Geochemical characterisation of geothermal reservoir rocks in the Upper Rhine Graben


Among European geothermal sites, the Upper Rhine Graben (URG) has a great potential for a lithium (Li) production from geothermal brines due to its high concentration and the significant water flows exploited by the geothermal power plants in this area.

Despite its great potential, certain gaps in the basic knowledge of the geochemistry of the URG rocks are persisting, as there is scarce conclusive investigation carried out in the past to estimate the Li content as well as the mechanisms of Li recharge in brine. Identifying Li-rich geological units are essential to target areas with higher Li concentrations for exploration and to ensure the sustainability of this resource.

In geothermal systems, hydrothermal fluids circulate through the fractured and porous rock formations, undergoing complex interactions with the surrounding lithology. Various processes, such as leaching, dissolution, and precipitation, can occur and they can significantly influence the concentration of Li in the circulating fluids. Knowing the chemistry of the reservoir rocks could help us understand chemical reactions occurring between the hydrothermal fluids and the rocks and therefore how Li is mobilised and transported into the geothermal brine.

In the LiCORNE project, ESG is conducting detailed geochemical analysis of several core drills including granite, sandstone, and limestone from geothermal wells drilled in Northern Alsace. Researchers finalised the rock sampling task at the beginning of 2024, while the chemical measurements are expected at the end of June, current year.

AN ES-Geothermie employee sampling of granite rocks in the core shelter.

Sampling of granite rocks in the core shelter. © ES-Géothermie (ESG)

In total, 57 samples were collected and closely studied, which facilitates understanding of the chemical elements behaviour in the rock before and after the hydrothermal circulation/alteration. Comparing the results of this on-going investigation with the few data available in literature and referring to the Li concentration in URG rocks could reveal an unexpected behaviour of Li in the geothermal reservoir rocks.

After careful analysis of the chemical composition, isotopic analysis of the same rock will follow which will show more accurately potential sources of Li in the geothermal brine.

A. Fresh monzogranite sampled at 1774.5 m depth); B. Hydrothermally altered granite showing argillic alteration sampled at 2159.30 m depth

A. Fresh monzogranite sampled at 1774.5 m depth); B. Hydrothermally altered granite showing argillic alteration sampled at 2159.30 m depth. © ES-Géothermie (ESG)