LONDON—Libya’s largest oil field, El Sharara, has been shut down since July 19 due to a suspected valve closure and an investigation has been launched, the National Oil Corp. (NOC) said July 20.

It was not immediately clear what or who had caused the shutting of the valve, but the field has been the target of numerous shutdowns by protesters or armed groups in the past few years.

The field was pumping around 290,000 barrels per day (bbl/d) prior to the shutdown, the NOC said, adding that the incident had occurred in Hamada, an area of desert in western Libya surrounded by a number of towns and cities.

Sharara's closure will cut Libya's oil production to a little over 1 MMbbl/d, a NOC official told Reuters.

It said production from the nearby El Feel oil field had not been affected.

Sources had earlier told Reuters that production at El Sharara had stopped due to a valve closure on the pipeline linking the field to Zawiya port, which is also home to a refinery.

"They have shut (it] down as the pressure in the pipe was building until they find out what is going on," one of the sources said.

Libya’s overall oil production before the Sharara outage stood at 1.2 MMbbl/d to 1.3 MMbbl/d, NOC chief Mustafa Sanalla said earlier this month.

NOC operates Sharara in partnership with Spain’s Repsol SpA, France’s Total SA, Austria’s OMV and Norway’s Equinor ASA.