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.

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