The Ventura Oil Field is a large and currently productive oil field in the hills immediately north of the city of Ventura in southern California in the United States. It is bisected by California State Route 33, the freeway connecting Ventura to Ojai, and is about eight miles (13 km) long by two across, with the long axis aligned east to west. Discovered in 1919, and with a cumulative production of over 1 billion barrels (160,000,000 m3) of oil as of 2023 out of its original 3.3 billion. It is the tenth-largest producing oil field in California,[1] retaining approximately 50 million barrels in reserve, and had 423 wells still producing.[2][3] As of 2009[update] it was entirely operated by Aera Energy LLC.[4]