The 3D geometry of the Zechstein Z3 carbonate/anhydrite member: implications for the study of salt structures and hydro-carbon production

The Late Permian Zechstein deposits of NW Europe contain a relatively brittle claystone, carbonate/dolomite, anhydrite member (stringer), fully enclosed within ductile halite/ potash-salts. The stringers can be productive for hydrocarbons and at the same time can form a drilling hazard when sub-salt targets are drilled. furthermore, the salt structures of Europe are considered for different kinds of geological storage. Despite this, little is known about the large-scale geometrical development and the early evolution of intra-salt brittle layers. The presence of a single stringer makes the stringer a simplified structural analogue of the producing stringers of the South Oman Salt Basin or other locations where multiple stringers are positioned in evaporites. Here, a description of the geometry of this brittle layer is presented, based on two seismic data sets from the Dutch subsurface. This study aims to give an overview of structures observed and to compare them with observations from salt mines, analogue and numerical models. This purely geometrical study has its application in deciphering salt tectonics. Structures observed include zones of increased thickness in the stringer that probably formed shortly after stringer deposition under the influence of early diagenesis, gravitational sliding and enhanced by locally increased sedimentation. This sliding is interpreted to have formed gaps in the stringer along zones of increased thickness, coeval with thrust- and fold-structures in the centre of the zone. Later, salt tectonic structures began to form in the evaporites. Overall the stringer folding is harmonic with repect to the shape of the top salt horizon, but different dis-harmonic fold-types (including steep isoclinal folds) and boudins formed as a result of the constrictional flow of salt towards the narrow stems of salt structures. This constrictional salt flow, rather then plane-strain deformation, resulted in very complex boudin and fold geometries. Stringer deformation was further complicated by the relatively stronger zones of increased thickness, but there are indications that the thicker zones also influence the locations of salt structures. There thus appears to be a feedback between the sedimentary and early diagenetic evolution of large salt deposits and their later tectonic development.