Variability in Syn-Rift Structural Style Associated with a Mobile Substrate and Implications for Trap Definition and Reservoir Distribution in Extensional Basins: A Subsurface Case Study from the South Viking Graben, Offshore Norway

The South Viking Graben (SVG), northern North Sea, hosts many large hydrocarbon accumulations. In the Norwegian sector, the main reservoir-trap pairs are; (i) Middle Jurassic shallow marine sandstones in structural traps; and (ii) Palaeocene Deepwater sandstones in structural or combination traps. Upper Jurassic, syn-rift turbidite sandstones form reservoirs in several fields in the UK sector, but the equivalent succession in the Norwegian sector remains relatively unexplored due to difficulties in predicting reservoir distribution and trapping configurations. These difficulties reflect the control that rift-related normal faults and salt movement has on the deposition of Deepwater reservoir sandstones. In this study we use potential field, 3D seismic and well data to investigate how normal fault growth and movement of the evaporite-dominated Zechstein Supergroup control spatial variations in syn-rift structural style, trapping styles and reservoir distribution in the SVG. In the north of the basin, syn-rift deformation is dominated by listric faults that detach downwards into the underlying evaporites. These faults formed in response to tilting of the hangingwall and break-up of the supra-salt units, and halokinesis in this area is restricted to low-relief salt rollers in the immediate footwalls of the listric faults. In the central part of the basin, rift-related normal faults are basement-involved and only rarely propagated up through the Zechstein Supergroup. In this location fault-propagation folds, which are cored by low-relief salt pillows, developed in the supra-salt cover strata. The southern part of the basin is dominated by a series of ‘minibasins’ developed in response to the collapse of older Triassic-age salt diapirs; normal faulting is rare, and limited to low-displacement structures overlying the crests of salt diapirs and a few basement-involved faults that breach the Zechstein Supergroup. This study demonstrates that the Late Jurassic, syn-rift structural evolution of the SVG varied markedly over relatively short (i.e. <20 km) length-scales. We interpret this variability is related to mobile halite distribution within the Zechstein Supergroup; ‘halite-poor’ parts of the basin are characterised by supra-salt, gravity-driven faults, whereas minibasins formed in ‘halite-rich’ parts of the basin. We conclude by demonstrating how these variations in structural style control the distribution and geometry of syn-rift reservoirs.  

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