25/5-4

Well 25/5-4 is located approximately in the centre between the 5/5-5 Discovery and the Frøy and Skirne Fields in the northern North Sea. The prospect was defined at Intra Statfjord Formation level, consisting of two compartments offset by an E-W fault. No significant closure was expected at Base Cretaceous. The main target was the Middle Jurassic Vestland Group sandstones. Early Jurassic Statfjord sandstones was secondary target. The expected fluid was most likely oil. No shallow gas was expected.

Operations and results

Wildcat well 25/5-4 was spudded with the semi-submersible installation Dyvi Stena on 22 December 1990 and drilled to TD at 3185 m in Early Jurassic sediments of the Statfjord Formation. Only minor problems were encountered during drilling the Hordaland Group. The well was drilled with sea water and bentonite down to 1208 m, with a KCl mud from 1208 m to 2805 m, and with lignosulphonate mud from to 2805 m to TD.

The results for the main seismic markers were close to prognoses (from 17 to 36 m higher than prognosis). Sandy water bearing reservoirs were encountered in the basal Tertiary Heimdal and Ty Formations. The Vestland Group reservoir (Hugin + Sleipner Formations) was encountered from 2895 to 2941 m and was completely filled with a retrograde condensate. Extracts of core material showed a significant increase in extractable organic matter below 2932 m, and the EOM was significantly more oil-like (saturates > 55%) than the upper part of the reservoir. The Statfjord Formation proved good reservoir quality but was water bearing. Organic geochemical data proved excellent source rock characteristics in several levels throughout the Jurassic, with oil prone kerogen in the Draupne and Heather Formations, and gas - condensate prone kerogen in the Vestland Group and Statfjord Formation. The source potential of Draupne is similar to the one known in the surrounding wells, while the Heather has better characteristics. The extreme base