Structural and chemical controls of deformation bands on fluid flow: Interplay between cataclasis and diagenetic alteration

We examined cataclastic shear bands with varying degree of deformation and alteration, which formed in uncemented, arkosic sediments under identical kinematic conditions. The investigated outcrop in eastern Austria exposes numerous closely spaced sets of cataclastic shear bands formed at low burial depth. The uncemented host sediment consists of detrital quartz, albite, micas and metamorphic lithoclasts. We distinguished three types of cataclastic shear bands which differ in macroscopic and microscopic properties as well as in influence on fluid flow, i.e. single bands, multistrand bands and band clusters. All band types show preferred fracturing of sericited albite grains and decomposition of biotite through mechanical deformation and subsequent chemical alteration. These mechanisms reduce the mean grain size, increase the amount of phyllosilicates in the matrix and facilitate later growth of authigenic clay minerals. The dominant deformation mechanisms and influence on fluid flow are controlled by the initial composition and intensity of diagenetic alteration. We identified different evolutionary stages from a high-porosity host rock (Φ = 35%) to a deformation band cluster (Φ = 6%) acting as fluid baffles. The measured reduction in porosity of up to 29 % is reflected by retention of fluids along band clusters, multistrand bands and between intersecting bands. The timing and direction of the specific fluid flows can be determined by the interaction with the deformation bands. These findings suggest that localized deformation and associated diagenetic alteration in feldspar-bearing sediments may promote reservoir compartmentalization.