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Badley, Ashton & Associates Ltd, Winceby House, Winceby, Horncastle, Lincolnshire LN9 6PB, UK
The structural evolution of the Viking Graben has been the fundamental control on the deposition of the Brent Group and on the development of trapping geometries. Major crustal extension in the early Triassic caused tilting of basement fault blocks, which can still be clearly seen at the basin margins. By the mid-Triassic a post-rift thermal-subsidence basin was established. Local fault control on Brent Group deposition indicates the onset of a second period of crustal extension, although the main control on subsidence at this time was still thermal relaxation following Triassic rifting. Extension during deposition of the Brent was of only c. 1% magnitude.
The second period of extension increased in magnitude following Brent deposition and peaked during the late Jurassic, creating the main structural traps for Brent Province oil and gas. Footwalls to major normal faults were uplifted and eroded. The amount of uplift on a given fault-block can be predicted using quantitative models (flexural and domino models). Subsidence in the adjacent half-graben generally outpaced sedimentation, leading to deep-water basins into which footwall material could collapse. On all but the largest fault-blocks it is likely that footwall uplift rates were low compared with erosion rates, and so footwall crests would have been degraded faster than they would have been uplifted above sea-level. Flushing by meteoric water during the late Jurassic is therefore expected only on the largest fault-blocks (e.g. Snorre, Gullfaks).
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