Structural Style and Evolution of a Half Graben Bounding Normal Fault Array, Oseberg Fault Block, North Viking Graben, Norwegian North Sea

This study integrates 3D seismic and well data to gain insights into the geometry and evolution of a half-graben bounding normal fault system geometry and related fault propagation folding. Integrating structure and stratigraphic analysis allows us to reconstruct the growth, interaction and linkage of normal fault segments and their associated fault perpendicular and parallel hangingwall folds during the formation of a crustal-scale tilted fault block. We focus on the 140 km long Brage - Oseberg Ost - Veslefrikk fault system, using a 7500 km2, merged 3D seismic survey and multiple wells. This normal fault system forms the eastern boundary of the Oseberg Fault-Block, on the eastern flank of the North Viking Graben, North Sea. The normal fault system has a dominantly N-S strike, with prominent NNE- and NNW-trending fault jogs. Overall, displacement decreases away from the centre of the fault system and local displacement anomalies (lows) occur associated with the fault jogs.

Analysis of sequential isopachs, stratal geometries and well data show that individual (fault parallel) depocentres formed during the early rift initiation stages (Bathonian; c. 168 Ma) and that these merge to form a laterally extensive depocentre in the immediate hangingwall of the fault system during later rift climax (Callovian -Kimmeridgian; c. 162 - 152 Ma). The early formed depocentres were associated with fault parallel folds above blind normal faults and their length was related to the length of the blind fault segments. In contrast fault-perpendicular hangingwall anticlines separated the initial synclinal depocentres along strike and are located adjacent to the short fault jogs and displacement lows. This evolving structural template allows inferences to be made on the timing and origin of sediment sources and main sediment transport system(s).

Major rift shoulder and local footwall uplift and erosion during rifting accompanied deposition of more than 5 km of Upper Jurassic syn-rift sediments on the western side of the normal fault system. Resulting from the evolving structural template, the orientations and interactions of normal faults and folds defined the location of the hangingwall depocentres, the fault scarp related sediment input and the associated relay ramps defined the local pathways for the coarse grained clastic sediments that have been delivered into adjacent basins and readjusted the regional rift shoulder drainages.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California