Mechano-chemical feeback processes in deformation bands

Deformation bands are common features of strain localization in granular, porous sediments in the uppermost kilometers of the Earth`s crust. In contrast to discrete slip surfaces, deformation bands are characterized by a tabular zone of reduced porosity by grain rotation and/or grain fracturing with usually diffuse borders to the undisturbed host material. As mechanical grain size reduction and contributing chemical alterations within these structures act at the scale of individual sediment grains, high-resolution analytical techniques are necessary to assess their petrophysical properties and thereby constrain the governing mechanical and chemical processes. This project plans to employ a state-of-the-art multi-analytical approach of microscopic, chemical and tomographic methods to investigate selected samples of deformation bands with well defined kinematics from the Eisenstadt- Sopron Basin, Austria. Different rock types, i.e. arkosic sands and carbonate grainstones, containing deformation bands will be analyzed with regard to the preferred chemical alteration of particular mineral phases, growth of diagenetic minerals and cement within the pore space, degree of fracturing of original sedimentary grains and cement, crystal-lattice deformation of fractured grains, and distribution/reduction of pore space. A special focus will be drawn to the interplay between mechanical and chemical processes, which either compete or cooperate in the modification of the sedimentary fabric by reduction of grain size and/or porosity. The insights gained from the broad range of analytical techniques enable a comprehensive understanding of micro- scale deformation processes in granular sediments, which can be further applied to constrain petrophysical properties of deformation zones in porous hydrocarbon or groundwater reservoirs.

When high-porosity sediments are subjected to tectonic forces, they frequently form diffuse zones of deformation with petrophysical properties deviating from the undeformed host rocks, commonly described as deformation bands. Numerous examples of deformation bands have been identified in friable sands and high-porosity limestones in the Eisenstadt-Sopron and Vienna Basins. The project investigated the interplay between tectonic and diagenetic processes in the deformation bands. Using scanning-electron microscopy and micro-computed tomography, synsedimentary normal faulting could be identified as the cause of a pronounced reduction in porosity (from 35 to 2%) and permeability in Leitha Limestone. The faulting occurred prior to the precipitation of a pore-coating cement throughout the rock, which conserved the high primary porosity outside the deformation bands. Mechanical experiments conducted on this material suggest that the degree and distribution of calcite cementation plays represents a key parameter for the mechanical strength of the material, rather than the bulk porosity of the rock. A similar degree of permeability reduction (by 1-2 orders of magnitude) is observed in deformation bands crosscutting feldspar and mica rich unconsolidated sands in a different outcrop. In contrast, the fluid baffling effect is caused by (1) the preferred mechanical destruction of weak feldspar grains rather than strong quartz grains, and (2) the growth of clay minerals from the generated fragments. For comparison, samples were taken from a hydrocarbon reservoir in the Vienna Basin at ca. 1600m depth. Also in these sandstones, deformation bands could be identified. In contrast to the samples taken at the Earths surface, the pore space in these bands is filled by a iron-rich dolomite cement. The different degree of oil staining at either side of the deformation bands confirms their impact on fluid migration in reservoir rocks. Since the displacement along the bands rarely exceeds a few centimeters, these structures are invisible for seismic waves (providing a resolution around 20 m), and for most petrophysical logging tools. However, due to their strong effect on fluid migration, deformation bands may cause compartmentalization of reservoirs and problems in the production of hydrocarbons.