The Statfjord area in the North Sea covers a few licenses, which include the Statfjord oil and gas field on the border between the Norwegian and UK sectors.
The company said Equinor gave updated information "which indicates a reduction in 2P and 2C volumes of 10% to 15% over the lifetime of the acquired assets" compared to the previous estimates.
2P stands for proven and probable reserves, and 2C stands for contingent resources.
OKEA had expected to complete the acquisition on Nov. 30 and has notified Equinor of the postponement.
Equinor was not immediately available to comment.
OKEA said preliminary assessments of the updated estimates indicated "a material reduction in fair value", which would likely require a significant impairment, or writing down of asset value, following the acquisition.
When announcing the acquisition in March, OKEA said it would add net 2P reserves of 41 million barrels of oil equivalent (boe) and net 2C resources of 8 million boe for the company.
It also expected the acquisition to add 13,000 to 15,000 barrels of oil equivalent per day in new production in 2023, rising to 16,000 to 20,000 boed in 2024.
OKEA said on Thursday it continued discussions with Equinor regarding the way forward.
(Reporting by Nerijus Adomaitis; Editing by Josie Kao)