7121/4-1

General

Exploration well 7121/4-1 was drilled in the Hammerfest Basin in the Troms I area. The primary objective was to test possible hydrocarbon accumulations in sandstones of Middle to Early Jurassic age.

Operations and results

Well 7121/4-1 was spudded with the semi-submersible rig West Vanguard on 6 August 1984 and drilled to TD at 2609 m in Late Triassic sediments (Fruholmen Formation). Loss of circulation occurred at TD in 12 1/4" section at 2285 m, and several times during drilling of 8 1/2" hole but otherwise operations went without significant problems. The well was drilled with spud mud down to 817 m, with gypsum/Celpol from 817 m to 2285 m, and with gel/chromium-lignosulphonate from 2285 m to TD.

Hydrocarbon accumulations were discovered in two separated sandstone sequences. The uppermost sequence between 2318 m and 2442 m in the Stø and Nordmela formations contained gas over oil, while the lower sequence was gas bearing between 2468.5 m and 2473 m in the uppermost few meters of the Tubåen Formation. The gas/oil contact in the upper reservoir was seen from RFT to be at 2425. The oil zone, 2425 m to 2442 m in the Nordmela Formation, consists of an interbedded sandstone/shale sequence with fair/poor reservoir properties. There is a sealing shale between 2444 and 2468.5 m that screens off the upper gas-oil system from the lower Tubåen gas reservoir. The lower gas zone has good reservoir properties. The sandstone continues to TD at 2587 m, interrupted by some minor shale beds and one major shale bed at 2532 m to 2560 m. Four cores were cut in the reservoir interval from 2321 m to 2416.35 m in the Middle Jurassic down into the Early Jurassic. Three segregated RFT samples were taken. For all three samples the 2 3/4 gallon chambers were bled off at well site. Sample 1 from 2365 m recovered gas, black condensate, and mud filtrate. Sample 2 from 2470 m recovered gas, brown condensate, and mud filtrate. Sample 3 from