Source: US. Bureau of the Census
Data for "Resident Population" are from estimates as of July 1. Data for 1970 and 1980 are annual census.
US. Bureau of the Census, Resident Population in Petroleum County, MT [MTPETR9POP], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/MTPETR9POP, December 3, 2016.