Note on the Chalk group nomenclature in the UK Offshore sector

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The development of the terminology in the offshore UK Sector. P895014.

Investigations into the hydrocarbon prospectivity of the North Sea has led to the development from an early, essentially down hole geophysical log and seismic, stratigraphy into a more refined lithostratigraphical nomenclature based not only on geophysical data but on biostratigraphy, cuttings and type cores. In the UK sector the first comprehensive lithostratigraphical schemes were established by Rhys (1974) [Fig.11/1] in the Southern North Sea and by Deegan and Scull (1977) [Fig.11/2] in the Central and Northern North Sea (east of the Shetland Islands and southwards). The terms, developed in those early studies, have been expanded with a considerable body of stratigraphical evidence and piecemeal additions were made such that the meaning of many lithostratigraphical terms became blurred. The BGS under the auspices of the UKOOA (UK Offshore Operators Association) published a rationalisation of this nomenclature in seven volumes (under the editorship of Knox and Cordey) with volume 2 (Johnson and Lott, 1993) [Fig.11/3], and volume 7 (Lott and Knox, 1994) [Fig.11/4] covering the Cretaceous strata of the Central and Northern North Sea, and the Southern North Sea respectively.

Three similar volumes were produced by BGS to cover the UK Sector North West Margin with volume 1 giving a correlation of the pre-Tertiary lithostratigraphy (Ritchie, Gatliff and Riding, 1996) [Fig.11/5]. This covered the area west of the Shetland Islands southward to the Orkneys Islands.

The most recent review of the stratigraphy of the offshore sector is contained in The Millennium Atlas: petroleum geology of the central and northern North Sea published by the Geological Society of London in 2002 (Surlyk et al., 2003). This contains a comprehensive overview of the stratigraphy within the Danish, Norwegian and UK sectors of the hydrocarbon producing regions of the North Sea, north of latitude 55˚ 20´ N, and updates the UKOOA and BGS volumes.

Data is also held within the Offshore Regional Reports of the BGS published between 1990 and 1996 although in general the terminology used reflects their compilation prior to the UKOOA volumes.

A detailed discussion is beyond the scope of this report but a correlation of the lithostratigraphical units given in the UKOOA publications is given in Figure P895014 for completeness. Two aspects of the offshore terminology should be noted.

The Hunstanton Formation is considered to be of Albian age and equivalent to the Rødby Formation in the Cromer Knoll Group.

In general the Hidra Formation and its lateral equivalents correlates roughly to the Grey Chalk Subgroup or Ferriby Chalk Formation with the Plenus Marl Formation and/or the Black Band at the base of the Herring Formation (and lateral equivalents) marking the base of the White Chalk Subgroup. However it should be noted that the use of the terms Plenus Marl Formation and Black Band within the offshore sector is confused and they are not necessarily mutually exclusive. Indeed in correlation charts the Black Band is shown spanning the Cenomanian/Turonian boundary and must therefore include the entirely Cenomanian Plenus Marls Member as defined onshore. It follows therefore that where the Black Band defines the base of the Herring Formation this may or may not include the Plenus Marls succession and similarly the Plenus Marl Formation may encompass the Black Band. It is quite possible therefore that the topmost part of the Hidra Formation should be considered as equivalent to the very basal White Chalk Subgroup.