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Role of Relay Ramp Evolution in Governing Syn-Rift Sediment Dispersal and Petroleum Prospectivity


Relay ramps are defined as zones of gentle dip and flexure that occur between overlapping en-echelon normal faults. Since being first defined, their role in accommodating normal faulting in other extensional provinces be they exposed rift systems or buried (subsurface) rift systems has become more widely appreciated and they are now recognised to be fundamental elements of an evolving rift systems. Whilst field studies have suggested that both undeformed and breached relay ramps have an important role in controlling sediment dispersal, relatively few ancient subsurface examples where this has been the case have been documented and their impact upon petroleum systems in rift basins remains uncertain. The results reported in this paper are based on a detailed interpretation of a large (300 km2) rectangular 3D seismic volume lying on the western margin of the East Shetland Basin. The survey was acquired by WesternGeco in 1998 and covers United Kingdom Continental Shelf (UKCS) license blocks 210/29 and 210/30. Twelve exploration or appraisal wells have been drilled in the block to date, eight of which are released into the public domain and were used to calibrate and constrain the resulting interpretations. The first six exploration wells all failed to find significant volumes of oil and it was only with the drilling of the 210/29a-4 well in 2008, its side-tracks, 210/29-4Z and 210/29a-4Y two years later and the subsequent 210/30a-4 appraisal in 2011 that commercial success of the Cladhan Discovery proven. The results demonstrate how the development and evolution of a relay ramp affects syn-sedimentary dispersal and petroleum migration pathways in the prospective Late Jurassic North Sea rift system. As such, this study provides important new insights regarding the factors that govern the success of failure of subtle stratigraphic plays in deep-water extensional settings in general and in the North Sea rift basin in particular.