25/4-1

General

Well 25/4-1 is the discovery well on the Heimdal Field. The primary objective was Paleocene sand development (which was confirmed by the well), while Jurassic sands were regarded as a secondary objective. Planned depth was 3500 m.

The well is Type Well for the Heimdal Formation.

Operations and results

Wildcat well 25/4-1 was spudded with the semi-submersible installation Neptune 7 on 1 July 1972 and drilled to TD at 4060 m in the Triassic Smith Bank Formation. From 2600 m to 3580 m the well built some angle (maximum 5 deg), resulting in a 3 m deviation between measured and true vertical depth at 3580 m. In addition, logger's depth is 6 m deeper than driller's depth from seabed to 3525 m. Below 3525 m driller's depth is equal to logger's depth. This history quotes logger’s depth if not otherwise stated. While drilling at 3178 m the well started to flow and a lost circulation situation followed. The situation was adequately dealt with. The well was drilled water based all through.

The well penetrated a 356 m thick Heimdal Formation from 2067 m to 2423 m. The reservoir was composed of more or less unconsolidated sands with interbedded shales and carbonates. The formation was gas filled down to a gas/oil contact at 2173 and contained oil in a thin zone down to a shale at 2177 m. The OWC could not be seen in the well. The upper Cenomanian rested unconformably on a 14 m thick Oxfordian/Callovian Draupne Formation sequence, which in turn rested unconformably on the Middle Jurassic Vestland Group at 3185 m. Several permeable reservoirs were penetrated from 3185 m to 3512.5 m (3179 m to 3506.5 m drillers depth) in the Middle Jurassic to Late Triassic. Four of these were hydrocarbon bear