FILE PHOTO: The logo of the Organization of the Petroleum Exporting Countries (OPEC) is seen inside its headquarters in Vienna, Austria, December 7, 2018. REUTERS/Leonhard Foeger/File Photo

By Ahmad Ghaddar and Alex Lawler

LONDON (Reuters) -OPEC+ is likely to leave output levels unchanged at its meeting on Sunday while focusing talks on a theoretical topic of how much oil its members can produce so the group can decide future policies, three OPEC+ sources said.

OPEC+, which includes the Organization of the Petroleum Exporting Countries and allies such as Russia, pumps about half the world's oil and has been discussing for years production capacity figures - or baselines - against which members' output targets are set.

Ministers on Sunday are expected to discuss a mechanism to assess countries' maximum production capacity, to be used as reference for 2027 output baselines, OPEC+ sources said. OPEC+ discussed the issue in September at a technical level.

The group has struggled to find a compromise with some members such as Nigeria, which want higher output quotas while having limited unused capacity, according to assessments such as the International Energy Agency.

On the other hand, the United Arab Emirates still holds plentiful spare capacity after it secured this year a small rise in its share of OPEC+ output. Angola quit the group in 2024 over a disagreement on its production target.

Any quota changes on Sunday would need to be approved by the full group, one OPEC+ source said.

OPEC+ had been reducing output for several years until April when eight members began to raise production to recover market share. The cuts had peaked in March, amounting to 5.85 million barrels per day, almost 6% of world output, in total.

The eight members have raised output targets by around 2.9 million bpd from April to December and, at their last meeting in November, paused hikes for the first quarter amid predictions of a looming oversupply.

OPEC+ is unlikely to make any changes to its first-quarter output policy on Sunday or to consider changes to group-wide 2026 output levels, agreed at its last meeting in May, the three sources said.

(Reporting by Alex Lawler and Ahmad Ghaddar, editing by Dmitry Zhdannikov and Tomasz Janowski)