The Soultz-sous-Forêts granite located in the Rhine graben (France) has been chosen for the European Enhanced Geothermal System (EGS). Three wells have been drilled to a minimum depth of 5000 m in order to reach a temperature of 200 °C. At Soultz, the main fracture network in the basement is orientated approximately N–S with moderate to steep dip. Its geometrical relationship with the in situ stress field suits fluid circulation, hence the EGS project. However, fractures are firmly sealed due to natural fluids (fossil and/or possibly present). The main sealing minerals, as identified during previous studies, are clay minerals, calcite and quartz. However, some fractured zones remain permeable due to the high porosity developed in the wall rocks through hydrothermal alteration. A circulation test was carried out from July to December 2005 to test the performance of the Soultz geothermal reservoir. This test showed a significant difference in the production rate between the two outflow wells (GPK2 and GPK4). In order to improve the fluid circulation within the underground heat exchanger, chemical stimulations have been scheduled. In this framework, quantifying the calcite content of the granite provides a basis for identifying the calcite-rich zones as well as choosing the most suitable chemical stimulation. Measurement of calcite ponderal concentration was carried out in the cutting samples of the 3 deep wells (GPK2, GPK3, GPK4) between 4000 and 5000 m depth using manocalcimetry. Based on detailed measurements in the 3 wells, this study shows that calcite is not only a very common and ubiquitous hydrothermal mineral but that its content seems to be also spatially correlated with most of the fracture zones bearing natural flow. The conductivity of these natural flow pathways seems anti-correlated with the calcite content. Calcimetry has proved to be an easy, efficient and low-cost method for a better knowledge of hydrothermal sealing of the Soultz reservoir. This method can be applied with great benefit to all types of basement reservoirs (oil, gas, water, heat) overlain by sedimentary rocks for a better understanding of the fracture network permeability.