Abstract. The growth of salt structures is controlled by the low flow strength of evaporites and by the tectonic boundary conditions. The potassium-magnesium salts (K-Mg salts) carnallite and bischofite are prime examples of layers with much lower effective viscosity than halite: their low viscosity presents serious drilling hazards but also allows squeeze solution mining. In contrast, intrasalt anhydrite and carbonate layers (stringers) are much stronger than halite. These rheological contrasts within an evaporite body have an important control on the evolution of the internal structure of salt, but how this mechanical layering affects salt deformation at different scales is not well known. In this study, we use high-resolution 3-D seismic and well data to study the evolution of the Veendam and Slochteren salt pillows at the southern boundary of the Groningen High, northern Netherlands. Here the rock salt layers contain both the mechanically stronger Zechstein III Anhydrite-Carbonate stringer and the weaker K-Mg salts, thus we are able to assess the role of extreme rheological heterogeneities on salt structure growth. The internal structure of the two salt pillows shows areas in which the K-Mg salt-rich ZIII 1b layer is much thicker than elsewhere, in combination with a complexly ruptured and folded ZIII Anhydrite-Carbonate stringer. Thickness maps of supra-salt sediments and well data are used to infer the initial depositional architecture of the K-Mg salts and their deformation history. Results suggest that faulting and the generation of depressions on the top Zechstein surface above a Rotliegend graben caused the local accumulation of bittern brines and precipitation of thick K-Mg salts. During the first phase of salt flow and withdrawal from the Veendam area, under the influence of differential loading by Buntsandstein sediments, the ZIII stringer was boudinaged while the lens of Mg salts remained relatively undeformed. This was followed by a convergence stage, when the K-Mg salt-rich layers were deformed within the inflating salt pillows. This deformation was strongly disharmonic and strongly influenced by folding of the underlying, ruptured ZIII stringer, leading to thickening and internal deformation of the K-Mg salt layers.

In fully developed evaporite cycles, effective viscosity contrasts of up to five orders of magnitude are possible between different layers, but the structures and mechanics in evaporites with such extreme mechanical stratification are not well understood. The Zechstein 3 unit in the Veendam salt pillow in the Netherlands contains anhydrite, halite, carnallite and bischofite, showing this extreme mechanical stratification. The Veendam Pillow has a complex multiphase salt tectonic history as shown by seismic reflection data: salt withdrawal followed by convergent flow into the salt pillow produced ruptures and folds in the underlying Z3-anhydrite–carbonate stringer and deformed the soft Z3-1b layerWe analysed a unique carnallite- and bischofite-rich drill core from the soft Z3-1b layer by macroscale photography, bulk chemical methods, X-ray diffraction and optical microscopy. Results show high strain in the weaker bischofite- and carnallite-rich layers, with associated dynamic recrystallisation at very low differential stress, completely overprinting the original texture. Stronger layers formed by alternating beds of halite and carnallite show complex recumbent folding on different scales commonly interrupted by sub-horizontal shear zones with brittle deformation, veins and boudinage. We attribute this tectonic fragmentation to be associated with a softening of the complete Z3-1b subunit during its deformation. The result is a tectonic mélange with cm- to 10 m-size blocks with frequent folds and boudinage. We infer that these structures and processes are common in deformed, rheologically strongly stratified evaporites.

In the northern part of The Netherlands the magnesium salts carnalite and bischofite are mined in a deep solution mine. For optimal production and also for environmental impact such as subsidence, it is vital to understand the process of salt production. To this aim, it is studied whether seismic in time-lapse mode may help to visualize and quantify the changes due to the salt production. The main questions addressed are: Can changes due to salt production be detected in seismic time-lapse mode? And if so, how big are these changes? To address these issues, a modelling study was performed. Seismic data were synthesized using acoustic and elastic finite-difference schemes, and further processed to obtain migrated seismic images. For the models themselves, different production scenarios were studied. Analysis of these data clearly shows that salt mining causes detectable changes in time-lapse seismic signals and can be quantified. The amplitude effect seems relatively larger than the induced time shift.

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Abstract. At the first order salt structures are controlled by the low flow strength of evaporites and by the tectonic boundary conditions. Rheological contrasts within an evaporite body have an important effect on the evolution of the internal structure of salt, but how this mechanical layering affects salt deformation at different scales is not well known. The potassium–magnesium salts (K-Mg salts) carnallite and bischofite are prime examples of layers with much lower effective viscosity than rock salt: their low viscosity presents serious drilling hazards but also allows squeeze solution mining. In contrast, anhydrite and carbonate layers (stringers) in salt are much stronger than halite. In this study, we used high-resolution 3-D seismic and well data to study the evolution of the Veendam and Slochteren salt pillows at the southern boundary of the Groningen High, northern Netherlands. Here the rock salt layers contain both the mechanically stronger Zechstein III Anhydrite–Carbonate stringer and the weaker K-Mg salts, providing an example of extreme rheological heterogeneities in salt structures. The internal structure of the two salt pillows shows areas in which the K-Mg salt-rich ZIII 1b layer is much thicker than elsewhere, in combination with a complexly ruptured and folded ZIII Anhydrite–Carbonate stringer. Thickness maps of supra-salt sediments and well data are used to infer the initial depositional architecture of the K-Mg salts and their deformation history. Results suggest that active faulting and the resulting depressions of the Zechstein surface above a Rotliegend graben caused the local accumulation of bittern brines and precipitation of the thick K-Mg salts. During the first phase of salt flow and withdrawal from the Veendam area, under differential loading by Buntsandstein sediments, the ZIII stringer was boudinaged while the lens of Mg salts remained relatively undeformed. This was followed by a convergence stage, when the K-Mg salt-rich layers were deformed with the evolving salt pillows. This deformation was strongly disharmonic and strongly influenced by folding of the underlying, ruptured ZIII stringer, leading to thickening and internal deformation of the carnallite–bischofite layers.