There are 1 US Post Offices in Petroleum County serving a total of 436 residents equating to 436 residents per post office. It's estimated that approximately 696 packages are handled across the 1 county post offices annually. Petroleum County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 494, making it the least populous county in Montana and the seventh-least populous in the United States. Its county seat is Winnett. The Montana Legislature approved the formation of the county by separating it from Fergus County; Petroleum County was thus created on February 25, 1925, as the last of Montana's 56 counties."