Several hundred people demanding jobs shut off access to the Nassiriya field, 300 kilometres (190 miles) south of Baghdad, which produces 82,000 barrels of oil per day, executives said.
The two-day-old blockade is the first to disrupt operations in OPEC's second largest producer since the start of the popular revolt set to enter its fourth month in early January.
The youth-led protests demand the ouster of the entire political class that has run the country in the aftermath of the 2003 US-led invasion that toppled dictator Saddam Hussein.
Demonstrators have vented their fury at what they consider inept politicians who have mismanaged the economy, enriched themselves and are beholden to powerful neighbour Iran.
Sit-in protests have shut down state offices and schools across the Shiite-majority south for weeks, and demonstrators again declared a "general strike" in Diwaniya on Sunday, the first day of the working week.
Mass rallies and picket lines also paralysed Kout, al-Hilla, Amara and the shrine city of Najaf, AFP correspondents said.