least populous county and the 6th smallest in the country. Its county seat is Winnett. It was created in 1926 and is the newest of Montana's 56 counties. According to the U.S. Census Bureau, the county has a total area of 1,674 square miles (4,336 km²), of which 1,654 square miles (4,284 km²) is land and 20 square miles (52 km²) (1.20%) is water. At the 2000 census, there were 493 people, 211 households and 136 families residing in the county. The population density was 0.29 per square mile (0.11/km²). There were 292 housing units at an average density of 0.17 per square mile (0.07/km²). The racial makeup of the county was 99.19% White, 0.20% Native American, 0.20% from other races, and 0.41% from two or more races. 1.22% of the population were Hispanic or Latino of any race. 37.0% were of German,14.3% American, 13.2% Norwegian and 10.9% English ancestry according to Census 2000. There were 211 households of which 31.30% had children under the age of 18 living with them, 55.90% were married couples living together, 5.70% had a female householder with no husband