(Adds details, context on PDVSA's operations)
June 26 (Reuters) - Venezuela's state-run oil firm PDVSA and U.S. company Chevron have begun to restart their 210,000-barrel-per-day Petropiar heavy crude upgrader after a nearly month-long repair shutdown, according to two sources close to the facility.
Venezuela's crude upgraders, which can convert near 700,000 barrels per day (bpd) of the country's Orinoco Belt extra-heavy crude into exportable grades, have been mostly out of service in recent weeks while PDVSA focused on easing a tanker backlog that has delayed exports.
The country's oil production fell to 1.39 million bpd in May, according to secondary sources cited by OPEC, the lowest level since the 1950s. Oil is Venezuela's main export and the decline has deepened a severe economic crisis.
Workers attempted to restart Petropiar earlier in June, but quality issues that were ultimately solved delayed the process, one of the sources said. The restart typically takes several days to be completed while the upgrader's performance is evaluated.
If Petropiar fully restarts in the coming days, the neighboring 190,000-bpd Petrocedeno would be the only upgrader completely shut for maintenance while the 160,000-bpd Petro San Felix works intermittently, according to the sources.
But the 150,000-bpd Petromonagas, operated by PDVSA and Russia's Rosneft, is expected to be out of service later this month due to a planned major maintenance project.
PDVSA did not immediately respond to a request for comment.
Reduced crude upgrading means PDVSA and its partners in the Orinoco Belt, the country's largest producing region, have to mix Diluted Crude Oil (DCO) for exports, but the volume of the replacement grade is typically lower.
That could help to ease a bottleneck of tankers waiting to export oil. As of June 26, there were more than 75 tankers anchored off Venezuelan ports waiting to load some 24 million barrels of crude and refined products, according to Thomson Reuters vessel tracking data, about the same as earlier this month. (Reporting by Deisy Buitrago, Mircely Guanipa and Marianna Parraga Editing by Bill Trott and Susan Thomas)