After decades of research on enhanced geothermal systems (EGS), induced seismicity is still a major obstacle in the development of these engineered systems. With this study, we draw attention to a key observation of the European EGS reference project in Soultz-sous-Forêts, the relationship between hydrothermal alteration of the crystalline basement and minor to aseismic deformation in natural fractures (Meller and Ledésert, 2017). Observations show that large seismic events are limited to unaltered granite, while only small seismic events occur in the hydrothermally altered zones. To systematically investigate hydrothermal alteration products from different zones of seismic response to hydraulic shear or fracturing, we investigate cuttings from the granitic section of the GPK1 well of the Soultz EGS site. We have selected about 120 rock samples from a depth between 2,800-3,597m in 1 to 10 m intervals with smaller intervals around the fractures that have been deformed during the 93OCT11 hydraulic stimulation. Here correlations between spectral gamma ray and temperature logs with the locations of seismic events and deformation were observed. During the 93OCT11 hydraulic stimulation of the open hole section of GPK1 well, aseismic deformation tended to preferentially develop in clay rich, high porosity – high permeability and relatively thick altered fracture zones. Here, we summarize and detail these observations and add new observations on the behavior of induced seismicity at the FORGE and Cape Modern sites that point to similar behavior.

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