Venezuela's massive 955,000-barrel-per-day (bbl/d) Paraguana refining center was operating at around 45% of capacity on Monday, according to an internal document from state oil company PDVSA seen by Reuters on Tuesday.
The 645,000-bpd Amuay refinery was running at about 308,000 bpd, while the adjacent 310,000-barrel-per-day Cardon refinery was at 118,500 bbl/d according to the document.
Venezuela's refineries have been plagued with blackouts, equipment issues and stoppages for years.
PDVSA often blames problems on "saboteurs" intent on bringing down socialist rule in Venezuela, and says its foes and hostile media try to exaggerate refinery issues. Critics say years of underinvestment and poor maintenance are the cause.
The Caracas-based company did not respond to a request for information on the refineries.
Meanwhile, workers were trying to restart the broadly idled Puerto La Cruz refinery, with capacity of 187,000 bbl/d, according to a worker.
But attempts since Monday night to get two crude distillation units and the catalytic cracker going have failed, he added.
"The order is to get the refinery going no matter what, but it looks like God's own hand is not letting the units operate in these bad conditions," the worker said, asking to remain anonymous because he is not authorized to speak to news organizations.
To compensate for the refining problems, U.S. refining firm Citgo Petroleum is sending more products to its parent company, PDVSA, according to sources and Reuters data.