Influence of Intra-Salt Lithological Variability on Salt Tectonics: A numerical modelling approach

Understanding the internal structure of intra-salt layers within deformed salt bodies is crucial for geo-energy storage in salt-bearing basins. This study integrates high-resolution 2D finite element numerical modelling to explore how variations in salt stratigraphy, lithological heterogeneity, and post-salt sedimentation patterns influence deformation processes and the internal architecture of diapiric salt structures across different basin geometries. Specifically, we examine the impact of lithological variability by systematically varying the position and thickness of frictional-plastic, relatively strong intra-salt layers (e.g., anhydrite or carbonates) within a viscous layered salt sequence. The position of the strong intra-salt layer within a salt body significantly influences salt flow dynamics, internal and external diapir morphology, and overburden deformation. When located at the top, the strong layer acts as a stiff cap, restricting upward salt flow and producing broader diapirs with limited overburden deformation. When located in the middle, it localizes strain within the salt, leading to sharper and more discrete diapirs. When located at the bottom, it enhances upward salt flow of the overlying weak salt layer, resulting in tall, narrow, and more intrusive diapirs with more pronounced overburden deformation. In all cases, the strong intra-salt layer breaks and forms boudins, which vary in dimensions, distribution and structural complexity according to their different position and thickness. These intra-salt boudins can be transported by the salt flow to the upper parts of salt structures, but are often trapped at diapir pedestals, beneath diapir flanks, or under minibasins, where they experience repeated folding and refolding as the weaker, less dense salt flows around them. The presence of this heterogeneous intra-salt layer alters the flow paths of the weaker salt and controls both the geometry of salt structures and associated deformation in the overburden. These findings underscore the critical role of stratigraphic and tectonic controls in shaping both the external and internal architecture of salt diapirs, patterns that are particularly relevant for the North Sea, where salt structures play a crucial role in emerging geo-energy storage.