An appraisal well was spudded by Byford Dolphin on 16 November at the southern end of Statoil's Mikkel gas find in the Norwegian Sea.
The outcome of this operation will be crucial for a decision by the group and its partners to continue work on a development plan for the field.
Earlier drilling has proven about 13 billion cubic metres of recoverable gas in Mikkel, which is not sufficient to sustain profitable production.
"We're hoping the appraisal well will prove enough additional gas to bring the recoverable total to 20-25 billion cubic metres," says Yngve Vassmyr, vice president for the Halten Bank/Nordland areas of the Norwegian Sea.
"That would give us sufficient resources to continue work on this promising development project."
Plans call for the rig to drill to a depth of roughly 2,800 metres, and the whole operation is expected to take just under 40 days.
Statoil will be applying in December to Norway's Gas Supply Committee to have Mikkel allocated to a gas sales contract, subject to the new well providing a basis for profitable development.
The field could be developed using a straight subsea solution, tied back either to Statoil's Åsgard field or to Norske Shell's Draugen platform.
Investment is put at roughly NOK 2 billion, with a start to production in 2002-2003.
Mikkel spans two production licences, PLs 092 and 121. Mobil has 40 per cent of PL 092, the state's direct financial interest (SDFI) 30, Statoil 20 and Saga Petroleum 10. Interests in PL 121 are 40 per cent for the SDFI, 20 for Mobil, 20 for Norsk Hydro, 10 for Statoil and 10 for Saga.