30/6-20

General

Well 30/6-20 was drilled on the Lambda structure west of the Oseberg Field in the North Sea. Lambda, Alpha North, Theta and Theta South are all rotated fault blocks with the Jurassic sequence dipping towards the east-northwest. The primary objective was to prove hydrocarbons in the Statfjord Group up-dip of well 30/6-16 Theta. The Statfjord Group is truncated by the base Cretaceous Unconformity to the west, and by faults to the south and east. Planned TD was ca 40 m into the Triassic Hegre Group, at a depth of ca 3025 m RKB.

Operations and results

Wildcat well 30/6-20 was spudded with the semi-submersible installation Treasure Scout on 10 March 1986 and drilled to TD at 3046 m in the Early Jurassic Statfjord Group. A pilot hole was drilled from 222 m to 620 m due to a shallow gas warning at 363 m. A thin sand with shallow gas was penetrated at this depth without creating any problem. Drilling proceeded without significant problems, but was delayed ca 8 days by bad weather. The well was drilled with sea water and viscous pills down to 620 m and with KCl/polymer mud from 620 m to TD.

The Cook Formation came in at 2747 m, and Statfjord Group at 2937 m. The seismic reflector that was interpreted as top Statfjord Group proved to be top Cook Formation. This implies that the fault bounding to the east has a larger throw than first assumed. Both Cook Formation and Statfjord Group were found water bearing, but with some oil shows in the Cook Formation. RFT data indicated pressure communication between Cook and Statfjord. The first oil show in the well was described on claystone at 1990 m in the Balder Formation. Oil shows in thin limestones and claystones were seen also in the Shetland Group.

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