Tectono-stratigraphic development of the southern part of UKCS Quadrant 15 (eastern Witch Ground Graben): implications for the Mesozoic–Tertiary evolution of the Central North Sea Basin

A detailed structural analysis of southern part of UKCS Quadrant 15 has been integrated with a regional tectonic evaluation of the eastern Witch Ground Graben to provide a geological framework for exploration. Seismic data show that the tectonic development of the Witch Ground Graben was controlled by multiple reactivation of W–E, NE–SW and WNW–ESE to NW–SE Palaeozoic basement structures of Caledonian and Variscan origin. These structures can clearly be seen on regional seismic attribute maps at Mesozoic reservoir levels. They were first reactivated during Middle Jurassic (Toarcian–Bathonian) plume-related uplift, extension and volcanism. Extension was accommodated along several major crustal detachment faults, which locally appear to underpin the southern Fladen Ground Spur. These detachments also acted as volcanic conduits on a range of scales during crustal thinning. Following a phase of ?Bathonian–Oxfordian weak rifting and subsidence, these fault systems were again reactivated during the early Kimmeridgian-Ryazanian transtensional opening of the Witch Ground Graben, which is interpreted to have occurred in a sinistral simple shear system. This new model explains observations which suggest there was extension on ~W–E faults, variable transtensional and transpressional activity on NW–SE and WNW–ESE faults, and reverse reactivation of N–S faults over this interval. In Quadrant 15, structures formed during this phase are clearly seen at the Base Cretaceous level, the Upper Jurassic sequences having behaved as a thin, mechanically weak cover to the deforming rigid basement. Further structural activity is evidenced during the Tertiary, which is significant to the understanding of overlying Palaeogene reservoirs. This structural model has implications for Jurassic and Lower Cretaceous sandbody generation and deposition, for structural trap formation and integrity and for the distribution of reservoir compartments.