A BP-led consortium of oil companies will pump more than £330 million into a drilling programme which it hopes will lead to a rapid expansion of the giant Clair oilfield, west of Shetland.
The consortium of BP, Shell, ConocoPhillips and Chevron said appraisal drilling was underway on the first of five wells planned over the next two years, which could rise to 12 wells if results are promising.
It is estimated that Clair holds eight billion barrels of oil, although it has so far proven uneconomical to retrieve much of it.
Despite its discovery 35 years ago, the field only started production in 2005.
BP announced another expansion last year, and hopes the new scheme will herald a third phase.
"This is a major milestone and a further big commitment to the west of Shetland by BP and its co-venturers," said BP North Sea regional president Trevor Garlick said:
"If successful, the appraisal programme could pave the way for a third phase of development at Clair - this is now a real possibility."