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Nature's own way of storing crude oil and gas in the pores of sedimentary bedrock gives us a hint that oil and gas can be stored in bedrock without risk that they will migrate.
In first instance the very large oil occurences are contained in the pores of sandstones. Oil is kept in place in the communicating systemes of pores due to impermeable surrounding bedrock and water pressure. Also within sedimentary limestones oil has been stored but it is frequently locked up to a large extent and it is necessary to acidize the limestone to get a proper production.
Oil does not occur in Precambrian rocks, but it is particularely in Precambrian bedrock, that man wants to store oil in rock, as this rock is the one best suited for artificial storage of oil. In Sweden mining operations in Precambrian bedrock has been carried out since a very long time, e.g. at the Falun copper mine which was mined at least already in the eleventh Century, and the mine is still in operation. In connection with mining, storage underground was in use already in Prehistoric time - and so it was at Falun as early as history tells us - and the medium stored was water in connection with unwatering the mines, step by step, from large depths.
At the end of the nineteenth Century small mine caverns started to be used in Sweden as storage for compressed air, at which the air was kept under pressure by water from a higher situated water storage cavern [see Fig. No. 1). When there was a surplus of compressor capacity the lower cavern was loaded. Today only two of these installations are in use.
It is interresting to record the reason for the closing down of the installations. In connection with the deepening of the mines the groundwater level was lowered, which means that the principle reason for the proper functioning of the storage was removed, i.e. the water pressure in the cracks in surrounding bedrock was lowered to such extent that the compressed air could escape through open cracks.
In 1937 the Swedish Department of Trade appointed a Board to investigate the possibilities for storage of petroleum products, safe of bombattacks. It was assumed that this could be done in rock caverns lined with concrete. Research was started and Tor H. Hagerman, Ph.D., chairman of Hagconsult, could prove that concrete lining on rock walls below the ground water level was fully tight against petroleum products of different kinds y as long as water fully filled up all the pores in the concrete.
At the end of the 1940-ies a closed down feldspar mine in the Stockholm archipelago was used for storage of Heavy Fuel Oil. The feldspar occurs in pegmatite veins in Precambrian granites and gneisses. Consequently, there was a solid wall rock comparatively free of cracks. On account of the vicinity to the sea there was also a certainty that the mine was surrounded by ground water.
