Oil production at the Kashagan oilfield has fallen to 180,000 barrels per day from 300,000 barrels per day around March 20, three industry sources told Reuters.
It was not immediately clear what the reason was for the sharp fall in daily production.
"It should be maintenance," - one source said, but added that it was not planned.
North Caspian Oil Consortium (NCOC), the operator of the Kashagan oilfield, has not answered a request for comment.
Oil production at Kashagan reached 300,000 barrels per day in January this year, while a rise in output to 370,000 barrels per day is expected during 2018.
Kashagan crude is exported via the Caspian pipeline consortium (CPC). In April CPC plans to export 5.08 million tonnes of CPC Blend.
The Kashagan consortium includes Kazakh
state oil firm KazMunayGaz, Eni,
ExxonMobil,
Royal Dutch Shell (RYDAF), Total, China's CNPC and Japan's Inpex.
(Reporting by Olga Yagova and Alla Afanasyeva, writing by Olga Yagova, editing by David Evans)