Statfjord Formation
(From NPD Bulletin no. 3)
Name
Named by Deegan and Scull (1977)
Well type section
Norwegian well 33/12-2 (Mobil), from 2700 m to 2951 m, coord N 61° 13'31.18", E01o51'25.97", (Fig. 6).
Well reference sections
UK well 211/24-1 (Conoco) from 3112 m to 3434 m, coord N 61° 11'46.2", E01°46'26.3", (Fig. 7), and Norwegian wells 30/6-1 (Statoil), from 2712 m to 3003 m, coord N 60°33' 15.10'' E 02°46'38.36", (Fig. 8), and 25/2-5 (Elf) from 3652 m to 3847 m, coord N 59°48'01.40", E02°28'18.30", (Fig. 9).
Thickness
251 m in the type well, 322 m in UK well 211/24-1, 291 m and 195 m in Norwegian wells 30/6-1 and 25/2-5 respectively. The formation is thinner on the crest of fault blocks and thicker on the downthrown side. It attains its fullest development in the central part of the Viking Graben. To the west, e.g. in the area of the Cormorant Field, only the upper member is present and this is reduced to a sandstone only a few metres in thickness. To the east, toward the bounding fault zone of the Fennoscandian Shield, the formation is reduced to tens of meters in thickness.
Lithology
The formation exhibits a transition from continental to shallow marine sediments. In the type well area it is a transitional "coarsening upward" sequence in the basal parts consisting of grey, green and sometimes red shale inter-bedded with thin siltstones, sandstones and dolomitic limestones. Above are massive white to grey sandstone bodies interbedded with greenish-grey to red-brown shales. The top part of the formation consists of thick, white to grey, fossiliferous and glauconitic sandstones. On the Horda Platform, east of the Viking Graben, the formation consists of massive, white, fine to coarse grained sandstones interbedded with light grey, and sometimes red, silty micaceous, lignitic shales. Towards the east the frequency of black, coaly shales and coal layers increases.
Boundaries
The base of the Statfjord Formation is very difficult to define, and different operators have established different boundaries. In the type well area the proposed boundary marks the base of the thin transitional unit (coarsening upward sequence), which marks the passage from the more shaly Lunde Formation of the Hegre Group to the massive sandstones of the Statfjord Formation. It should be emphasized that this transition can only be clearly recognized in some wells in the area of the Brent and Statfjord Fields. Towards the east, e.g. in the Viking Graben and the Horda Platform, the lower boundary is chosen at the base of the lowest massive sandstone unit before passing downwards into dominantly red brown shales.This boundary is often associated with a sonic log break.
The top of the formation is at the contact between the uppermost medium to coarse-grained calcareous sandstones and the dark shales and siltstones of the overlying Dunlin Group. It should be noted that the top part of the calcareous sandstones in the UK sector (e.g. in the Brent Field and in UK part of the Statfjord Field), passes laterally into calcareous shales and siltstones in the Norwegian sector. The formation boundary, being a lithological one, however, is placed at the top of the sandstones, regardless of age.
Distribution
The Statfjord Formation can be recognized in the entire area between East Shetland Platform to the west and the bounding fault zone of the Fennoscandian Shield to the east. The formation is identified in the Viking Graben as far south as Norwegian blocks 25/8 and 11 (e.g. in wells 25/8-1 and 2 (Esso)).
Age
The formation ranges in age from Rhaetian to Sinemurian.
Depositional environment:
The lower transitional unit in the type well area
appears to represent an upward passage from the dominantly continental deposits of the Liin-de Formation of the Hegre Group to lower alluvial plain and braided stream deposits which make up most of the Statfjord Formation (Kirk, 1979, Chauvinand Valachi, 1980). Towards the top of the formation coarse sandstones with pebble beds, crossbedding and channel structures appear to have been deposited in a coastal environment. The uppermost sandstones are relatively structureless but the presence of marine fossils and glauconite suggests a shallow marine environment (Deegan and Scull, 1977).
Subdivision:
The Statfjord Formation is divided in the type well area (Statfjord Field) into three members, the Raude Member (base) the Eiriksson Member and the Nansen Member (top). (Deegan and Scull, 1977). It should be emphasized that this subdivision can only be applied west of the Viking Graben. Even within this area application of the subdivision is often difficult. No workable subdivision has been established in the few well penetrations of the Statfjord Formation east of the Viking Graben. The currently limited scope for subdivision of the Statfjord Formation indicates that it would serve no purpose to elevate the unit to group status (c.f. the underlying Hegre and overlying Dunlin Group). For definition and detailed description of the members of the Statfjord Formation, see Deegan and Scull (op. cit).
