Venezuela's Amuay and Cardon refineries are halted following a blackout in the early morning hours, two oil industry sources with knowledge of the issue said on Sunday.
The twin refineries together form the 955,000 barrel-per-day (bpd) Paraguana Refining Center, which has been operating well below capacity for years because of chronic operational problems that have been aggravated by Venezuela's economic crisis.
"Blackout in both refineries," one employee who works in the complex wrote in a text message. "It's going to be difficult to get the system back up."
State oil company PDVSA did not immediately respond to a request for comment.
Social media users in the area reported that power was out in nearby communities that rely on refinery generators for electricity.
Outages are frequent at both facilities.
Cardon, with a capacity of 310,000 bpd, halted operations in May after damage at some of its units.
The 645,000 bpd Amuay refinery was shuttered because of a technical failure in November.