Pre-salt and post-salt fault trends and regional fault coupling mechanisms and their relationship with salt structural trends in the Southern North Sea basin
Authors: Preiss Anna , Adam Jürgen 
Published: March 2020

&lt;p&gt;In the Southern North Sea Basin, Zechstein evaporites act as a basin-wide viscous detachment layer, partly decoupling thick-skinned (basement-involved) and thin-skinned (gravity-driven) salt tectonic deformation, producing coupling relationships ranging from completely coupled to completely decoupled systems. This study attempts to define sub-areas inside the basin characterized by different types of coupling both qualitatively through analysis of correlations between regional pre-salt and post-salt fault patterns and salt structural trends and quantitatively by analysing fault and salt structure orientations. It also aims at relating those regional fault coupling changes to variations in salt thickness and basement structure.&lt;/p&gt;&lt;p&gt;Using a supra-regional Southern North Sea MegaSurvey 3D seismic dataset provided by the Oil and Gas Authority and Petroleum GeoServices, pre- and post-salt fault networks have been mapped with a combination of edge detection and edge enhancement seismic attributes and automated vector tracing techniques. Analysis of fault pattern correlations between the resulting fault datasets allowed to identify three types of pre-salt and post-salt fault correlations, which can be used to identify and map the types of mechanical fault coupling in the basin: A. Strong correlation, full mechanical coupling in area of thin, undeformed salt, B. Moderate correlation, all types of coupling present; thick salt and Central Graben rift dominance, C. Moderate to weak correlation with predominantly no mechanical coupling on thin to thick deformed salt. Additionally, an area of a noticeable rotation of salt structural trends, C&amp;#8217; was isolated within area C.&lt;/p&gt;&lt;p&gt;Fault and salt structure orientation analysis has shown an overall high similarity between pre-salt and post-salt fault systems with a dominant NW-SE trend and a weaker secondary SW-NE trend, implying a strong mechanical link between pre-salt and post-salt structures in general. The dominant NW-SE basement trend characterizes areas A, C and C&amp;#8217; but is strongly suppressed in area B (Central Graben rift dominance), where NE-SW and N-S trends dominate. The dependence of post-salt structural trends on pre-salt structural trends decreases in areas with an increasing role of gravity-driven processes. In area B with basement-involved deformation, salt structural trends correlate with Upper Jurassic basement structural trends. In subarea C&amp;#8217;, a rotation of c. 30&amp;#176; towards the N-S orientation of NW-SE and NE-SW salt structural trends can be observed with respect to the underlying basement faults. This can be interpreted as gravity-driven pure-shear deformation of the post-salt overburden into the basin centre with regional E-W shortening and N-S stretching.&lt;/p&gt;&lt;p&gt;As salt thickness increases in general from basin margin to the basin centre, the amount of pre- and post-salt fault coupling in the SNS decreases, however, in the area of the Central Graben rift dominance, basement fault displacement takes over the role of the main controlling factor. Further studies are required to quantify the effects of salt thickness and pre-salt fault displacement on the post-salt fault systems and salt structural trends in the basin.&lt;/p&gt;