The Cook Formation, an offshore sand ridge in the Oseberg area, northern North Sea

Abstract

In the Oseberg area, the early Jurassic Cook Formation has been subdivided into three units. The lower unit (Cook A) was deposited as a prograding subtidal sand body and the upper unit (Cook C) was deposited as an offshore sand ridge. A thin mudstone unit (Cook B) separates these two sandstone units. The Cook Formation is enclosed by mudstones in which resistivity log peaks have a high degree of correlatability. The stratification visualized by the log correlation provides useful information, revealing the sedimentary history of the Cook Formation; in particular, it is used as a chronostratigraphic framework to describe synsedimentary fault activity at the margin of the Viking Graben. The Cook A Unit thickness from 0 m in the west to 45 m in the east, implying a clastic source area at the eastern margin of the northern North Sea and deposition in response to marine regression. The Cook C Unit shows maximum thicknesses (up to 37 m) centred upon the palaeostructural highs at the margin of the Viking Graben and defines N-S-oriented sand bodies. The sand bodies are interpreted as offshore sand ridges deposited during the marine transgression associated with fault block crest erosion. Subsequent sea-level rise resulted in rapid deepening of marine conditions and the establishment of the lowe-nergy mudstone depositional environments of the Drake Formation. Passive mudstone infilling of the lows levelled the relief created by the sand ridges.