The COSC-2 scientific drilling project: summary of science, operations, management and legacy

The Collisional Orogeny in the Scandinavian Caledonides (COSC) multi-disciplinary scientific drilling project characterises the structure and orogenic processes involved in a major collisional mountain belt. Located in western central Sweden, the project drilled its second fully cored borehole, COSC-2, during spring and summer 2020. It extends the COSC composite geological section, which above is composed of outcrops at Åreskutan mountain and the COSC-1 scientific borehole (drilled 2014), through the nappes of the Caledonian Lower Allochthon, the main décollement and the upper kilometre of basement rocks. In summary, the retrieved geological section differs partially from the expected geological section with respect to the depth to the main décollement and the expected rock types. COSC-2 targets include the characterisation of orogen-scale detachments, the impact of orogenesis on the basement below the detachment, and the Early to Lower Ordovician(?) palaeoenvironment on the outer margin of palaeocontinent Baltica. This is complemented by research on heat flow, groundwater flow, gas compositions and characterisation of the microbial community in the present hard rock environment of the relict mountain belt.

COSC-2 successfully, and within budget, recovered a continuous drill core to 2276 m depth. On-site scientific investigations on the drill core by experts were impeded by travel restrictions due to the Covid-19 pandemic. Thus, the core was first completely described in late 2021 at the BGR Core Repository for Scientific Drilling (Berlin Spandau, Germany). After further delay, the sampling party was held in mid-2022.

The entire operations, technical and scientific, were conducted on a 1600 m² drill site. COSC-2 was drilled by the Swedish national research infrastructure for scientific drilling, Riksriggen, with a core recovery close to 100 %. Drilling was performed with water as drilling fluid. Biodegradable polymers were added for drilling in greater depth to reduce friction. Down to 1576 m, HQ triple tube drilling was used (96 mm hole diameter, 61 mm core diameter), followed by NQ triple and double tube drilling to total depth (76 mm hole diameter, 45/48 mm core diameter). Drilling was directly followed by extensive downhole surveying. In autumn 2021, a major surface and borehole seismic survey was conducted, covering approximately an area of 20 km² around the borehole. In 2022, fluid-conducting zones were investigated and fluids sampled with different methods for geochemical (gas and fluid) and microbiological analysis.

The drill site was restored in 2022, leaving a 35 m long and 4 m wide access road to the borehole. This is sufficient for COSC-2 long-term downhole investigations. The borehole is also available for research that is not part of the original COSC project. However, observations during recent downhole investigations suggest that sedimentation with a rate of several tens of meters per year successively is limiting access to the deepest part of the borehole. Scientific results from the COSC project are presented in session TS6.4 "The Caledonian Orogen of the North Atlantic region: insights from geological and geophysical studies".