Late Jurassic half-graben control on the siting and structure of hydrocarbon accumulations: UK/Norwegian Central Graben

Abstract The late Jurassic structure of the Central Graben is discussed, both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures. The regional kinematics of the North Sea rift suggest that during the late Jurassic the Central Graben opened approximately orthogonally to the main NNW—SSE fault-trend within the basin. Some evidence that Triassic extensional faults were reactivated at this time is seen towards the basin margins, and a significant Triassic extensional history is inferred. The late Jurassic basin comprises the axial c. 100 km of the Central Graben ( s.l. ). The basin axis is offset progressively to the SE by both transfer zones and discrete transfer faults. Footwall uplift of the basin margins during extension provided the source for the Upper Jurassic reservoir sands encountered by exploration drilling. These sands were probably preferentially fed into the basin at transfer zones in the margins. Salt structures are ubiquitous within the Central Graben, and their siting is everywhere controlled by the underlying fault-block topography. Salt structures form preferentially towards the crests of half-graben or buttressed against their boundary faults. Inversion structures in the Norwegian sector reactivated, with a reverse sense of displacement, late Jurassic extensional faults. A model is proposed which shows the two main inversion structures (Feda Graben, Norway; Tail End Graben, Denmark) to be detached on antithetic reverse faults riding in the salt. These faults terminate at tip-folds within major salt walls. The geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late Jurassic half-graben.