HR: 08:30h
AN: MR21A-03    [Abstracts]
TI: Anisotropy in Experimentally Compressed Kaolinite-Illite-Quartz Aggregates: Microstructure, Preferred Orientation and Acoustic Velocities
AU: * Voltolini, M
EM: voltolini@berkeley.edu
AF: Dept. Earth and Planetary Science, University of California., 307 McCone Hall, Berkeley, CA 94720-4767, United States
AU: Wenk, H
EM: wenk@berkeley.edu
AF: Dept. Earth and Planetary Science, University of California., 307 McCone Hall, Berkeley, CA 94720-4767, United States
AU: Mondol, N H
EM: nazmul.haque@geo.uio.no
AF: Department of Geosciences, University of Oslo., P.O. Box 1047, Blindern, Oslo, N-0316, Norway
AU: Bjørlykke, K
EM: knut.bjorlykke@geo.uio.no
AF: Department of Geosciences, University of Oslo., P.O. Box 1047, Blindern, Oslo, N-0316, Norway
AU: Jahren, J
EM: jens.jahren@geo.uio.no
AF: Department of Geosciences, University of Oslo., P.O. Box 1047, Blindern, Oslo, N-0316, Norway
AB: Shales and mudstones composed of clay minerals and quartz are important sedimentary rocks that frequently display anisotropy of physical properties. This study investigates anisotropy in experimentally compressed kaolinite-illite-quartz aggregates by determining preferred orientation (texture) of component phases and comparing results with acoustic anisotropy. Sample were prepared compressing clay (81% kaolinite, 14% illite and 4% K-feldspar)-silt (~99% quartz) mixtures (0 to 100% clay) at 5 and 50 MPa vertical effective stress to explore the role of clay content and compaction stress on the elastic properties. Optical and scanning electron microscopy have been used to characterize microstructures. Texture patterns are quantified based on synchrotron X-ray diffraction patterns analyzed with the Rietveld method. Preferred orientation of quartz is more or less random. Clay minerals are strongly oriented. Pole figures display axisymmetric (001) maxima parallel to the compression direction, ranging in strength from 1.6 to 8 multiples of a random distribution. Texture strength strongly increases with compaction pressure and clay content. Both microstructure and preferred orientation are essential contributions to aggregate elastic properties that have been calculated by averaging single crystal properties over the orientation distributions. Calculated P-wave velocity anisotropies range from 0% (100% quartz) to 44% (100% clay, 50 MPa). Anisotropy roughly doubles by increasing the vertical effective stress from 5 to 50 MPa. In experiments only P- and S-wave velocities parallel to the compression direction were measured and values (2-3 km/s) are much lower than those predicted by single crystal averaging (5-7 km/s) which we attribute mainly to the influence of porosity that was not considered in the model. With these experimental compaction data we are currently refining a model to determine macroscopic properties of mudstones and shales based on detailed information of the microscopic structure.
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting