The impact of fault-controlled hydrothermal silicification on the petrophysical properties of sandstones: insights from the Kornos-Aghios Ioannis Normal Fault (Lemnos Island, Greece)

Circulation of silica-bearing hydrothermal fluids along faults affects petrophysical and mechanical properties of fault-related rocks by modifying their texture and mineralogy, with strong implications on geofluid storage and seismicity in the shallow crust. However, in the subsurface, it is extremely difficult to predict the geometry of silicified rock volumes along and around fault zones as well as their petrophysical properties and, therefore, outcrop analogues can provide important insights. The Kornos-Aghios Ioannis Fault (KAIF) on Lemnos Island (Greece) is a silicified extensional fault system active at shallow depth (<1 km) that is well exposed over 10 km length and juxtaposes volcanic rocks against turbidite sandstones. In this study, we investigate the distribution, petrophysics and mineralogy of silicified rocks along two across-fault transects through a multi-analytical approach that combines data from X-ray diffraction analysis, Hg-intrusion porosimeter, digital image analysis, X-ray micro-computed tomography and unsteady-state gas permeameter. The permeability of silicified fault cores (i.e. breccias, cataclasites, ultracataclasites), characterized by quartz contents >70 wt. %, decayed of 3 orders of magnitude (from 10⁰ to 10^-3 mD) with respect to pristine host rocks as pore space was occluded by silica cements. In fault damage zones, porosity of massively silicified sandstones strongly varies in the range 2-13% because of the presence of dissolution intragranular and intercrystalline pores whose formation is strongly controlled by the mineralogy (i.e. microcrystalline silica, sulphides and feldspars are preferentially dissolved). However, permeability of these massively silicified rocks remains low (<0.01 mD), regardless of their porosity, due to the low connectivity of the pore network. In the silicified volume characterized by reduced permeability, that extends 100’s of meters from the master fault plane being locally greater than the damage zone, the permeability drop produced by cementation is partially counterbalanced by higher fracture density and connectivity because of increased rock brittleness (UCS increases up to 30% compared to pristine host rocks). Moreover, all the samples analyzed show that porosity values are sensitive to pressure and strongly decrease with increasing confining pressure (up to 17 MPa). Our results show that hydrothermal silicification along faults may strongly degrade the reservoir quality in the surrounding area (100’s of meters from the master fault plane) where its effect is only locally counterbalanced by an excess permeability produced by dissolution, fractures and subsidiary faults. However, the intensity and extension of silicification are heterogeneous along-fault strike and fault segments not affected by hydrothermal silicification can interrupt the along-strike continuity of low-permeability silicified fault rocks.