Hydrogeological characteristics of the Outokumpu Deep Drill Hole

Extensive hydrogeological studies on the Outokumpu Deep Drill Hole R2500, drilled in 2004-2005, have been carried out during both the drilling phase as well as the post-drilling period. The present paper introduces the main results and characteristics of deep fluids and gases in the 2516 m deep hole drilled into a Paleoproterozoic formation of metasediments, ophiolite-derived altered ultrabasic rocks and pegmatitic granite. The main hydrogeological experiments during drilling were the daily monitoring of drilling fluid electrical conductivity, pH, composition and consumption (loss) of drilling fluid, as well as targeted fluid sampling and hydraulic testing during drilling breaks with the drill stem method. Hydrogeological sampling of the drill hole water with a tube method has been carried out three times to up to 1500-2350 m depths in the post-drilling period, and undisturbed formation fluid was pumped for several weeks from a packer-isolated fracture system at 967 m. The loss of drilling water during drilling was very heavy in the uppermost 1000 m of the hole (1-4 m3 of water per 1 m of drilling), but it decreased to a low level (<1 m3/m) at lower depths, indicating that hydraulically conductive fractures are more frequent in the first kilometre of bedrock than beneath. The hydraulic testing carried out at approximately 500-m depth intervals in 40-70 m thick sections indicated a similar pattern of hydraulic conductivity decreasing with depth: about 7.5 ·10-6 m/s at 500 m, 5.3 · 10-7 m/s at 1000 m, and practically impermeable rock at deeper levels. The electrical conductivity of the drill hole fluid rapidly increased in the post-drilling period due to the discharge of saline water from several fracture systems, and was monitored with repeated down-hole logs. In the uppermost 1000 m, fluid salinity has been in a semi-stable condition since about 2006, but in the deeper parts of the hole electrical conductivity continued to gradually increase in 2008 and 2009, when the most recent downhole logs and fluid sample profiles were obtained. The fluids are Ca-Na-Cl fluids with elevated Mg concentrations at the depths of the ophiolite-derived rocks of the Outokumpu assemblage. The fluids contain abundant gases, with methane and nitrogen being the main components. The stable isotope compositions (d2H, d18O) of the saline fluids indicate that they are not meteoric fluids but probably result from long-term water-rock interaction. The results indicate distinct water bodies isolated in fracture zones with minimal hydraulic connections.