CNOOC has confirmed that it has started drilling at the Howick prospect west of Shetland.
An exploration well was drilled at the site on August 31 by the Island Innovator semi-submersible rig.
Howick is 100% owned by CNOOC, with drilling expected to last about 49 days.
Meanwhile the firm said operations are ongoing at the Cragganmore appraisal well, also in the west of Shetland, which is spudded in July.
Diamond Offshore’s Ocean GreatWhite rig is currently stationed there.
CNOOC, then Nexen, said last year it has “large volume potential” with “uncertain extent and reservoir quality”.
The prospect lies nearby to Total’s Glenlivet and Edradour fields, as well as its flagship Laggan-Tormore development.
Cragganmore was discovered in 2013 by Dong Energy.
The discovery is operated by CNOOC with a 70% stake, while the remaining 30% is held by Ineos.
A CNOOC spokesman said: “Well operations are ongoing at the Cragganmore appraisal well.
“The Howick exploration well was spudded on 31 August and operations are going.”
In January CNOOC made the largest North Sea discovery in a decade with Glengorm in the central North Sea, holding up to 250million barrels of oil equivalent.