No oil rig flanges gulf coast production data was immediately available, though the engineer said that output could recover to 300,000 barrels per day (bpd) as soon as Tuesday.

The field, located deep in Libya's south, had been producing about 308,000 bpd until the shutdown and has capacity of about 340,000 bpd.

Libya's National Oil Corporation (NOC) operates Sharara in partnership with Repsol, Total, OMV and Statoil.

The valve that had been closed near the west Libyan town of Zintan was reopened before any formal declaration of force majeure on loadings of Sharara crude, one oil source said.

NOC said the closure had cost the BOP Blow Out Preventer repair company gulf coast about $30 million in lost revenue and that it intended to press charges against the landowner.

Sharara reopened after a long blockade in late 2016 but has suffered a number of stoppages since then because of localised protests by guards and other groups amid the wider turmoil that has gripped Libya since the toppling of Muammar Gaddafi in 2011.

Repeated and lengthy shutdowns cause pressure in the oilfield's wells to drop, reducing oil rig flanges gulf coast production capacity. Boosting capacity at Sharara and other fields requires investment that NOC has been struggling to secure.

As conflict and falling oil revenues caused living standards to slide in Libya in recent years, local groups have tried to press demands by blockading oil facilities.

A week ago NOC declared force majeure on the 70,000 bpd El Feel after a protest by guards closed the field, which is operated by a joint venture between NOC and Italy's Eni .

(Reporting by Ahmad Ghaddar, Ahmed Elumami, Ayman al-Warfalli and Aidan Lewis Writing by Ulf Laessing Editing by Dale Hudson and David Goodman)