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Energy Systems and Technologies for the Coming Century ...

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slightly dipping packages, to undulating sequences, usually lying at depths of betweentwo to five kilometres. They are overlain by younger sediments consisting of limestones,claystones <strong>and</strong> s<strong>and</strong>stones. The rock salt <strong>for</strong>med approx. 250 million years ago within<strong>the</strong> Zechstein Sea that covered large areas, including <strong>the</strong> Ne<strong>the</strong>rl<strong>and</strong>s, <strong>the</strong> whole of NorthGermany, large parts of Denmark <strong>and</strong> of Pol<strong>and</strong> <strong>and</strong> <strong>the</strong> area presently covered by <strong>the</strong>North Sea.Because rock salt has a low specific gravity, geological changes over <strong>the</strong> course ofgeological time (including <strong>the</strong> development of faults) caused <strong>the</strong> salt to rise up towards<strong>the</strong> surface: this salt movement lifted up <strong>the</strong> overlying sediments as it <strong>for</strong>med a saltpillow. This process can proceed so far that <strong>the</strong> salt breaks through <strong>the</strong> overlying beds<strong>and</strong> pushes <strong>the</strong>m to <strong>the</strong> side, giving rise to a salt diapir. These commonly have amushroom-like shape, extending upwards <strong>for</strong> several kilometres in some cases. Theyhave moved upwards to within a few hundred metres of <strong>the</strong> ground surface.Up to 200 separate salt diapirs are known to exist in Northwest Germany. However,<strong>the</strong>re are only a few isolated salt diapirs in Nor<strong>the</strong>ast Germany. Salt diapirs often havecircular to oval horizontal cross sections with diameters of 5 to 10 km on average. Thesituation in <strong>the</strong> approx. 100 *130 km large area between Wilhelmshaven <strong>and</strong> W-Hamburg, as well as Flensburg <strong>and</strong> Kiel, is <strong>for</strong> three to five salt domes to be joined up to<strong>for</strong>m several elongated salt walls with lengths of 50 to 100 km. Salt walls <strong>and</strong> numeroussalt domes are also present in <strong>the</strong> sou<strong>the</strong>rn, western <strong>and</strong> nor<strong>the</strong>rn parts of <strong>the</strong> Germansector of <strong>the</strong> North Sea.The tops of <strong>the</strong> salt <strong>for</strong>mations in <strong>the</strong> diapir salt structures in North Germany usually lieat depths below ground level of 400 to 1,200 m. The top salt in some salt domes or partsof salt walls can be deeper (>1,200 m) – especially in <strong>the</strong> central part of <strong>the</strong> Germansector of <strong>the</strong> North Sea. On <strong>the</strong> o<strong>the</strong>r h<strong>and</strong>, <strong>the</strong>re are salt structures with top salt at depthsas shallow as 200 – 400 m along <strong>the</strong> North Sea coast of Schleswig-Holstein in particular,as well as near Bremen <strong>and</strong> Hamburg.With <strong>the</strong> exception of North Jutl<strong>and</strong>, <strong>the</strong> isl<strong>and</strong> of Fyn, <strong>and</strong> <strong>the</strong> area to <strong>the</strong> east ofSjæll<strong>and</strong>, as well as a zone between Ringkøbing, Grindsted, Gram <strong>and</strong> Esbjerg, <strong>the</strong>re arealso salt deposits beneath Denmark. Salt <strong>for</strong>mations have been confirmed undergroundbeneath large parts of <strong>the</strong> Danish sector of <strong>the</strong> North Sea <strong>and</strong> <strong>the</strong> Baltic Sea. 14 saltdiapirs have been located in an area covering around 90 * 120 km between Aalborg,Århus <strong>and</strong> Nissum Fjord. The maximum diameter of <strong>the</strong>se salt diapirs is between approx.4 <strong>and</strong> 11 km. Top salt in <strong>the</strong>se structures usually lies at depths from approx. 200 to 500m. To <strong>the</strong> north of Ringkøbing, <strong>the</strong>re are 5 offshore salt diapirs at distances of 1 to 25 kmfrom <strong>the</strong> coast, with depths to top salt of 100 m. Experience hasshown that <strong>the</strong> rock salt zones most suitable <strong>for</strong> <strong>the</strong> construction of underground saltcaverns, i.e. those parts of <strong>the</strong> salt <strong>for</strong>mation with <strong>the</strong> lowest concentrations of <strong>for</strong>eignlithologies (anhydrite, clay), lie in <strong>the</strong> central parts of salt domes.Figure 3 <strong>and</strong> Figure 4 display <strong>the</strong> prevalence of salt structures in Denmark <strong>and</strong> Germany.For Germany, <strong>the</strong> offshore wind farms <strong>and</strong> <strong>the</strong>ir connection corridors illustrate thatcavern storage would be possible in <strong>the</strong>ir proximity.Risø International <strong>Energy</strong> Conference 2011 Proceedings Page 4

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