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Geological evolution
Last updated on 31. October 2016
Lille Torup: The salt dome is cylindric and its diameter and height are approx. 5 km.

Stenlille: The aquifer formation is a large pillow formation 1500-1600 m below the surface, covering an area of 14 km2.


In the Permean period 250-290 million years ago, the area around Lille Torup was covered by desert and tropical primeval seas that were extremely salty. Over time, in the so-called Zechstein periods Z1, Z2 and Z3, the water evaporated leaving a settlement of salt. This happened three times and, after the last settlement, the salt had been deposited much like a stack of pancakes.

The sparingly soluble anhydrite (gypsum) was deposited first, followed by dolomite (lime). Then the rock salt, which made up > 90 per cent of the material,  and lastly the easily soluble red potassic salt were deposited.
Millions of years went by, and the salt was exposed to increasing pressure from settlements of clay, sand and lime. The salt sank deeper and deeper into the ground, where its temperature rose. In the middle of Triassic period 225 million years ago, the salt layers began to merge, "mushrooming" like huge salt cushions. In this process, the deposited layers of salt got mixed up, which poses a challenge in relation to flushing of caverns because the individual settlements are not equally soluble.
In the Jurassic period 135 million years ago, the salt domes finally settled into the structures we know today.
During the "mushrooming", pockets or traps are sometimes formed on top of the dome, where oil and gas will often collect.

The Stenlille gas storage facility is located c. 70 km SW of Copenhagen, near the town of Stenlille. In contrast to Lille Torup, the Stenlille gas storage facility is an aquifer reservoir, created in the Gassum Sandstone Formation. The good storage quality of the Gassum Sanstone Formation was formed during the late Triassic period 209-201 million years ago. In this age, Pangea (the supercontinent) began to break up, though the Atlantic Ocean was not yet formed.

The gas storage replaces saline formation water in some of the layers in the Gassum Sandstone Formation with a vertical closure of c. 35 m. The overlaying 300 m thick clay layer of the Lower Jurassic Fjerritslev Formation serves as an efficient caprock and holds the gas captured into the porous sandstone structure. The whole formation is pushed upwards in a slight arch of a large salt pillow. The reservoir is divided into six horizontal zones of porous sandstone separated by hard claystone. Only four of the zones are in operation as gas storage.
The storage came into operation in 1989. Twenty wells have been completed to date; 14 wells for injection and withdrawal of gas and 6 observations wells for monitoring pressure variations in the aquifer around the gas reservoir and in the caprock above. It is estimated that a total of 3 billion km3 of natural gas can be stored in the Gassum Sandstone Formation.