In the 1980s, scientists drilled over 6 kilometers into this ancient meteor crater, not for impact data, but to test a radical theory: that oil is not a fossil fuel.
NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. Story by Kathryn Hansen., Public domain, via Wikimedia Commons
The deep gas hypothesis
The Siljan Ring is a 52-kilometer-wide impact structure, the largest in Europe, formed around 377 million years ago when a meteorite struck the Earth. This massive impact fractured the Precambrian granite bedrock to great depths. In the 1980s, this ancient scar became the testing ground for the abiogenic petroleum hypothesis, a theory championed by astrophysicist Thomas Gold.
Gold's idea, building on earlier work by Russian and Ukrainian scientists, proposed that hydrocarbons are not the product of decayed organic matter. Instead, he argued they are primordial substances, mainly methane, that were trapped deep within the Earth during its formation. According to this "deep gas hypothesis," this methane continuously migrates up from the mantle. Gold reasoned that the Siljan Ring's heavily fractured granite, which lacked the typical sedimentary source rocks for conventional oil, was the location to test his theory. The cracks, he believed, would act as conduits and the overlying rock as a trap for this deep, non-biological gas.
The Gravberg-1 well
The Swedish State Power Board, Vattenfall, initiated the Deep Gas Drilling Project to test Gold's hypothesis. In 1986, drilling began on the Gravberg-1 well, located within the crater. The objective was to penetrate the granite basement in search of a commercially viable reservoir of abiogenic gas. The project was an ambitious and costly undertaking, with total expenditures for it and a subsequent well exceeding $60 million.
Engineers bored through solid granite and diabase sills, eventually reaching a true vertical depth of 6,779 meters (22,241 feet). The operation faced immense technical challenges. They encountered traces of methane and other hydrocarbon gases, along with significant amounts of hydrogen and helium. However, the quantities never approached commercial levels. After several years of drilling, the project recovered only about 80 barrels of a black, oily sludge. Analysis of this sludge revealed biomarkers consistent with the fossil-rich sedimentary rocks that were pushed into the granite during the meteorite impact, which supported the conventional biogenic theory of oil formation. The project was declared a commercial failure and the well was suspended. While the Siljan experiment did not find the sought-after reserves, the abiogenic hypothesis prompts scientific discussion about the various ways hydrocarbons can form within a planet.
💡Fun Facts
The Siljan Ring impact occurred during the Devonian period and has been proposed as a possible cause of the Kellwasser event, a major extinction.
Proponent Thomas Gold also co-developed the "Steady State" theory of the universe, an alternative to the Big Bang model.
The project's final scientific report noted that the recovered oil was likely derived from diesel and additives used in the drilling mud.
The temperature gradient in the borehole was found to be a relatively cool 16 degrees Celsius per kilometer of depth.
The area is an open, natural landscape accessible at all times.
Admission
Free to visit the area. Specific sites within the Geopark may have fees.
Accessibility
The drill site is in a rural, forested area. Access requires a vehicle, and the final approach may be on unpaved roads. The terrain is not developed for accessibility.
