Tectonostratigraphic evolution of the North Sea rift basin

Abstract

The North Sea is a failed rift, comprised of the Viking Graben, Central Graben, and Moray Firth, that underwent multiple phases of extension during the Devonian (RP0), Permo-Triassic (RP1), and Late Jurassic-Early Cretaceous (RP2). The North Sea is a prolific petroleum province with a substantial amount of seismic and well data, therefore making it a prime candidate to study rifting on a regional scale. The goal of this research was to examine the tectonostratigraphic evolution across the rift using both seismic interpretation and regional restorations to address some lingering questions, including the amount of extension attributed to different rifting events, whether thermal re-equilibration of the lithosphere was achieved between these events, and the applicability of various stretching and tectonostratigraphic rifting models. Devonian extension (RP0) had a major impact on the evolution of the Horda Platform and Moray Firth, with the former showing large amounts of extension due to the relatively low angle of these faults, while the latter showed a thick Devonian section, but faults that were planar and higher angle. This could imply a different origin of the Moray Firth faults as newly formed normal faults rather than reactivated thrusts as on the Horda Platform. Activity during the Permo-Triassic rift phase (RP1) was focused over the Horda Platform, which was almost entirely inactive during the Late Jurassic rift phase (RP2). Rifting shifted west during RP2 and was focused on a relatively narrow zone along the Viking and Central Graben axis, as well as in the Moray Firth. A thick section of post-rift developed over the Horda Platform after RP1 rifting and deposition continued through the Lower Cretaceous. Part of this succession included the Upper Jurassic package, which is syn-rift in the Viking Graben, but post-rift on the Horda Platform and deposited due to thermally generated accommodation, indicating that the effects of RP1 were ongoing during RP2. Beginning in the Upper Cretaceous, the post-rift focused over the RP2 rift axis, indicating that this area was now the focus of thermal subsidence related to RP2. The tectonostratigraphic model of Prosser (1993) was applied, but on a regional scale it did not prove possible to differentiate the S2 and S3 of the syn-rift or the S4 and S5 of the post-rift. Instead, a simple distinction between pre-rift, syn-rift, and post-rift strata was most useful. Additionally, as the North Sea is a multiphase rift, it is important to add rifting event, accommodation mechanism, and overall basin activity qualifiers to the tectonostratigraphic packages. Also, while the northern North Sea is considered a textbook example of the McKenzie (1978) model for rifting and while this model is still broadly applicable, what is being observed is a summation of multiple rifting events creating the “steers head” geometry seen on regional profiles. Additionally, the lithospheric necking model proves important for understanding why some lines demonstrate footwall uplift as predicted by the flexural cantilever model, but some do not. A potential explanation for this difference in isostatic response of the footwall blocks is lithospheric necking occurring at different crustal levels, resulting in dramatically different fault block geometries at the margins of the rift. Understanding the rifting evolution of the North Sea is important for the future development of petroleum plays in underexplored parts of the basin. One such example is the giant Johan Sverdrup field on the Utsira High, where the previously understudied tectonostratigraphic evolution led to the development of accommodation space through oblique extension and generation of source areas for sandstones in subaerially exposed basement highs. This discovery came long after it was considered that all large fields in the North Sea had been discovered, and application of the restoration techniques used in this research could lead to further discoveries in the region.