Evolution of rheologically heterogeneous salt structures: a case study from the NE Netherlands

Abstract

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.

Deformation of intrasalt competent layers in different modes of salt tectonics

“…A 3-D depth-migrated seismic profile from the Groningen High of the Netherlands (Raith et al, 2016) illustrates some of the resulting geometries. It crosses two pillows: the Slochteren Pillow is in the common footwall to west-dipping and southdipping presalt extensional faults, and the Veendam Pillow is located partly over the southern flank of an E-W trending graben and partly over the graben itself (Fig.…”

“…2d). Modern seismic data, especially 3-D depth-migrated data (e.g., Gamboa et al, 2008;Van Gent et al, 2011;Fiduk and Rowan, 2012;Cartwright et al, 2012;Strozyk et al, 2012Strozyk et al, , 2014Gvirtzman et al, 2013;Jackson et al, 2014Jackson et al, , 2015Raith et al, 2016;, can reveal the full three-5 dimensional geometry but lack the resolution to image the small-scale structure, especially in mobile lithologies ( Fig. 2e).…”

Multiphase Structural Evolution and Geodynamic Implications of Messinian Salt‐Related Structures, Levant Basin, Offshore Israel

“…However, when salt or evaporites are composed of “stringers” of carbonate, anhydrite, or clay, they provide good reflectors within the salt and opportunities to investigate internal salt structures and deformation (Geluk, ; C. A.‐L. Jackson et al, ; C. A. Jackson et al, ; M. Jackson et al, ; Peters et al, ; Raith et al, ; Schléder et al, ; Strozyk et al, , ; Talbot & Jackson, ; Van Gent et al, ).…”

Structural and microstructural analysis of K–Mg salt layers in the Zechstein 3 of the Veendam Pillow, NE Netherlands: development of a tectonic mélange during salt flow

“…While the external morphology of salt bodies is relatively well understood and can be imaged using 3D reflection seismic data, internal structures in 3D have only recently begun to be studied, using internal reflectors with high acoustic impedance contrasts (anhydrite + carbonate stringers) (Van Gent et al, 2011;Cartwright et al, 2012;Fiduk & Rowan, 2012;Jackson et al, 2014Jackson et al, , 2015Strozyk et al, 2014). A recent study (Raith et al, 2016) using high-resolution 3D seismic data, described the large-scale structure of the K-Mg salt-rich Zechstein 3, inside the Veendam salt pillow in the north European Zechstein basin. Nevertheless, the spatial resolution of 3D reflection seismic is limited and structures smaller than 25-30 m cannot be imaged.…”