40°19′31″N 124°17′13″W / 40.32528°N 124.28694°W
Petrolia is an unincorporated community in Humboldt County, California, United States, 10 miles (16 km), southeast of Cape Mendocino. The site of the first oil well drilled in California, it lies at an elevation of 121 feet (37 m) above sea level, within ZIP Code 95558, with an area code 707.
The Mattole River is a river on the north coast of California, that flows northerly, then westerly into the Pacific Ocean. The vast majority of its 62 miles (100 km) course is through southern Humboldt County, though a short section of the river flows through northern Mendocino County. Communities, from north to south, closely associated with the Mattole River include: Petrolia, Honeydew, Ettersburg, Thorn Junction, and Whitethorn. The river enters the ocean at the Mattole Estuary about 4 miles (6.4 km) west-southwest of Petrolia and 10 miles (16 km) south of Cape Mendocino.