Effect of strain geometry on the petrophysical properties of plastically deformed aggregates: experiments on Solnhofen limestone

Abstract

Abstract Specimens of Solnhofen limestone were deformed under conditions where calcite deforms plastically using four experimental configurations: extension, torsion, direct shear and axisymmetric shortening. All experiments were run on dry specimens at the same temperature (600 °C), confining pressure (200 MPa) and comparable strain rates ( c. 10 −4 s −1 ). The different experimental settings and the heterogeneity of deformation within some of the specimens provided a large range of strain geometries. They allowed locally imposed strain geometries to be related to the crystallographic preferred orientation (CPO) patterns of calcite and the orientation of the shape fabric of calcite grains. CPO in calcite was measured using electron back-scattered diffraction (EBSD) in scanning electron microscopy. The development of CPO during deformation under the dominance of intracrystalline plasticity contains information about the strain geometry accumulated in rocks in 3D, although in nature the strain geometry can be modified by dynamic recrystallization that was not seen in the experiments. The different CPO patterns have a significant effect on the velocity structure of the deformed aggregates. Seismic properties inferred from CPO show that the orientation of the fastest V p wave aligns with principal strain directions that are not equivalent in different strain geometries.