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18-630: The lake is located around the southwestern perimeter of the Siljan Ring (Swedish: Siljansringen ), a circular geological formation which was formed 377 million years ago in the Devonian by a major meteorite impact. The original crater, now mostly eroded, is estimated to have been about 52 km (32 mi) in diameter and is the largest known impact crater in Europe (excluding Russia ). The Cambrian , Ordovician and Silurian sedimentary rocks deformed by

36-573: A cause of the first Devonian extinction, the Kellwasser Event or Late Frasnian extinction, due to it being believed by some researchers to coincide around the time of the Kellwasser event at 376.1 Ma ± 1.6 Ma, although the timing of this extinction event has since been pushed forward to 371.93–371.78 Ma. The effects of the impact can clearly be seen in the bedrock in the area. The Cambrian , Ordovician and Silurian sedimentary rocks deformed by

54-695: A halo around the ring coincident with vanadium and nickel trace elements; the sediment was shown to not be the source of the hydrocarbons due to their location, ice movement pattern, and low level of maturity. Although the greatest gas leakage was in the north-northwestern corner, the lack of leakage in the central area suggested an underlying cap. Drilling began on 1 July 1986. By September 1989 it reached 6,957 m (22,825 ft) in depth. Four seismic reflectors were detected at 1,440 m (4,720 ft), 2,800 m (9,200 ft), 4,740 m (15,550 ft) and 7,400 m (24,300 ft). The first three were penetrated and found to be dolerite sills, but

72-664: Is a prehistoric impact structure in Dalarna , central Sweden. It is one of the 15 largest known impact structures on Earth and the largest in Europe , with a diameter of about 52 kilometres (32 mi). The impact that created the Siljan Ring occurred when a meteorite collided with the Earth's surface during the Devonian period. The exact timing of the impact has been estimated at 376.8 ± 1.7 Ma or at 377 ± 2 Ma. This impact has been proposed as

90-560: The Solar System . Impact structure is synonymous with the less commonly used term astrobleme meaning "star wound". In an impact structure, the typical visible and topographic expressions of an impact crater are no longer obvious. Any meteorite fragments that may once have been present would be long since eroded away. Possible impact structures may be initially recognized by their anomalous geological character or geophysical expression. These may still be confirmed as impact structures by

108-514: The area. Drilling was carried out in the late 1980s and early 1990s but proved inconclusive. Drilling for natural gas was resumed in the late 2000s and continued as of mid-2012. The scientific premise for prospecting of this sort is based on the work of physicist Vladimir Kutcherov, who is cooperating with Igrene, the company financing the drilling operations. In 2019, a study of gases and secondary minerals revealed that long-term microbial methanogenesis and methane oxidation have occurred deep within

126-695: The boundary between the Paleoproterozoic and the Mesoproterozoic . They were emplaced shortly after the Svecokarelian orogeny . The oldest sedimentary rocks that outcrop in the Siljan area are of Ordovician age. The sequence is dominated by limestone formations with one prominent black shale, the Fjäcka Shale , which is bituminous and has generated petroleum, sourcing the oil found in limestone cavities in

144-461: The context of theory of abiogenic petroleum origin ). The Gravberg-1 well location, in the north-northeast section of the ring, was chosen because a gravity anomaly indicates less dense rock below, and four seismic reflectors may represent cap rock above a natural gas reservoir. Electrical resistance is lower in the ring than outside it, suggesting the rocks contain fluid. Soil in the area showed clear traces of methane and heavy hydrocarbons , with

162-456: The dolerite sills points to a probable mantle origin. Hydrogen was found in most sections of the hole, correlated with helium . The helium isotopic pattern was consistent with a radiogenic, crustal origin. A black sticky substance was found to be blocking the drill pipe when circulation was stopped for several days at a depth of 5,520 m (18,110 ft). The substance was black, sticky when wet, powdery when dry, showed no fluorescence, and

180-516: The fourth had been identified before drilling as having the best potential for gas production. The dolerite is believed to have existed as much as 1/2 billion years before the meteorite impact. The granite is extensively fractured, with calcite as the principal cement down to the 3,200 m (10,500 ft) depth, with lesser amounts to 5,000 m (16,000 ft). Traces of methane were found at all depths, with higher levels in dolerite. Although they appear to be of abiogenic origin, only that near

198-403: The fracture system of the crater (for at least 80 million years). and in 2021 a study revealed findings of fossilized anaerobic fungi that had lived in consortium with methanogens deep in the crater. Impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface , whatever

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216-522: The impact are rich in fossils . Some people suspect that there might be oil in the area, but drilling has been unsuccessful so far. There are large deposits of lead and zinc in the Boda area. In the 1980's, state-owned power company Vattenfall drilled a deep well in search of natural gas. The project began as a deep commercial wildcat well , and its unusual location in fractured granite attracted scientific attention (e.g., as potentially significant in

234-462: The impact are rich in fossils . The area around the Siljan Ring has been the site of recent prospecting for oil and natural gas, though none of the projects has so far been commercially viable. There are large deposits of lead and zinc near Boda at the eastern edge of the Ring. There are several lakes in the vicinity, the largest of which is Siljan on the south-southwestern edge of the crater, with

252-694: The same area. The Ordovician sequence is overlain by rocks of the Llandovery Series (Lower Silurian ). During the last ice age the area was covered by a thick icesheet. The bedrock was sculpted by the ice, with the softer Palaeozoic sedimentary rocks being preferentially eroded. These erosional hollows are now occupied by lakes Siljan , Orsa , Skattungen and Ore. In accordance with theories about abiogenic petroleum (that hydrocarbons can be formed without involving material from dead plants and animals), astrophysicist Thomas Gold suggested that there might be major deposits of oil and natural gas in

270-532: The smaller Orsa Lake to the west and Skattungen and Ore on the northeastern margin. The Siljan Ring consists of an annular outcrop of Lower Paleozoic sedimentary rocks within Proterozoic granites of the Dala series. The impact event had an energy of 1.94 × 10 J, with the diameter of the impactor being 5 km. In Dalarna, the basement rocks consist of granites dated as 1.6 billion years old, putting them at

288-509: The stage of erosion of the structure. In contrast, an impact crater is the surface expression of an impact structure. In many cases, on Earth, the impact crater has been destroyed by erosion, leaving only the deformed rock or sediment of the impact structure behind. This is the fate of almost all old impact craters on Earth , unlike the ancient pristine craters preserved on the Moon and other geologically inactive rocky bodies with old surfaces in

306-521: Was later shown to originate from organic additives, lubricants and mud used in the drilling process. A second hole, Stenberg-1, was drilled in the center of the ring, 12 km (7.5 mi) distant from the ring sediments and from Gravberg-1. This well went to a depth of 6,500 m (21,300 ft). The hole produced gases and oily magnetite sludge similar to those found in Gravberg-1. Siljan Ring The Siljan Ring ( Swedish : Siljansringen )

324-495: Was strongly magnetic. None of the drilling materials showed gas chromatographic patterns which corresponded with the black substance. It contained no ordinary rock minerals, none of the granite fines expected in the drilling fluid, but is made up of microscopic magnetite held together by oils. The well did not penetrate the fourth seismic reflector, did not produce large quantities of fuel, and never entered commercial production. Only 84 barrels (13.4 m) of oil were produced; it

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