Controls on the genesis and prospectivity of Paleogene palaeogeomorphic traps, East Shetland Platform, UK North Sea

Abstract

A seismic stratigraphic interpretation of the Bressay area of the East Shetland Platform demonstrates the key role that fluctuations in relative sea-level had in the development and evolution of Paleogene deposition in proximal parts of the Viking Graben. Relative sea-level fall, probably driven by events associated with the early development and evolution of the Iceland plume, enabled a Late Palaeocene–Early Eocene coarsening-up deltaic system to initially prograde and offlap as part of a forced regressive wedge. Coeval and subsequent erosion led to deep (>250 m) incision of the delta and formation of a significant drainage system consisting initially of a dendritic incised valley network and a major, deep, low sinuosity channel, all of which fed sediment into distal parts of the basin. The highly localised nature of major incision, the area's situation above the known occurrence of a buried Late Caledonian granitic intrusion (the Bressay Granite) and coincident fault reactivation combine to suggest that the regional transient plume-related uplift was locally enhanced by Late Palaeocene–Early Eocene tectonic uplift of a previously suppressed crustal root. Subsequent Early–Mid Eocene sea-level rise, coeval with North Atlantic opening, caused transgressive backfill of the erosional (palaeogeomorphic) relief and drape of the clastic wedge by tuffaceous marine mudstones of the Balder Formation. Two, distinctive and mutually exclusive palaeogeomorphic play types have resulted. Structural relief on the delta top incision surface combines with either the onlapping, fine-grained tuffaceous valley fill or compactional drape above the coarse axial fill of the main low-sinuosity channel system to form good reservoir-seal pairs. A third closure exists in association with activity on a prominent reverse fault that also appears to have initiated in response to the rejuvenation of the Bressay Granite. Present-day hydrocarbon charge from the basin and the occurrence of heavy oil and gas in both of the subtle stratigraphic trap types and the independent structural closure, may encourage further exploration for similar features on the East Shetland Platform. Comparison with neighbouring areas suggests that similar palaeogeomorphic play types might be expected to occur in other Early Cenozoic basin margin locations in the North Sea and the West Shetlands, albeit on a smaller scale than the locally enhanced, tectonically driven incision seen in the Bressay area.