Towards quantifying the matrix permeability of fault damage zones in low porosity rocks

Abstract

In nature, permeability is enhanced in the damage zone of faults in crystalline rocks, where fracturing occurs on a wide range of scales. Understanding this permeability structure is paramount for predicting crustal fluid flow. We combine quantitative field and laboratory measurements to predict microfracture damage zone permeability in low-porosity granitic rocks as a function of distance from the fault core and displacement. Microfracture controlled matrix permeability exerts an increasingly dominant role on fluid flow with increasing depth. In the field we analysed the scaling relationships of microfracture densities surrounding strike-slip faults developed in granodiorite within the Atacama fault system in northern Chile. Displacements ranging over 5 orders of magnitude (∼0.012–5000m), allow the variation of microfracture damage with increasing distance from faults to be determined empirically as a function of displacement. We reproduce microfracture damage in the laboratory in a suite of triaxial deformation experiments by inducing cyclic damage in initially intact samples while continuously measuring permeability. Combining field and laboratory datasets through the microfracture density allows the permeability profile with distance from the fault to be predicted from fault displacement.