The impact of hydrothermal silicification on fault zone porosity and permeability: insights from the Kornos-Aghios Ioannis normal fault, Lemnos Island, Greece

Abstract

In fault zones, silica-bearing hydrothermal fluids may strongly affect petrophysical and mechanical properties of rocks with significant implications on fluid storage and flow potential. However, it is extremely difficult to predict the geometry and petrophysical properties of silicified rocks around km-long fault zones affecting reservoirs. The Kornos-Aghios Ioannis Fault (KAIF) is a 10-km long silicified extensional fault system juxtaposing volcanic rocks against turbidite sandstones. In this study, we investigate the distribution, petrophysics and mineralogy of silicified rocks through a multi-analytical approach that combines X-ray diffraction analysis, Hg-intrusion porosimetry, digital image analysis, X-ray micro-computed tomography and unsteady-state gas permeametry. Silicification is mostly localized in the footwall sandstones and extends 50–300 m from the master fault. The porosity of silicified fault cores and silicified sandstones varies over a wide range (1–13 %) depending on the degree of post-silicification dissolution that is strongly controlled by the mineralogy. However, the permeability of silicified rocks always decayed by 2–3 orders of magnitude with respect to pristine host rocks. In silicified volumes, permeability drops imparted by cementation are partially counterbalanced by higher fracture density and connectivity because of increased rock brittleness. Our results show that hydrothermal silicification along fault zones may severely degrade reservoir quality in the surrounding areas, where its effect can be locally counterbalanced by an excess permeability produced by silica dissolution, fractures, and subsidiary faults. However, the intensity and extension of silicification and dissolution are spatially variable, controlling the along-strike distribution of potentially sealing and non-sealing areas.