--> ABSTRACT: Fault Evolution during Polyphase Extension: Horda Platform, North Sea, by Bell, Rebecca; Jackson, Christopher A.; Whipp, Paul; Clements, Benjamin ; #90142 (2012)

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Fault Evolution during Polyphase Extension: Horda Platform, North Sea

Bell, Rebecca *1; Jackson, Christopher A.1; Whipp, Paul 2; Clements, Benjamin 3
(1) Imperial College London, London, United Kingdom.
(2) Statoil ASA, Sandsli, Norway.
(3) Statoil ASA, Stavanger, Norway.

Not all petroleum systems in rifts have been produced by a simple history of single phase faulting followed by thermal subsidence. Many hydrocarbon-bearing rift basins have experienced multiple phases of extension, and the superimposition of normal faults has resulted in complicated basin and trap geometries (e.g. South Atlantic, North Sea). Such a complex rift history causes problems in predicting the distribution and quality of reservoirs.

We investigate the effect of multiple rift phases on the evolution of normal faults in the Horda Platform area of the North Sea, including those that bound the Troll field. Pre-rift basement and deep structures are imaged by 2D seismic reflection data and shallower structural and stratigraphic features within the overburden are imaged by high-quality 3D seismic data. The lateral and vertical distribution of fault throw is assessed and a fault evolution model is presented for the eastern margin of the rift basin.

Rifting in the Horda Platform started during the Permian-Triassic (PT) and involved initiation of originally isolated, W-dipping normal fault segments, which eventually linked to form ~100 km long fault systems that bound three half-graben and terminate to the north against a granite-cored structural high. These PT faults were not reactivated during the Middle Jurassic rift phase, despite the initiation of normal faulting in the nearby Viking Graben. During the Middle Jurassic, however, a series of N-S striking faults began to develop directly to the north of the PT fault population, and deformed the previously un-faulted granitic basement. These faults terminate to the south near the northern tips of the PT faults, in the location of a suspected crustal shear-zone (the Nordfjord-Sogn detachment). Reactivation of PT faults did eventually occur in the Early Cretaceous, at a time when subsidence is commonly thought to have been driven by thermal relaxation.

The style of faulting in the Horda Platform appears to be controlled by an older crustal fabric. Large PT faults developed south of a suspected crustal shear zone, and show limited Middle Jurassic reactivation. A large granitic footwall basement high may have supplied sediments southward to pre-Middle Jurassic reservoirs within these tilted fault blocks. New faults initiated during the Middle Jurassic to the north of the shear zone and have experienced significantly greater Early Cretaceous reactivation than the larger PT faults.

 

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