Abstract: Structural Decoupling by the Zechstein Salt During Multiphase Tectonics in the Southern Norwegian North Sea

Guerin, G. - Elf Petroleum Norge as, and B. Vendeville - Bureau of Economic Geology, The University of Texas at Austin

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 décollement layer decoupling the post-Zechstein sequence from the basement. Because most hydrocarbon fields and prospects are associated with salt structures, understanding the salt tectonics, illustrated here using seismic examples and analogue modeling, is the key for exploration in this province.

The Zechstein salt acted as a décollement during Triassic basement-involved extension: salt allowed the overburden to deform by thin-skinned extension and form listric faults with growth sequences, turtle back structures and overburden rafts. Translation of the Triassic cover is locally blocked near the southern edge of the salt basin, which lead to formation of Triassic-age buckle folds. Most importantly, the salt was already redistributed by the end of the Triassic including formation of salt welds below rafts, salt pillows, walls, diapirs, and salt-cored anticlines. This structural salt redistribution strongly influenced the response to later deformation.

During rifting (Jurassic and Lower Cretaceous), salt played a major role in decoupling the overburden from the normal-faulted basement. Generally, deep-seated faults caused forced folding and down-slope gliding of the overlying sediments. Above the hanging wall, deformation was accommodated by faults located above the crest of pre-existing Triassic-age diapirs that were extended, hence subsided, during extension. Locally, the offset between the basement fault and the nearest reactivated diapir can be as large as 20 km. The main upper Jurassic sand reservoirs could be trapped along these crestal grabens and some of them are hydrocarbon discoveries.

From Cretaceous to Tertiary, the Southern North Sea was affected by several pulses of regional tectonic inversion. Numerous anticlines above salt structures occur within the inverted basins, and simple section restoration indicates that these anticlines correspond to pre-inversion diapirs that have been laterally squeezed and rejuvenated during tectonic inversion. Contraction and upward squeezing of the diapir induced folding of the roof, forming potential anticlinal traps. Locally, contraction on pre-existing salt structures can be attributed to downslope gravity- driven gliding above the thin salt during Tertiary time. This contraction is linked with extension and normal faulting upslope.

In conclusion, salt tectonics history in the southern Norwegian North Sea can be summarized as follows: (1) The structural redistribution of the Zechstein Salt had occurred by the end of Triassic, in other words by the end of the first large scale tectonic event after the salt deposition; (2) The distribution of the Triassic-age salt structures has strongly influenced the fault pattern within the overburden during the later deformation phases (Jurassic/Cretaceous extension, Cretaceous/Tertiary inversion phases); (3) The main reactivation models are falling diapirs (during extension) (Fig. 1a), and diapir reactivation by squeezing (during contraction) (Fig. 1b); (4) Local gravity-driven deformation induced also rejuvenation of pre-existing salt structures (Fig. 2). This suggests that even a drastically thinned salt sequence can decouple the cover from the basement and allow for translation of large overburden blocks.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil