34/7-21

General

Exploration well 34/7-21 was drilled the south-western part of block 34/7 on Tampen Spur, on the H-prospect northwest of the Tordis Field. The main objective was to test the hydrocarbon prospectivity within the Late Jurassic (Viking Group) interval. The target was a Late Jurassic sand wedge, truncated in the up-dip direction towards the east by the Base Cretaceous Unconformity. Sufficient amounts of reservoir sand were conditioned by erosion and re-sedimentation from the underlying Middle Jurassic Brent Group, which is progressively truncated up-dip in the same direction. The secondary objective was to test the Paleocene in which an oil discovery had been made in well 34/7-18.

Operations and results

Well 34/7-21 was spudded with the semi-submersible installation Treasure Saga on 19 October 1992 and drilled to TD at 3015 m in the Early Jurassic Drake Formation. Tight and sticky formation was reported in the 12 1/4" section, but no significant technical problems occurred in the operations. Based on the site survey, possible shallow gas was predicted at 298, 548 and 612 m. The MWD confirmed a gas bearing sand at 541 m - 548 m. A flow check proved negative. The well was drilled with spud mud down to 1113 m and with KCl polymer mud from 1113 m to TD.

In the Nordland, Hordaland, and Upper Rogaland Groups, the well penetrated mainly clay/claystone with minor sand, except for the sandy Utsira Formation between 929-1040 m. At the base of the Rogaland Group, the Lista Formation sandstone was encountered and proven dry, but with shows. In the Shetland and the condensed Cromer Knoll Group, claystone with limestone beds and massive marls/limestones were penetrated, respectively. A hydrocarbon-bearing interval was proven in a sand in the Cromer Knoll at 2498-2501 m. The top of the Late Jurassic reservoir was reached at 2508 m, which was 29 m shallower than prognosed. Within the Draupne Formation, two separate oil bearing sandstone i