Abstract
Hydrogeological properties of fluid reservoirs in the brittle continental crust at 5 km have been deduced from hydraulic and chemical data provided by the Deep Heat Mining well Basel-1 in the south of the Upper Rhine rift valley (central Europe, Switzerland). The investigation was challenging because no direct temperature logs or fluid samples from the undisturbed reservoir exist. However, the properties of the undisturbed reservoir have been reliably reconstructed from short time hydraulic tests and the evolution of outflow water composition. The rock of the open hole sections (4629–5000 m) is predominantly coarse-grained undeformed poorly fractured quartz-monzodiorite. The permeability k = 5.8 × 10–18 m2 is characteristic for plutonic basement at 5 km depth. Fluid flow is restricted to few steeply dipping fracture zones in this section. Outflow water triggered by massive injection of river water contains predominantly NaCl. The total of dissolved solids (TDS) in the pristine reservoir at depth is about 45 g kg−1. The origin of the high salinity is probably fossil seawater. The water has been modified in the reservoir by desiccation reactions related to the partial and local hydration of the igneous reservoir rock. The estimated reservoir temperature of 185 °C using three different calibrations of standard fluid geothermometers is in excellent agreement with measured and extrapolated temperatures in the borehole. The consistent application of different fluid geothermometers confirms the rock control of the fluid composition.
Highlights
Fractures, faults, joints, and veins, i.e. features of brittle deformation, are the principal water-conducting structures in basement rocks and provide the dominant conduits for fluid flow in the brittle upper continental crust
During later injection the reservoir fluid was displaced along the main fractures to regions farther away from the borehole and into small fissures intersecting the main fractures
Since undisturbed original reservoir fluids were not sampled, the hydrochemical analysis refers to outflow water samples taken after the massive hydraulic stimulation with river water
Summary
Faults, joints, and veins, i.e. features of brittle deformation, are the principal water-conducting structures in basement rocks and provide the dominant conduits for fluid flow in the brittle upper continental crust. Measured permeability of the upper continental crust varies over a very large range (nine log-units) from 10–14 to 10–4 m s−1 (Stober & Bucher, 2006), depending on the predominant rock type at the studied site and the geological history of the drilled crystalline basement. This large variance and the permeability itself decrease with depth Saline brines have been described from several km depth in the continental basement worldwide (e.g. Banks et al, 1996; Edmunds & Savage, 1991; Emmermann et al, 1995; Frape & Fritz, 1987; Frape et al, 2004; Pauwels et al, 1993; Stober & Bucher, 1999a; Bucher & Stober, 2010; Sanjuan et al, 2016)
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