A holostratigraphic approach to the chalk of the North Sea Eldfisk Field, Norway

Abstract A biostratigraphic review, conducted on 34 wells from the chalk of the Eldfisk Field, Norwegian Central Graben, has been integrated with petrophysical, geophysical and sedimentological information resulting in a revised lithostratigraphic framework for the chalk on this structure. Chalk of Danian to Turonian age is divided into five formations: the established Ekofisk Formation of Danian age and Tor Formation of Maastrichtian age, together with a new three-fold division of the Hod Formation, namely the Magne Formation of Campanian age, the Thud Formation of Santonian age and Narve Formation of Coniacian to Turonian age. This work demonstrates the application of this three-fold division of the Hod Formation. Internal field specific subdivisions of all formations are also presented for the Eldfisk Field. This lithostratigraphic framework is applied across the Eldfisk Field, together with the recognition of erosional features, unconformities, areas of non‐deposition, reworking and lateral changes in biofacies. The results have also allowed recognition of the following regionally synchronous tectonic phases for the first time on a Norwegian chalk structure: Stille9s Ilsede phase (Late Turonian–Coniacian) and Wernigerode phase (Late Santonian–‘earliest’ Campanian), Mittel–Santon phase (Middle Santonian) of Niebuhr et al. and Reidel9s Peine phase (‘latest’ Early Campanian), together with un-named phases of ‘latest’ Campanian, intra Mid Maastrichtian and (previously unrecognized?) intra Danian age. Evidence for these tectonic phases is compared with work from Denmark, Germany and the Anglo-Paris Basin. An innovative approach to mapping lateral biofacies (principally water depth) variations has been applied using the microfaunal database. This enhances understanding of the timing of structural phases when integrated with time lines generated by nanoplankton data. Biofacies proxies for silica content in the sediment may also correlate with changes in reservoir quality. Biofacies interpretations have also facilitated the identification and mapping of allochthonous bioclastic rich debris flow deposits. The fully calibrated biostratigraphic, lithostratigraphic and tectonostratigraphic frameworks presented can be applied to chalk structures regionally.