Micropalaeontological biostratigraphy of the Magnus Sandstone Member (Kimmeridgian-Early Volgian), Magnus Field, UK North Sea
Authors: Morris P. H. , Payne S. N. J. , Richards D. P. J. 
Published: April 2008

The micropalaeontological biostratigraphy of the Magnus Sandstone Member of Kimmeridgian-Early Volgian age is described for the first time using data from 27 Magnus Field wells (blocks 211/12a and 211/7a). Despite extensive re-sedimentation within the Magnus Sandstone Member turbidite reservoir, a consistent sequence of nine bioevents is recognized field-wide. These include both regionally extensive bioevents and localized extinction and acme bioevents. The microfaunas are documented and key taxa, which include numerous radiolarian species, are described and figured.

The bioevent scheme provides the basis for the chronostratigraphic and genetic sequence stratigraphic subdivision (
sensu
Galloway) of the Magnus Sandstone Member. It allows the recognition and biostratigraphic calibration of three maximum flooding surfaces (MFS) which in ascending stratigraphic order comprise: the pre-Magnus Sandstone Member top J62-basal J63 ‘
Eudoxus
’ of Partington
et al.
(upper limit of the ‘Lower Kimmeridge Clay Formation’); the intra-reservoir top J63-basal J64 ‘
Autissiodorensis
’ (‘B Shale’); and the supra-reservoir top J64-basal J66 ‘
Hudlestoni
’ (upper limit of the ‘H Unit’).

The erection of a field-specific biostratigraphic framework that characterizes and discriminates between the individual shales associated with the Magnus Sandstone Member has impacted on reservoir modelling, primarily in the area of fluid pressure distribution, where shales such as the ‘B Shale’ and those of the ‘F Unit’ form significant intra-reservoir pressure barriers, placing sands in pressure isolation. Using a bioevent-constrained well correlation in the central field area this pressure compartmentalization is illustrated with reference to repeat formation tester tool (RFT) pressure depletion data.

At a broader scale, the biostratigraphically-defined ‘time slices’ are used in conjunction with three-dimensional (3D) seismic data to model Magnus Sandstone Member submarine fan evolution, which is interpreted in terms of three main depositional phases: (1) a latest Kimmeridgian (J63) single submarine fan lobe with a depocentre in the mid-field, sourced from the northwest and displaying laterally confined, fault-controlled canyon-fill; (2) Early Volgian (J64 lower) bilobate accretion with two depocentres forming either side of the N-S-trending Brent High; and (3) late Early Volgian (J64 upper) as before but with the southward migration of the northern lobe depocentre, with fault confinement.

Micropalaeontology has also been used at the well site in the Magnus Field where it has influenced drilling operations in ‘real time’. The primary role to date has been the precise picking of casing points above high-pressured or low-pressured sands (e.g. ‘A Unit’) and calibration of supra-reservoir angle-build in deviated wells.

It is concluded that the application of biostratigraphy to integrated reservoir description has considerably enhanced our knowledge of submarine fan heterogeneity in Magnus, which has in turn led to more effective reservoir management and an extended production life for the field.