The Koillismaa Deep Hole – Solving the mystery of the hidden dyke

Featured Image: The Arctic Drilling Company Ltd. (ADC) are conducting the drilling on a self-built and designed rig which, theoretically, can reach depths of 3500m.

The Geological Survey of Finland (GTK) announced in September of this year that they would be drilling a 3km deep hole in southern Kuusamo, Finland.  The intention behind the ambitious exploit is to gain an understanding of the bedrock geology, and to hopefully facilitate deep-hole research for many disciplines such as survey technology, mineral systems, 3D modelling, geothermal energy, deep groundwater, and bedrock stability. This week I spoke to Aleksi Salo, the Project Manager for the Koillismaa project.

The project, known as the Koillismaa Deep Hole Project, is focussing on an area of southern Kuusamo. The area has been previously studied, due in part to the presence of a significant geophysical anomaly deep in the bedrock. In the 1970’s interpretation of a regional gravimetric survey identified a distinct geophysical anomaly (Figure 1) with no outcrops, connecting to major intrusions: The Näränkävaara and Koillismaa intrusions. The anomaly of focus for the deep hole is approaching 60km in length and sits between 1 and 3 km deep.

Figure 1 - Geological map of the Koillismaa metallogenic area, the boundaries of which are marked here by a green line. The intrusive blocks of the 2.44 Ga layered intrusions and the main metallic mineral deposits of the area are named. The 'Gravity anomaly zone' is located inside the green boundary. The map is based on Karinen (2010).

Investigation and speculation on the anomaly over the years has favoured the theory that it is an extension of the Näränkävaara and the Koillismaa Intrusive Complexes at depth, surrounded by lighter Archaean gneisses. These two complexes are comprised of 2.4 Ga old mafic to ultra-mafic layered intrusions and they sit ~60km part, with this mysterious gravity anomaly between. Other less probable suggestions for the origin of the anomaly include an Archaean greenstone belt,  granulites, Ni-Cu-PGE mineralisation, or even an Archean-age impact crater.

Using updated gravity and seismic data, GTK geophysicists conducted new and improved interpretation into the anomaly and determined that the centre would be intersected at a depth of 1.5km. Previous attempts to drill the anomaly during the early 2000s were unsuccessful but with improved drilling technology and capability, this project is looking promising. The Arctic Drilling Company Ltd. (ADC) are conducting the drilling on a self-built and -designed rig which, theoretically, can reach depths of 3500m (Figure 2).

Figure 2 - ADC Drilling site in Southern Kuusamo. ADC K5 rig before the change to K10.

Prior to beginning the drilling, GTK surveyed the local environment, over 60 water, soil and humus samples were taken. This will allow them to report back afterwards on the environmental impact of the project, which they anticipate being minimal as no harmful substances are expected to be encountered in the sub-surface which would contaminate the returned drilling fluid. Secondary testing is pencilled in for January, or maybe even before Christmas is all runs smoothly from here on. Ongoing passive seismic surveys are also being used to monitor ground vibrations from the drilling, and active seismic survey will also be used in this capacity.

After receiving the go-ahead for the project in September, drilling commenced quickly. The first stage was to drill through the till to the top of the bedrock with a 122mm and then 96mm drill bit, case the hole and fill it with cement. The purpose of this is to anchor the rig to reduce resonance when large depths are achieved, and the weight of the rods is impacting the rig. The rig is additionally anchored to the bedrock with 96mm cemented and cased hole.

The next step was to drill back through the cement with an NQ drill bit, and progress through the bedrock. While the lithology immediately below the surface was expected to be largely Archean gneiss the quality of the rock was unknown to GTK. The first ~300m transpired to be heavily fractured, causing a reduction in water pressure. The solution was to use a wider 96mm drill bit to re-drill through the problematic sections and leave the rods as casing, which proved to be effective and all ran smoothly to ~1km as rock quality increased.

At this point water pressure decrease again. It was decided between ADC and GTK that stabilising the hole was the solution, as is often the case with problematic long holes. Aleksi suggests that pressurised groundwater at depths of ~900m was the root of the problem and was preventing using injector tools and chemicals successfully.

As it stands now 1.5km of the hole has been completed, and relatively smoothly. The impressive feat of engineering that produced the drill rig and problem-solving prowess of both teams is proving to be effective in completing this ambitious project and a new rig has just been installed that is more efficient. So, what have they discovered so far?

Driller send photographs of retrieved core regularly, the GTK team are remotely assessing the downhole lithologies, and it appears from a marked change in colour that the cause of the gravitational anomaly has been intersected at 1423m . Initially the change from a reddish grey granitic rock to a dark green to black rock was treated with caution and thought to be a small intrusion or vein, but so far it has continued for over 100m indicating a substantial unit (Figure 3). The interpretation currently sits at a metamorphosed ultramafic cumulate rock in which olivine has altered to talc, and pyroxene altered to amphibole.

Figure 3 - Metamorphosed Olivine-pyroxene-cumulate rock from the depth of 1540m.

Looking forward, GTK and ADC will hopefully be completing the hole in the coming months. They will then log all the drill core which will allow them to greatly advance their geological understanding of this area. Regional-scale 3D models will be developed from the data, and Aleksi hopes that it may also help in mineral exploration by facilitating the development of exploration vectors.

However, Geology is only one of the faculties GTK are hoping will benefit from this research project. Once the hole is completed and stabilised, it will offer a unique environment for research and innovation in geothermal energy and deep geothermal gradients, deep rock mechanics, and the development of downhole technologies. In addition, the process and technology of drilling such a deep hole is of interest to the private exploration and mining sector. They hope to attract interest from Finnish and international researchers and companies, which will help pay back the initial investment.

The significance this project doesn’t stop there either. Working remotely to assess the geology will provide understanding of the limitations and advantages of doing so, and what innovation is required to optimise this new working environment. This has been in part forced by the pandemic which has provided an opportunity for positive change. Developing efficient remote working environments increases accessibility to the world of work by people with families, as Aleksi points out, a change which I think we can all get behind.

For now, its cross fingers, continue the hard work, and complete the drill hole for Aleksi, his team, and ADC.

Esme

Further Information & Sources

Project Website here

Detailed write-up in Materia Magazine (In Finnish) here