Print this pageStructural Decoupling by the Zechstein Salt during Multiphase Tectonics in the Southern Norwegian North Sea
GUERIN, G. and B. C. VENDEVILLE
The Southern Norwegian North Sea underwent multiple rift phases, later overprinted by tectonic inversion. The structural pattern was further complicated by the presence of Zechstein evaporites that acted as a decollement layer decoupling the post-Zechstein overburden from the basement. Because most hydrocarbon fields and prospects are associated with salt structures, understanding the salt tectonics, illustrated here using seismic examples and analog modeling, is key for exploration in this province.
The Zechstein salt first acted as a decollement during Triassic basement-involved extension: salt allowed the overburden to deform by thin-skinned extension and form listric faults, tilted blocks, and overburden rafts. The structural redistribution of the salt, including salt welds below rafts, and salt pillows, walls, and diapirs, was acquired mostly at this time. During Late Jurassic rifting of the Central Trough, salt played a major role in decoupling the overburden from the normal-faulted basement. On the eastern margin of the rift, most faults formed above the crest of Triassic-age salt structures as they subsided. The main Upper Jurassic sand reservoirs can be trapped along these crestal grabens.
From Cretaceous to Tertiary time, several pulses of tectonic inversion squeezed and rejuvenated the diapirs, forming potential anticlinal traps above the diapir crest. Although inversion involved the basement, some contractional overburden structures formed by gravity gliding above the salt and linked updip (eastward) with normal faults. That suggests that even a drastically thinned salt sequence can decouple the cover from the basement and allow for translation of large overburden blocks.