Britannia Sandstone Formation

updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman,  Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016

Cromer Knoll Group

Introduction

The term Britannia Sandstone Formation was introduced by Johnson & Lott (1993). The formation was formerly known as the Kopervik Formation (informal in oil company reports; Bisewski, 1990; Guy, 1992), the Bosun Sands (informal oil company reports; Andrews et al., 1990) and Bosun Sand Member, Sola Formation (Crittenden et al., 1991).

Name

Named by Johnson & Lott (1993) after the Britannia Field in block 15/30, where the formation is a hydrocarbon reservoir.

Lithology

The formation comprises sandstones with interbedded mudstones. Sandstones are pale grey or tan and mainly fine- to medium-(locally coarse) grained.  They vary from hard to friable, with patchy calcareous cement.  Interbedded mudstones are typical of the Carrack and Valhall formations, into which they laterally pass.

Thickness

Up to 180m in the Fisher Bank Basin (Johnson & Lott, 1993).

Geographical distribution

The formation occurs in the southeast of the Outer Moray Firth, extending westwards into the Glenn Horst, and is most thickly developed in the Fisher Bank Basin (Quadrants 15-16, 21-22) of Central North Sea. The formation onlaps the Fladen Ground Spur, which is believed to have been a source area for the sands, along with the Glenn Horst and Jaeren High (Johnson & Lott, 1993).

 

Type well

Well name: 15/30-9

WGS84 coordinates: Lat. 58° 03’44.162”N   Long. 00° 56’ 42.070”E
UTM coordinates:
UTM zone: 31
Drilling operator name: Conoco (UK) Limited
Completion date: 02.09.1991
Status: P & A
Interval of type section & thickness in type well: 4027-4124.5 m below KB (13212-13764ft) (Johnson & Lott, 1993).

 

UK Reference wells

Central North Sea (Johnson & Lott, 1993)

16/26-16: 3995-4195.5 m (13106-13764ft)
21/2-6: 3342-3450 m (10964-11319ft)
22/3a-1: 4140-4200 m (13583-13779ft)

 Lat. 58° 02’18.42”N
Lat. 57° 58’ 07.184”N
Lat. 57° 58’50.05”N		 Long. 01° 07’ 06.45”E
Long. 00°12’ 29.786”E
Long. 01° 29’ 21.68”E

 

Upper and lower boundaries

Upper Boundary

The upper boundary is normally defined by the downward change from mudstones (Carrack Formation) to sandstones (Britannia Sandstone Formation). Locally the Britannia Sandstone Formation is overlain by the Valhall Formation (e.g.22/1a-3, Johnson & Lott, 1993)

Lower Boundary

The base is usually marked by a downward change from sandstones (Britannia Sandstone Formation) to mudstones and chalky mudstones (Valhall Formation). Locally (e.g. Well 16/27a-2, Johnson & Lott, 1993), the Britannia Sandstone Formation rests unconformably on the Valhall Formation.

Well log characteristics

The thick, massive sandstones display a blocky wireline log signature; the thinly interbedded sandstones and mudstones are characterised by serrated motifs and the upward fining units have gradually upward increasing gamma values. On wireline logs, the upper boundary is shown by low velocity of the mudstones to higher velocities of the sandstone and a downward decrease in gamma-ray values. The base of the formation corresponds with a downward increase in gamma-ray values and a decrease in velocity.

Biostratigraphy

The formation generally yields abundant palynofloras. The Cerbia tabulata biomarker occurs in the upper part of the formation, whereas Ctenidodinium elegantulum, Heslertonia heslertonensis, Hystrichodinium ramoides, Batioladinium longicornutum and Pseudoceratium anaphrissum biomarkers occur in the lower part. A diverse fauna of agglutinated foraminifera are present at the top of the formation, where the Verneuilinoides chapmani biomarker is recognized (Johnson & Lott, 1993).

Age

Mid Barremian to late Aptian

Correlation

The formation passes laterally into the argillaceous Carrack and Valhall formations.

Depositional environment

The formation consists largely of marine mass-flow sandstones and interbedded marine mudstones. The presence of small amounts of skeletal debris and glauconite indicates the sands were derived from contemporaneous shallow shelf sands (Downie & Stedman, 1993). Guy (1992) interpreted much of the sandstone succession as representing high and low density turbidites, whereas Downie & Stedman (1993) postulated emplacement mainly as debris flows or liquefied deposits.

Subdivisions

In the Kilda Field (Block 16/26), the formation has been divided into six informal sandstone facies (Guy, 1992), but Johnson & Lott (1993) informally subdivide the formation into two (Upper and Lower).