The Tromm Region: Suitability for GeoLaB Research

With the decision on geological suitability, the exploration phase has been completed. The results show that the subsurface in the Odenwald offers favorable conditions for geothermal research. The detailed planning of the rock laboratory and its approval phase will now begin.

Geological Conditions for GeoLaB

The rock laboratory is planned to be built in the crystalline basement.

What is the crystalline basement? The crystalline basement consists mainly of granites and gneisses. It contains the oldest rocks on Earth and generally lies beneath all younger rock layers. Crystalline rocks are made up of many small crystals, mainly feldspar, quartz, and mica. Every continent rests on this crystalline foundation, which has a very similar composition everywhere. Over millions of years, additional rocks have been deposited in depressions on top of this foundation. These are known as sedimentary rocks. The crystalline basement can be used for geothermal heat production at depths of about 2 to more than 5 kilometers.

On both sides of the Upper Rhine Graben, rock layers have been uplifted over thousands of years. Today, these so-called rift shoulders form the well-known low mountain ranges of the Odenwald, Black Forest, Palatinate Forest, and Vosges. During this uplift, some sedimentary layers were eroded. As a result, crystalline rock is widely exposed at the surface here.

This is a major advantage for research. In the deep subsurface, investigations can only be carried out through boreholes. In the uplifted areas, by contrast, a rock laboratory gives direct access to the rock. There, experiments can be carried out under controlled conditions and observed over space and time. The rift shoulder and the deep rock were expected to share many geological features, so that key insights from the rock laboratory could be transferred to geothermal reservoirs. However, being close to the surface also creates differences. One example is temperature: in a rock laboratory just a few hundred meters below ground, it is significantly lower than at depths of 2 to 5 km. Even so, a rock laboratory could not be built at higher temperatures anyway, since work on site would hardly be possible above 40 °C.

The Tromm is located in the Odenwald, on the eastern shoulder of the Upper Rhine Graben. Here, crystalline rock has been uplifted to the surface. Beneath the graben itself, the same rock lies several kilometers deep. The Tromm rock is therefore closely related to the rock that can be used for geothermal energy in the Upper Rhine Graben. The Tromm is also of interest for GeoLaB research because a largely uniform granite body (the rock of the Tromm pluton) borders a fault zone here. Along this Otzberg fault zone, blocks of rock have shifted against each other, and the rock is heavily fractured. The juxtaposition of these zones is especially interesting for geothermal research.

These large-scale geological conditions were already known when the Tromm was first considered as a research area for GeoLaB. The exploration phase was intended to clarify whether the rock on site is actually suitable for a rock laboratory. The result: the Tromm offers good conditions for research. The next section explains this in more detail.

Results of the Exploration Phase

During the exploration phase, extensive data on the rock in the subsurface was collected. The data showed that the Tromm rock is moderately fractured overall and consists of three units. These rocks lie side by side and have different properties because of tectonic processes over many millions of years.

Geological structure: What does this structure look like in detail? The upper 400 meters of the subsurface consist of granite. Below this lies a 10-meter-thick transition zone. Here, millions of years ago, the rock was first deformed like modeling clay at depth and later broken up close to the surface. Geologists refer to this as a “mylonitic-cataclastic” boundary zone. Below it are metamorphic rocks, mainly gneisses. These rocks have been transformed by high pressure and high temperature.

Mechanical properties: Differences in the rock also affect its strength. Laboratory tests show that some areas are significantly “weaker” than others. These differences in mechanical strength also affect flow paths, because water appears to move preferentially along the boundaries between mechanically different rock layers.

Measurements from the borehole and from the surface also show that a geomechanical stress with a particular orientation prevails in the deeper subsurface. You can picture this like a book being pressed together between your hands: the pressure from one direction is stronger than from the others. In the subsurface, these forces are caused by tectonic movements, as continental plates shift against each other over long periods of time. Similar stress conditions are also found at other locations in Central Europe where geothermal heat could be extracted from deep, solid rock. This is an important aspect for the transferability of GeoLaB’s results.

Fractures: The stress conditions in the rock also influence how fractures and faults form. Measurements to date show that many of these structures in the subsurface run mainly in a north–south direction.

In the rock units investigated in the Tromm, some areas are more heavily fractured and altered than others. However, fractures in the rock do not automatically mean that water can flow through them. This depends mainly on how wide their openings are and whether they are connected to a network of water-bearing fractures. Most of the fractures examined in the boreholes are small or even closed. In GeoLaB-2, for example, many fractures have small openings of less than 4 mm, a few measure 5–10 mm, and only six are wider than 1 cm. Overall, this confirms the original assumption that the Tromm massif is a crystalline rock with varying degrees of fracturing.

Water system: Hydraulic borehole tests were used to investigate the immediate surroundings of the two exploratory boreholes. These tests show where water flows through the rock, in other words, how permeable it is. They revealed that water moved mainly in certain sections. In the GeoLaB-2 borehole, for instance, the rock was most permeable at a depth of about 280 m and moderately permeable at about 420 m. The lowest permeability was found in GeoLaB-1 at a depth of about 440 m, where the rock was virtually impermeable. Permeability around the boreholes is therefore highly variable.

Very precise temperature measurements in the borehole were also particularly revealing. They detect differences of around one tenth of a degree. Where water enters the borehole from the rock, the temperature changes very slightly. This made it possible to document open, water-bearing fractures indirectly.

The water pressures measured at depth indicate that the deep water is not directly connected to the surface. There are also some signs that the Tromm rock is not crossed by a single continuous fracture network. Instead, the fractures appear to be divided into separate compartments, like road networks in villages that are barely connected or not connected at all.

In the Tromm region, springs discharge water after rainfall and then dry up again. This suggests that the near-surface areas are largely hydrogeologically decoupled from the deeper parts of the Tromm. After rainfall, most of the water therefore probably flows toward the valley close to the surface, within the weathered layer, which is at most about 30 m thick. There it emerges at springs or is pumped from wells in the weathered layer or in young deposits in the Weschnitz valley. We are not aware of any wells in the granite that produce relevant quantities of water.