An oil worker at the large southern field said there was shooting, but no further details were available, according to a Reuters report.
Hours after the initial attack, workers said they were still trapped inside company buildings.
The field produced at least 200,000 barrels per day before the shutdown, oil ministry sources told the news wire.
The area near the field, which is deep in Libya’s lawless south, has been the scene of fighting between members of the Tuareg and Tibu, two minorities who have complained about neglect and discrimination in the North African country.
Protesters have closed the field twice in the past 12 months to press authorities into accepting their financial and political demands.
Tuarag protesters closed the field in October, demanding a bigger local council representation and national identity cards needed to get full rights and state benefits.
The government had twice renegotiated the reopening of the field but state authority has eroded since summer when an armed group seized the capital, forcing the internationally recognised Prime Minister Abdullah al-Thinni to move to the east.
Western powers worry that Libya is heading towards civil war as authorities prove too weak to control former rebels who helped oust Muammar Gaddafi in 2011 but now defy state authority to grab power and a share of oil revenues.
The rapid return of Libyan crude oil to the market in recent months has added to a glut of crude that has driven down prices by more than 25% since June, though growing political instability has increased uncertainty over the country’s levels of production and supply.