Caprock genesis in hydrothermal systems via alteration‐controlled fault weakening and impermeabilization

The mechanical and hydraulic behavior of faults in geothermal systems is strongly impacted by fluid-induced alteration. However, the effect of this alteration on fault properties in geothermal reservoirs is under documented. This affects our ability to model the properties of subsurface structures, both in reservoirs and caprocks, and potential hazards during geothermal exploitation. We investigated fault rocks from the caprock of a fossil hydrothermal system in the Apennines. We combined field structural observations with mineralogical and microstructural analyses, friction experiments and permeability tests on rocks from representative faults that guided the circulation of hydrothermal fluids and the caprock formation. We document fault weakening induced by the effect of hydrolytic alteration leading to the enrichment of clay minerals along the slip surfaces of major faults. Alunite-clay-rich rocks, are much weaker (friction coefficient 0.26 < µ < 0.45) than the unaltered protolith (trachyte, µ = 0.55), favoring strain localization. The late-stage enrichment of clays along faults induces a local decrease in permeability of three orders of magnitude (1.62x10-19 m2) with respect to the surrounding rocks (1.96x10-16 m2) transforming faults from fluid conduits into barriers. The efficiency of this process is demonstrated by the cyclic development of fluid overpressure in the altered volcanic rocks, highlighted by chaotic breccias and hydrofracture networks. Permeability barriers also enhance the lateral flow of hydrothermal fluids, promoting the lateral growth of the caprock. Velocity-strengthening frictional behavior of alunite-clay-rich rocks suggests that hydrolytic alteration favors stable slip of faults at low temperature.