Aberdeen oil firm Siccar Point Energy has started drilling a final appraisal well on the Cambo field, north-west of Shetland.
The Cambo field, which lies around 75miles north-west of Shetland, contains at least 600million barrels of oil, according to Siccar.
Results of the appraisal well are expected in the summer.
Meanwhile, the company has confirmed it has completed a sale of various working interests in the Greater Cambo Area to Shell.
The deal was first announced at the end of March, however details have not been disclosed on the value.
The energy giant has now acquired a 30% non-operated working interest in the P1028 and P1189 licenses, including the Cambo discovery.
Shell also holds a 22.5% interest in the P1830 license, including the Blackrock prospect where an exploration well is planned to be drilled next year.
Jonathan Roger, CEO of Siccar Point said: “We are pleased to have reached this important milestone on Cambo with work now underway on the final appraisal well.
“The partnership with Shell brings further extensive subsurface and development experience, supporting the expertise of the Siccar Point team as we progress towards an anticipated field development next year.”