Seismic Interpretation, Large-Scale Structures and Stability of the Interior of Evaporite Bodies

Salt structures are illustrated in two strikingly different ways. Publications on the over- and underburden deposition or deformation generally depict salt as structureless. On the other hand, detailed field-, well- and gallery mapping have shown an amazing spectrum of complexly folded, faulted and boudinaged interlayers, but these studies are on a smaller scale. The interlayers have different properties to halite, making evaporites rather heterogeneous.

We compare a 3D seismic interpretation of large-scale, complex internal structures of the complex internal structure of Zechstein salt bodies in NW-Europe, with data from salt mines and analogue and numerical models to better understand the large scale geometry of salt bodies. Focus is on a 40 m thick, relatively brittle claystone- carbonate- anhydrite member (the lowest part of the Leine Formation, called “Z3 stringer” here), which is fully encased in ductile salt and forms an excellent seismic reflector. This or similar stringers form HC reservoirs in some basins, while constituting a drilling risk elsewhere. At the same time, the density contrast between anhydrite/carbonate blocks and halite forms a potential risk in the stability of storage caverns and drill strings.

After an extensive seismic mapping over the entire northern Netherlands, structures observed include an extensive network of thicker zones, interpreted as early karstification. Later, this template of relatively strong zones was deformed into large scale folds and boudins as the result of salt tectonics. Non-plane-strain salt flow produced complex geometries that overprint each other. There are indications of a feedback between the early karsts and the position of later salt structures.

The stringer has a higher density then the surrounding halite. In the literature there is some controversy concerning the sinking rates. We observed no structures indicative of sinking, but conclude that the present-day position of the blocks is controlled by internal folding of the salt. This conclusion is corroborated by observations from mines, and by better understanding the effect of the distribution of grain boundary water in evaporite microstructures on deformation mechanisms and rates.

This work has shows that the internal geometry of the Zechstein evaporate rival the internal structure of mountain belts, both in complexity and size, but can be studied using high-quality 3D reflection seismic datasets.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California