A Kimmeridgian time-slice through the Humber Group of the central North Sea: a test of sequence stratigraphic methods

Abstract

Reservoir quality of the Fulmar Formation in the UK Central Graben depends on the original distribution of facies as well as subsequent modification by partly facies-dependant diagenesis. Numerous sequence stratigraphical analyses of the Humber Group have been published recently, each of which has elements which are convincing geologically. Nevertheless, it is our experience that facies and reservoir prediction within the Humber Group remain difficult. Sequence stratigraphical interpretations in the Group are clearly non-unique due to major uncertainties in the data. (1) Biostratigraphical uncertainties include species concepts, recognition of agediagnostic taxa in thermally altered floras, validity of repeated micropaleontological events and equivocal paleoenvironmental information. (2) Sedimentological uncertainties are due to the gross facies similarity of the Fulmar Formation over a large geographic area and throughout the Upper Jurassic, which make the detection of potential maximum flooding surfaces and especially sequence boundaries difficult even in many cored wells. Furthermore strong syndepositional subsidence variations due to both salt movement and tectonics, lead to aliased sampling of the subsurface in wells, which are mostly situated on highs. (3) Seismic correlation difficulties are due to persistent seismic imaging problems below the Late Cimmerian ('X') Unconformity. (4) The depositional and sequence stratigraphical models for Humber Group deposition need to be set in the context of a transgressively skewed, back-stepping system with strong local subsidence control. The overall transgression is thought to be due to the combination of sea-level rise, widening and deepening of the basin due to tectonics, and reduced sediment supply through time. In this situation existing models have to be applied with even more care than normal if they are not to be misleading (e.g. concepts of forced regressions). These difficulties are clarified in the context of a Kimmeridgian time slice through the Humber Group. Multiple options exist in well log correlations. As a result palaeogeo-g~aphic reconstructions of the time-slice have an error margin of + 15 km, which has serious implications on the validity of reservoir quality predictions. Recently published sequence stratigraphical analyses of the Humber Group include both Vail-type schemes (e.g. Donovan et al. 1993) and Galloway-type schemes (e.g. Partington et al. 1993a). The framework presented in this paper (Fig. 1) combines elements from both approaches. There is general agreement on the age of maximum flooding surfaces between Shell nomenclature and Partington et al. (1993a), although we have not been able to recognize all their surfaces in our datasets. Sequence boundaries indicated by Donovan et al. (1993) are all recognized by ourselves. Donovan et al. have tied their sequence boundaries to the global sea-level chart of Haq et al. (1988). However biostratigraphical datings indicate that at least some sequence boundaries found in the Humber Group have no ...

Fulmar: a mature field revisited

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Sequence stratigraphic subdivision of the Humber Group in the Outer Moray Firth area (UKCS, North Sea)

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The tectonic and stratigraphic framework of the United Kingdom's oil and gas fields

“…The Fladen Group itself consists of the following four formations: the Pentland, Brora Coal, Beatrice and Hugin Formations (Richards et al 1993), the naming of which depends upon their geographical location. Recent biostratigraphic studies (primarily palynology and other forms ofmicropalaeontology) have led to a far better understanding of the temporal and spatial relationships between these various lithostratigraphic units and accurate correlations can now be made in many offshore areas (Partington et al 1993a;Veldkamp et al 1996;Jeremiah & Nicholson 1999) and extended to the onshore, non-marine equivalents of the Ravenscar Group in East Yorkshire (Fisher & Hancock 1985).…”