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Structural and stratigraphic relationships of the Palaeocene mounds of the Utsira High

Authors


John Wild, Mobil North Sea LLC, Grampian House, Union Row, Aberdeen, AB10 1SA, UK. E-mail: john.wild@exxonmobil.com.

ABSTRACT

The Balder and Ringhorne Tertiary oilfields of the Utsira High are a cluster of prominent Palaeocene mounds, whose presence has confounded geologists since they were first observed on 2D seismic data back in the 1960s and 1970s. Until recently, the Balder Field geologic model consisted of distal, deep-water Heimdal, Hermod and Balder Formation sandy-debrite and turbidite sands, compensationally stacked along the flanks of the Utsira palaeo-high, with intervening layers of hemipelagic shale. Remobilization of the sands by large-scale fluidization accentuated the high-relief mounds and sand injections linked reservoirs that were originally isolated. Reprocessed seismic data show strong primary reflectors that cannot be reconciled with this model; continuous sand bodies are observed to cross-cut normal, biostratigraphically constrained sequence-stratigraphic reflectors. The implication is that many, if not most, of the Hermod and Balder sands are not depositional, but were emplaced by injection. Furthermore, most of the Palaeocene mounds are associated with major ‘tears’ in the normally smooth Top Cretaceous chalk seismic reflector, and overlying chalk ‘rafts’. The chalk rafts were formerly thought to be detached glide-blocks, shed from distant scarps to the east, but the improved data show that they are always positioned above matching depressions in the main Chalk, implying a local origin. Some of the rafts are immense, exceeding half a kilometre in width and millions of tonnes in mass. We believe the association of the chalk features with the mounds is more than a coincidence and suggest that they are genetically related. A number of driving mechanisms have been proposed for the formation of the Palaeocene mounds of the Utsira High, including normal post-depositional compaction, fluidization by earthquakes, overpressuring due to gas migration and gravity-sliding back into the Viking Graben. Our observations are, however, more consistent with the mounds resulting from episodic supra-lithostatic pressure escape from beneath the Chalk.

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