Vestland Group (new)
(From NPD Bulletin no. 3)
Name
Derived from the name used to denote the western part of Norway.
Type area
The group is widespread in the southern parts of Norwegian waters. It occurs in the southern Viking Graben, the Central Graben and the Norwegian-Danish Basin, but it is thin or absent over structural highs.
The group is illustrated in Norwegian wells
9/4-3 (Conoco) and
15/9-2 (Statoil).
Thickness
The thickness of the group varies considerably. Thick sections with more
than 450 m of Vestland Group are penetrated in the southern Viking Graben.
The thicknesses in the above selected wells are 123 m (9/4-3) and 216 m (15/9-2).
Lithology
The lower part of the group consists of a predominantly sandy deltaic sequence with shaly and silty layers and coal horizons. In the deeper parts of the basins the sequence seems to be influenced by marine conditions with thicker and more homogeneous sands.
The upper portion of the group generally consists of fairly clean, marine sands with minor shale interbeds.
Boundaries
The lower boundary of the Vestland Group is easily picked where it overlies
Lower Jurassic shales of the Dunlin Group or
Fjerritslev Formation. Where these shales are absent, the boundary may only show a slight increase in the gamma ray response and an overall increase in velocity marking the appearance of Triassic sediments. At this point the lower velocity peaks of the coal layers disappear suddenly and high velocity peaks from anhydrite, dolomite and limestone in the Triassic are often seen below the boundary.
The upper boundary is easily picked where the sandy sections pass into overlying shales giving clear log breaks on both sonic and gamma ray logs.
Distribution
The Vestland Group is widely distributed in the southern part of the
Norwegian North Sea. It passes northwards into the Brent
Group, north of the Frigg Area. The presence of a basal marine sandstone is
considered to be the distinguishing feature of the Brent Group.
The Vestland Group includes sediments formerly assigned to the Haldager
Formation by Deegan and Scull (1977).
Age
The group ranges from Bajocian to Volgian, or possibly Ryazanian.
Subdivision
Five formations are recognized within the group, namely the
Sleipner, Hugin,
Bryne, Sandnes, and
Ula Formations.
