Controls on salt tectonic styles and basement-overburden coupling in the Southern North Sea

The overall aim of this project was to analyse the coupling relationships between thin- and thick-skinned tectonics and to understand their distribution and controlling factors across the Southern North Sea (SNS) basins. Improving our understanding of these relationships on a regional scale is important for both resource exploration and energy transition considerations, such as subsurface energy storage or exploration for and extraction of geothermal energy.
The MegaSurvey 3D seismic dataset provided by Petroleum GeoServices and the North Sea Transition Authority was analysed at regional scale using original methods: 1) seismic-attribute-based sub-salt basement fault mapping and trend analysis, 2) seismic-attribute-based mapping and classification of salt structure morphologies, 3) analysis of fault coupling based on sub- and supra-salt fault trend correlations, 4) quantitative analysis of fault coupling using fault displacement ratios.
A dominant NW-SE basement fault trend was identified across southwestern basins. Salt thickness was determined to be the main factor controlling the amount of sub- and supra-salt fault coupling in the majority of SNS basins, except in the Central Graben, where basement fault displacement is the dominant control.
Three zones of fault coupling were identified across SNS area: A – complete coupling, B – mixed degree of coupling, C – complete decoupling. The zones correlate well with occurrences of Westphalian gas in reservoirs above Zechstein in zones A and B, with no occurrences in zone C. In parts of zone C, salt structures orientated towards the N from dominant basement trends may suggest a pure-shear style of thin-skinned deformation of the overburden with N-S stretching and W-E shortening. The general distribution of zones of coupling was confirmed by quantitative analysis. Areas of inversion are characterized by overestimated displacement ratio values.
The presented methods were developed for large seismic data sets and they have the potential to be applicable in other salt basins affected by tectonic reactivation of the basement. Main applications may include basin exploration in search for areas with good seal or, conversely, zones of hydrothermal fluid or hydrocarbon leakage from the basement into the supra-salt section.