Lake Siljan

Lake Siljan, nestled in the heart of Dalarna, central Sweden, is renowned as the country’s seventh largest lake. Its coordinates are 60°51′N 14°48′E. The combined area of Siljan and its neighboring smaller lakes, Orsasjön and Insjön, covers an impressive 354 square kilometers. The lake’s maximum depth reaches 134 meters, and it is positioned 161 meters above sea level, placing its basin’s lowest point at 27 meters above sea level. The most prominent town along its shores is Mora.

Impact Crater: The Siljan Ring

One of the lake’s most fascinating features is its location around the southwestern edge of the Siljan Ring, a circular geological formation created by a significant meteorite impact 377 million years ago during the Devonian period. This original crater, now largely eroded, was once about 52 kilometers in diameter, making it the largest known impact crater in Europe, excluding Russia. The deformed Cambrian, Ordovician, and Silurian sedimentary rocks around the lake are a treasure trove of fossils.

Geological Explorations

The region surrounding Lake Siljan has been the subject of speculation regarding oil presence, although drilling efforts have not yielded successful results. However, the area, particularly around Boda, is known for substantial deposits of lead and zinc.

Deep Drilling Project

In a notable venture, Vattenfall, a government-owned power company, embarked on a deep drilling project in the lake area in search of natural gas. The Gravberg-1 well, located in the north-northeast section of the Siljan Ring, was selected based on a gravity anomaly indicating less dense rock beneath. The site also presented seismic reflectors, which were hypothesized as cap rock layers over a natural gas reservoir.

The geological composition of the area, including lower electrical resistance within the ring indicating fluid-filled rocks and soil showing traces of methane and heavy hydrocarbons, further fueled these explorations. Interestingly, the distribution of these hydrocarbons, along with vanadium and nickel trace elements, formed a halo around the ring. However, the sediment’s characteristics suggested that it was not the source of these hydrocarbons. The highest gas leakage was noted in the north-northwestern corner of the ring, while an absence of leakage in the central area pointed towards the presence of an underlying cap rock.