Using Sedimentology, Biostratigraphy and Tectonics to Interpret a Complex Rift-Graben: The Middle-Late Jurassic South Viking Graben, North Sea

The Jurassic Viking Graben in the North Sea is one of the important HC provinces in Europe. The main play model in the southern part of the graben has been used for more than 20 years. In such a setting an improvement in the depositional model will open up for sub-optimal or untested targets. Examination of basic data in terms of sedimentology, biostratigraphy and tectonics was undertaken to reach this ambition.

The Bathonian rifting in the North Viking Graben (NVG) accompanied and influenced the drowning of the HC-rich Brent Delta. The rifting propagated southwards into the Southern Viking Graben (SVG) where the shallow marine and HC bearing Hugin Fm was deposited. The SVG experienced initial rifting in the M Jurassic (Bathonian - Callovian) and the rift-activity escalated through Middle Jurassic and into the Late Jurassic.

The Hugin Fm was deposited during differential subsidence and early part of the rift-phase and show in places sedimentary wedges that suggests rotated fault blocks, especially in the upper half of the formation. The increase in tectonic signatures upwards in the formation illustrates the escalation of rift-activity and formation of low-relief half-graben topography. The sedimentary wedges show characteristic facies pattern with dominance of wave-reworked facies at the fault crest and tidal facies and a general decrease in net to gross in deeper parts of the half-grabens. The rift topography amplified the tidal currents and preservation of mudstones and tidally-dominated heterolithics due to wave-sheltering at the elevated fault crests. Such a complex basin topography caused by the rifting led to pronounced environmental differences with a large span in water-chemistry that is reflected in the biostratigraphic data.

The continued rifting of the SVG into the Late Jurassic led to pronounced rift topography. The rifting and an eustatic sea level fall operating at the same time (Kimmeridge) led to a geological setting facilitating the formation of source rock - the hot shale Draupne Formation. The SVG was at this time characterised by rotated half grabens with exposed footwall islands, giving restricted basins where anoxic shale formed. Locally derived gravity flows from the eroded crests of the footwall islands were in places shed into the sub-basins. These sub-basins therefore also acted as sedimentary traps for local sands delivered from the surrounding elevated areas.
 

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