Controls on and the effect of extensional fault evolution in a transected rift setting, Northern North Sea

Abstract

The East Shetland Basin is a superb natural laboratory in which to study the role that normal fault growth and linkage has in determining petroleum prospectivity. Use of several high density 3D seismic volumes and over 250 boreholes permits key aspects of the Late Jurassic rift and its Permo-Triassic precursor to be analysed and its role on hydrocarbon trap formation, reservoir distribution and migration determined. The regional interpretation has revealed the generation of a North Sea archipelago of Upper Jurassic islands, the role of relay ramps in controlling syn-rift sediment dispersal patterns and the impact of normal faults of the later episode crossing and offsetting those generated by the earlier phase. The uplift, erosion and meteoric flushing of Upper Jurassic and older strata within the exposed fault blocks could potentially have huge consequences for the Brent play by enhancing reservoir properties and hence, help identify new play opportunities down-dip of major structures. Fault control on sediment dispersal can also be documented in a more localized study on the Cladhan Field, the site of a pronounced basin-margin relay ramp. This recent discovered set of syn-rift density flows illustrates how the development and distribution of depositional gradients and transport pathways form subtle play types. The Cladhan area is just one of several locations throughout the East Shetland Basin where the interaction of multiple rift phases is influential in the structural feedback after the Upper Jurassic rifting event. The delicate interaction and reactivation of underlying structural trends creates a series of multi-tiered fault block systems which can define several aspects of a petroleum system, depending upon the strike, polarity and level of reactivation of faults from one rift to another. The observations of fault growth and linkage in the Northern North Sea may provide generic lessons that help in determining petroleum prospectivity in other hydrocarbon rift basins (e.g. E. Africa and the N. Atlantic seaboard of North America)