A significant satellite: the Nevis South Field in Block 9/13

Abstract

The Nevis South Field is an outstanding field in a number of respects. It contains four stacked reservoirs, two contrasting oil types, a high quality Triassic reservoir and a Beryl reservoir of such quality that one of the producing wells held the North Sea’s record for productivity index. The field is operated by Mobil North Sea Limited (ExxonMobil) with co-venturers Amerada Hess Limited, Enterprise Oil Limited and OMV (UK) Limited. It is a sub-sea development tied back to the Beryl Alpha platform that lies 7 km to the east-northeast. First oil was produced in 1996 but the field is still actively being drilled to new reservoirs and to maximize recovery from already producing reservoirs. Structurally the field is a relatively simple rotated fault block with beds dipping to the southwest and a crestal depth of 8700 ft. The Base Cretaceous Unconformity cuts down into the crest of the fault block such that strata are progressively eroded towards the northeast. Consequently, the core of the field contains Triassic strata with younger, Jurassic, strata on the flank. The oldest reservoir in the field is the Triassic Lewis I sequence of fluvio-lacustrine origin. It contains an unusual heavy oil which is currently being produced via a single, high-angle well. Above this unit lies the Lewis III/IV reservoir which contains the bulk of the hydrocarbons in the field. These sediments are ephemeral stream deposits which are exploited via four producers and a pair of water injectors. The Middle Jurassic Beryl reservoir is a high net:gross tidal delta succession of multi-Darcy permeability. Drainage of the Beryl oil rim is by three horizontal wells with pressure support supplied by the large aquifer. A Beryl gas blowdown phase of development is scheduled for 2005. Drilling and workover activity is guided by fine-scale geological modelling and accompanying simulation. The youngest reservoir comprises mass-flow sandstones of the Middle–Late Jurassic Heather Formation. These relatively thin sandstones contain a comparatively small volume of oil and gas which it is currently uneconomic to develop.