LARGE SCALE CHEMICAL STRATIFICATION OF GROUNDWATER IN THE CRUST, DATA FROM THE GEOTHERMAL RESEARCH SITE “URACH3” GERMANY

The Urach 3 research borehole in SW Germany has been drilled through the sedimentary cover and reached gneisses of the Variscian crystalline basement at 1600 m below surface (Black Forest basement). An additional 2800 m have been drilled through fractured crystalline rocks, and the borehole has been used for hydraulic tests in the context of a “hot-dry-rock” (HDR) project. The sedimentary rocks range from the Carboniferous to Dogger in age and comprise mostly clastic sediments in the Paleozoic and limestone and shale in the Mesozoic.

Water composition data are available from 11 different depths and include samples from all major lithologies. There is a gradual increase in the total of dissolved solids (TDS) from the surface to about 800m where it reaches 4.3 g/l in Triassic limestone (Muschelkalk). In lower Triassic sandstones, TDS increases very sharply to 26.7 g/l and the water is saturated with pure CO2 gas. With increasing depth TDS decreases first sharply then gradually to a low of 8.2 g/l at 1400 m in the lower Permian clastic sediments. The top of the crystalline basement at 1600m is marked by a very sharp discontinuous increase in TDS to 61 g/l. The water in fractured granite and gneiss remains at nearly constant composition to > 4000 m depth (TDS = 67.7 g/).

The major component signature of the waters is variable in the upper part of the section to the Muschelkalk strata (Cl/Br = 672) and reflects the local lithology at the water sampling point. The waters from the lower Triassic Buntsandstein (800m) to the bottom hole at 4400 m in the basement have similar compositional signatures although widely differing TDS. The waters are dominated by NaCl, Cl/Br ratios (mass) range from 120 to 90.

The water composition features along the 4.4 km profile suggest that there is very limited vertical chemical communication over long periods of time. The CO2-gas cap in the lower Triassic sandstones requires a gas-tight cover. The similar overall composition of the deep-water samples relates to similar overall mineralogy of the quartzo-feldspathic rock matrix. The sharp increase of the TDS in the basement can be related to the abundance of reactive plagioclase feldspar in gneiss and granite, lacking in the Permian clastic sediments.