A sequence stratigraphic study of lower cretaceous deposits in the northernmost North Sea

Abstract

The prospectivity of Cretaceous deposits in the northern Norwegian North Sea, particularly in the Agat area, has been evaluated using a sequence stratigraphic approach. The study integrates high-resolution biostratigraphic data, biofacies analyses, well logs, core description and seismic data, all incorporated with the tectonic history of the basin. Two supersequences, K1-1 and K1-2 are informally defined within the Lower Cretaceous succession. Supersequence K1-1 ranges from Ryazanian to late Barremian age, whereas Kl-2 has an Aptian to late Albian age. Palaeoenvironmental maps for the two supersequences, using the biofacies analyses from the wells, suggest that Kl-1 supersequence was deposited mainly in a shelf environment, mostly within highstand and transgressive systems tracts. Supersequence Kl-2, on the other hand, was deposited largely in a bathyal environment and contains more prominent lowstand systems tracts. A more detailed sequence stratigraphie subdivision is suggested within the two main genetic units. The prospective deposits within the Lower Cretaceous interval are the Asgard Sand Unit (Hauterivian-Barremian), a highstand systems tract, and the Agat Formation (Albian), a lowstand systems tract. Within the Agat area, seismic anomalies/mounds have been recognized on an Intra-Albian unconformity. The sedimentological analyses of cores from the Agat Formation indicate, in contrast to published interpretations of depositional environments, that this unit was deposited on an upper slope environment by slump/mass flow processes, causing the sand bodies to be of laterally restricted extent. A well correlation, based primarily on the identification of maximum flooding surfaces, allowed the Agat Formation to be subdivided into at least three stacked sandy units of lowstand origin. Reservoir quality and seal capacity are the major risks in the evaluation of the proposed Agat lead.