High-resolution biostratigraphy and sequence development of the Palaeocene succession, Grane Field, Norway

Abstract

Abstract A detailed biostratigraphic zonation and sequence stratigraphic framework has been developed for the Grane Field (PL 169), situated in Block 25/11 in the North Sea. The study is focused on the Upper Palaeocene interval, where a high level of chronostratigraphic resolution has been achieved and where the depositional history can be understood in terms of a finite set of depositional cycles and subcycles. This work documents that the Grane Field reservoir succession, which consists of alternating turbidite sandstones and marine basin plain mudstones, fall within two main biozones, corresponding to a general Late Palaeocene age. The reservoir succession is developed between two regional datums: last stratigraphical common occurrence of A. gippingensis and last stratigraphical occurrence of Isabellidinium? viborgense . This corresponds to our major cycles T40 and T50, in which the reservoir sands comprise lowstand units filling in local accommodation during stepwise encroachment of the submarine fan system into this area. Within the reservoir interval the zonation is based on the appearance and extinction of species with a regional distribution that also have marked frequency variations. These include A. gippingensis, P. bulliforme, C. striatum, P. pyrophorum and I? viborgense . In addition, events representing quantitatively marked peaks in the frequency of long-ranging taxa are used for correlation purposes. These events need to be constrained by other events, but are good for local correlations. Examples from the Grane Field include influxes of Deflandrea denticulata, Cometodinium comatum and Subtilisphaera spp. The high-resolution biostratigraphy serves as a template for the sequence stratigraphic framework in the study area. Sixteen correlatable zones have been established by linking each of the fossil events to a specific sequence stratigraphic surface. As shown by previous workers, surfaces associated with condensation (regional and subregional maximum flooding surfaces) prove to be the most useful ones for correlating the Tertiary of the North Sea. In addition, some of the biostratigraphic events seem to coincide with transgressive surfaces and sequence bounding unconformities. Application of the sequence stratigraphic framework indicates that several reservoir sand units, which appear correlatable on well logs, actually are of different age. They may therefore contain potential flow barriers, formed in periods of condensation between the lowstand events.