Large‐scale chemical stratification of fluids in the crust: hydraulic and chemical data from the geothermal research site Urach, Germany

Abstract

The Urach 3 research borehole in SW Germany has been drilled through a sedimentary cover sequence and reached gneisses of the Variscan crystalline basement at 1604 m below surface. An additional 2840 m has been drilled through fractured basement rocks. The borehole has been used for hydraulic tests in the context of a ‘hot dry rock’ (HDR) project. The sedimentary cover ranges from the Carboniferous to the Middle Jurassic (Dogger) in age and comprises mostly clastic sediments in the Paleozoic and limestone and shale in the Mesozoic. Water composition data from 10 different depths include samples from all major lithological units. The total dissolved solids (TDS) increases from the surface to about 650 m where it reaches 4.1 g l−1 in Triassic limestone. In lower Triassic sandstones, TDS increases very sharply to 28.5 g l−1 and the water is saturated with pure CO2 gas. With increasing depth, TDS does not change much in the clastic sediments of the Permian and Carboniferous. The crystalline basement is marked by a very sharp increase in TDS to 55.5 g l−1 at about 1770 m depth. TDS increases within the basement to more than 78.5 g l−1 at about 3500 m depth. The data suggest that there is limited vertical chemical communication over long periods of time. The CO2 gas cap in the lower Triassic sandstones requires a gastight cover. The chemical stratification of the fluids relates to the permeability structure of the crust at the Urach site and fits well with hydraulic and thermal data from the site.