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.