(Adds details on upgraders, context)
CARACAS/HOUSTON, July 17 (Reuters) - Two of Venezuela's four crude upgraders are scheduled to undergo previously delayed maintenance in the next few weeks, according to three sources close to the facilities, further reducing state-run PDVSA's exports of upgraded crude.
The upgraders, which convert the extra-heavy oil produced in Venezuela's Orinoco Belt into exportable grades, have a combined 700,000 barrel-per-day (bpd) capacity.
The maintenance projects, including one that had been expected to begin in April, have been deferred due to lack of spare parts and outages. The work temporarily will reduce the amount of upgraded oil available, but will help ease tanker congestion at the country's main port of Jose.
The OPEC-member's oil production fell to 1.34 million bpd in June, according to secondary sources cited by OPEC, the lowest level since the 1950s. Oil is Venezuela's main export and the decline has deepened an already severe economic crisis.
The 150,000-bpd Petromonagas upgrader, operated by state-run PDVSA and Russian joint venture partner Rosneft, is expected to start major maintenance work by early August, according to the sources.
Repairs to the 210,000-bpd Petropiar upgrader operated by PDVSA and U.S. company Chevron Corp will also start in coming weeks, the sources said. There was a fire at that facility in June during its restart process.
When the upgraders are out of service, extra-heavy oil is blended with imported naphtha to produce diluted crude (DCO). But the volumes of DCO exported typically are lower than the upgraders' capacity.
In a meeting this week with some Orinoco Belt partners, Venezuelan officials complained about delayed maintenance affecting the upgraders, which has led to more exports of less-valuable DCO, according to a person that attended the meeting.
PDVSA did not reply to a request for comment.
Venezuela's oil exports this year have been limited due to output declines. Asset seizures by ConocoPhillips to enforce an arbitration award have also crimped sales by depriving PDVSA of most of the Caribbean terminals it once used to store and ship oil.
As the two upgraders are taken out of service for maintenance, PDVSA aims to restart the 160,000-bpd Petro San Felix upgrader in the coming weeks, after several interruptions in recent months for repairs.
The 190,000-bpd Petrocedeno, operated by PDVSA, Total and Equinor, is working at reduced rates due to repairs and spare parts. A major maintenance project there is expected to start in early 2019, the sources said. (Reporting by Deisy Buitrago and Marianna Parraga)