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

A seismic stratigraphic interpretation of the Bressay area of the East Shet land Platform demonstrates the key role that fluctuations in relative sea-l evel had in the development and evolution of Paleogene deposition in proxim al parts of the Viking Graben. Relative sea-level fall, probably driven by events associated with the early development and evolution of the Iceland p lume, enabled a Late Palaeocene-Early Eocene coarsening-up deltaic system t o initially prograde and offlap as part of a forced regressive wedge. Coeva l and subsequent erosion led to deep (>250 m) incision of the delta and for mation of a significant drainage system consisting initially of a dendritic incised valley network and a major, deep, low sinuosity channel, all of wh ich fed sediment into distal parts of the basin. The highly localised natur e of major incision, the area's situation above the known occurrence of a b uried Late Caledonian granitic intrusion (the Bressay Granite) and coincide nt fault reactivation combine to suggest that the regional transient plume- related uplift was locally enhanced by Late Palaeocene-Early Eocene tectoni c uplift of a previously suppressed crustal root. Subsequent Early-Mid Eoce ne sea-level rise, coeval with North Atlantic opening, caused transgressive backfill of the erosional (palaeogeomorphic) relief and drape of the elast ic wedge by tuffaceous marine mudstones of the Balder Formation. Two, distinctive and mutually exclusive palaeogeomorphic play types have re sulted. Structural relief on the delta top incision surface combines with e ither the onlapping, fine-grained tuffaceous valley fill or compactional dr ape 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 hydroca rbon charge from the basin and the occurrence of heavy oil and gas in both of the subtle stratigraphic trap types and the independent structural closu re, may encourage further exploration for similar features on the East Shet land Platform. Comparison with neighbouring areas suggests that similar pal aeogeomorphic 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 see n in the Bressay area. (C) 2001 Elsevier Science Ltd. All rights reserved.