Characterization of fluids in the Lower Allochthon and Baltican basement of the Scandinavian Caledonides (COSC-2 borehole, central Sweden)

Abstract

The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project studies mountain building processes in a major mid-Paleozoic orogen in western Scandinavia by means of two boreholes (COSC-1 and COSC-2) in Åre municipality, Jämtland, central Sweden. The 2276 m deep COSC-2 borehole was completed in 2020. Subsequently, rising gas bubbles were observed in the borehole, rendering COSC-2 a target for downhole fluid sampling to better understand gas and fluid migration in the subsurface.Seven downhole fluid samples were collected from the COSC-2 borehole with a Leutert Positive Displacement Sampler (PDS) at depths of potentially fluid-conducting fracture zones between 810 and 2081 m. Target depths for fluid sampling were determined by borehole seismic surveys and downhole acoustic logging conducted at COSC-2 from 2020 to 2022.Downhole fluid samples were analyzed for their gas-to-water ratio, chemical gas composition (N2, H2, CH4, CO2, He, Ar, O2), noble gas isotopes (He, Ne, Ar), and water composition (cations and anions). Gas analyses were also performed on two borehole headspace gas samples. The characterization of the fluids also includes determination of their age based on U/Th-He and K-Ar dating methods, as well as depth of phase separation (degassing) of fluids in the subsurface. These analyses provide valuable information for tracking fluid migration at different scales, i.e., from the microscale (core studies, mm-cm) and mesoscale (borehole studies, dm-m) to the macroscale (seismic, tens of metres-km). The fluid studies are accompanied by mineralogical studies on drill core samples from matching depths to constrain fluid-rock interaction by comparing solid and liquid (gas and aqueous) phases.Our study of the chemical composition of fluids in the deep crust, as well as their age and interaction with rocks, will provide unique insights into fluid migration processes in a Paleozoic orogen and help understand similar processes in modern/current analogs such as the Himalaya.