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The west of Sweden is dominated by the Scandinavian Mountains. During the Palaeozoic a continental collision between Scandinavia and Greenland produced the Caledonian mountain range. This underwent an extensional collapse during the Devonian.
Further extension then occurred at the Scandinavian margin during Permian and Mesozoic time, followed by continental breakup. Over long periods of time the mountains were partially eroded and uplifted again whilst the depression currently expressed by the Baltic Sea developed. This depression accumulated continental sediments that underlie the southern and eastern coastal and offshore areas of Sweden. However Globalshift recognises that the sedimentary fill in the western Baltic Sea was not conducive to creation of a rich petroleum system.
Nevertheless oil has been drilled in the island of Gotland off the mainland of southeastern Sweden. Gotland is made up of a sequence of limestones and shales of Silurian age, dipping to the southeast overlying a thick Ordovician sequence. It was deposited in a shallow sea on the edge of an equatorial continent which shallowed as carbonates and terrestrial sediments filled the basin.
Reef growth started and dominate the sedimentary record although some sandstones are present in the youngest rocks towards the south, representing sand bars close to the shore line. The limestones are weathered into karsts. The oil in Gotland, produced since 1974, occurs in these porous and fractured limestone reefs and to some extent in underlying sandstones. Further undrilled reef structures have been identified on seismic and by geochemical data.
Gas is also present in alum shales deposited in Cambro-Ordovician times in an anoxic inland sea in the area of Östergötland and Öland. Alum-shale, with its high content of organic material, can be broken down by bacteria to produce biogenic methane.