Kazakhstan's biggest oil field Tengiz, operated by U.S. major Chevron, boosted output to a record high in October, sources told Reuters, potentially complicating the country's future efforts to comply with its OPEC+ quota.

OPEC+ has named top 10 global oil producer Kazakhstan along with Iraq and Russia as countries that have repeatedly failed to comply with its pledges to curb oil production this year.

Tengiz boosted daily production to 699,000 bpd in early October from 687,000 bpd in September, when output increased by 30% from August after the completion of maintenance, said two industry sources familiar with the data.

The field's operator Tengizchevroil, which has invested more than $70 billion since the project's inception in 1993, said its operations were continuing as usual and declined further comment.

Chevron owns a 50% stake in the venture. Exxon Mobil controls 25%, KazMunayGaz has a 20% stake, and Russia's Lukoil owns 5%.

The Kazakh energy ministry did not reply to a request on comment about oil production plans for 2024 and 2025.

Kazakhstan - which relies on Tengiz and two other major fields, Karachaganak and Kashagan, for most of its production - is subject to output targets as a member of OPEC+, an alliance of OPEC and other top producers led by Russia

The country's oil production quota under the OPEC+ deal stands at 1.468 million bpd, a target it exceeded in September by around 170,000 bpd, according to Reuters calculations.

It is likely to remain within its targets this month because it will shut down the Kashagan field for maintenance, sources said.

Overall October production data from Kazakhstan is not yet available, but sources said Karachaganak will produce its regular volumes of 228,000 bpd, while maintenance at Kashagan will entail a complete stoppage of its 400,000 bpd facility.

While that means Kazakhstan will be able to achieve its October quota, the sources said, complying with OPEC+ quotas might become problematic again when the field returns from maintenance in November.

"Taking into account the expansion of Tengiz, compliance with the quota could become impossible," one of the sources said.

Chevron and its partners plan to expand output at the Tengiz project to 850,000 bpd in the first half of 2025. Expansion costs at the project stand at around $49 billion.

OPEC will release estimates of its members' September oil output next week.

The group's leader Saudi Arabia has repeatedly called on rival producers to improve compliance, saying it was the most paramount immediate task before OPEC+ embarks on releasing more barrels from December.

(Reporting by Reuters; Editing by Jan Harvey)