The Valhall Field: a geological overview

Abstract The Valhall Field is located within the Central Graben in the southern part of the Norwegian North Sea. The discovery well, 2/8-6, was drilled in 1975 and the field went on production in 1982. Production is currently 55 000 barrels of oil per day. The Valhall Field structure trends NW-SE and has an areal closure in excess of 60 000 acres. The hydrocarbon column measures ∼200 m. The structure is located on an elongate anticlinal feature called the Lindesnes Ridge, which developed during Late Cenomanian to Oligocene tectonic movements. The geophysical evaluation of the field is complicated by the existence of a gas-charged Tertiary section which obscures the deeper seismic reflections. Vertical seismic profiles (VSPs) have been utilized in an attempt to resolve the reservoir configuration. Reservoirs occur both in the Tor Formation (Maastrichtian), and in the underlying Hod Formation (Turonian-Coniacian). The Tor Formation is represented by a largely allochthonous chalk facies which thins regionally to the southwest. A crestal graben is developed in which the Tor Formation is locally thick. Autochthonous chalks are characteristic of the Hod Formation. Primary porosity of the chalk varies between 36 to 50% on the crest of structure. Preservation of such high porosity is primarily due to formation overpressure inhibiting mechanical compaction of the chalk. The field contains paraffinic oil of 36 API gravity. It originated from the Upper Jurassic Kimmeridge Clay Formation, which is at peak oil generating capacity under the field at present day.