Preferred orientation and elastic anisotropy in shales

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. D33-D40 ◽  
Author(s):  
Ivan Lonardelli ◽  
Hans-Rudolf Wenk ◽  
Y. Ren
Keyword(s):  
Seismic Anisotropy ◽  
Elastic Anisotropy ◽  
Rietveld Method ◽  
Orientation Distribution ◽  
Preferred Orientation ◽  
Crystallographic Preferred Orientation ◽  
Clay Particles ◽  
Crystal Properties ◽  
Overlapping Peaks ◽  
Structure And Microstructure

Anisotropy in shales is becoming an important issue in exploration and reservoir geophysics. In this study, the crystallographic preferred orientation of clay platelets that contributes to elastic anisotropy was determined quantitatively by hard monochromatic X-ray synchrotron diffraction in two different shales from drillholes off the coast of Nigeria. To analyze complicated diffraction images with five different phases (illite/smectite, kaolinite, quartz, siderite, feldspar) and many overlapping peaks, we applied a methodology based on the crystallographic Rietveld method. The goal was to describe the intrinsic physical properties of the sample (phase composition, crystallographic preferred orientation, crystal structure, and microstructure) and compute macroscopic elastic properties by averaging single crystal properties over the orientation distribution for each phase. Our results show that elastic anisotropy resulting from crystallographic preferred orientation of the clay particles can be determined quantitatively. This provides a possible way to compare measured seismic anisotropy and texture-derived anisotropy and to estimate the contribution of the low-aspect ratio pores aligned with bedding.

Preferred orientation and elastic anisotropy of illite-rich shale

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. E69-E75 ◽  
Author(s):  
Hans-Rudolf Wenk ◽  
Ivan Lonardelli ◽  
Hermann Franz ◽  
Kurt Nihei ◽  
Seiji Nakagawa
Keyword(s):  
Grain Size ◽  
Seismic Anisotropy ◽  
Elastic Anisotropy ◽  
Rietveld Method ◽  
Transverse Isotropy ◽  
Orientation Distribution ◽  
High Energy ◽  
Quantitative Information ◽  
Preferred Orientation ◽  
X Rays

Shales display significant seismic anisotropy that is attributed in part to preferred orientation of constituent minerals. This orientation pattern has been difficult to quantify because of the poor crystallinity and small grain size of clay minerals. A new method is introduced that uses high-energy synchrotron X-rays to obtain diffraction images in transmission geometry and applies it to an illite-rich shale. The images are analyzed with the crystallographic Rietveld method to obtain quantitative information about phase proportions, crystal structure, grain size, and preferred orientation (texture) that is the focus of the study. Textures for illite are extremely strong, with a maximum of 10 multiples of a random distribution for (001) pole figures. From the three-dimensional orientation distribution of crystallites, and single-crystal elastic properties, the intrinsic anisotropic elastic constants of the illite aggregate (excluding contribution from aligned micropores) can be calculated by appropriate medium averaging. The illitic shale displays roughly transverse isotropy with [Formula: see text] close to [Formula: see text] and more than twice as strong as [Formula: see text]. This method will lend itself to investigate complex polymineralic shales and quantify the contribution of preferred orientation to macroscopic anisotropy.

Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models

2021 ◽  
Author(s):  
John Keith Magali ◽  
Thomas Bodin ◽  
Navid Hedjazian ◽  
Yanick Ricard ◽  
Yann Capdeville
Keyword(s):  
Seismic Tomography ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Texture Evolution ◽  
Scaling Law ◽  
Preferred Orientation ◽  
Elastic Model ◽  
Effective Anisotropy ◽  
Crystallographic Preferred Orientation ◽  
Radial Anisotropy

<p>Large-scale seismic anisotropy inferred from seismic observations has been loosely interpreted either in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) development of mantle minerals or extrinsic anisotropy due to rock-scale Shape Preferred Orientation (SPO). The coexistence of both contributions misconstrues the origins of seismic anisotropy observed in seismic tomography models. It is thus essential to discriminate CPO from SPO in the effective anisotropy of an upscaled/homogenized medium, that is, the best possible elastic model recovered using finite-frequency seismic data assuming perfect data coverage. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and highly-heterogeneous Earth's mantle. The problem is applied to a 2-D marble cake model of the mantle with a binary composition in the presence of CPO obtained from a micro-mechanical model. We compute the long-wavelength effective equivalent of this mantle model using the 3D non-periodic elastic homogenization technique. Our numerical findings predict that overall, upscaling purely intrinsically anisotropic medium amounts to the convection-scale averaging of CPO. As a result, it always underestimates the anisotropy, and may only be overestimated due to the additive extrinsic anisotropy from SPO. Finally, we show analytically (in 1D) and numerically (in 2D) that the full effective radial anisotropy ξ<sup>*</sup> is approximately just the product of the effective intrinsic radial anisotropy ξ<sup>*</sup><sub>CPO</sub> and the extrinsic radial anisotropy ξ<sub>SPO</sub>:</p><p>ξ<sup>* </sup>= ξ<sup>*</sup><sub>CPO </sub>× ξ<sub>SPO</sub></p><p>Based on the above relation, it is imperative to homogenize a texture evolution model first before drawing interpretations from existing anisotropic tomography models. Such a scaling law can therefore be used as a constraint to better estimate the separate contributions of CPO and SPO from the effective anisotropy observed in tomographic models.</p>

Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab

Nature ◽  
2008 ◽  
Vol 455 (7213) ◽  
pp. 657-660 ◽  
Author(s):  
Rei Shiraishi ◽  
Eiji Ohtani ◽  
Kyuichi Kanagawa ◽  
Akira Shimojuku ◽  
Dapeng Zhao
Keyword(s):  
Seismic Anisotropy ◽  
Preferred Orientation ◽  
Crystallographic Preferred Orientation

scholarly journals Preferred orientation of ettringite in concrete fractures

2009 ◽  
Vol 42 (3) ◽  
pp. 429-432 ◽  
Author(s):  
Hans-Rudolf Wenk ◽  
Paulo J. M. Monteiro ◽  
Martin Kunz ◽  
Kai Chen ◽  
Nobumichi Tamura ◽  
...  
Keyword(s):  
Single Crystal ◽  
Elastic Properties ◽  
Elastic Anisotropy ◽  
Rietveld Method ◽  
Orientation Distribution ◽  
Sulfate Attack ◽  
X Ray ◽  
Fracture Surfaces ◽  
Concrete Fractures ◽  
Loss Of Strength

Sulfate attack and the accompanying crystallization of fibrous ettringite [Ca6Al2(OH)12(SO4)3·26H2O] cause cracking and loss of strength in concrete structures. Hard synchrotron X-ray microdiffraction is used to quantify the orientation distribution of ettringite crystals. Diffraction images are analyzed using the Rietveld method to obtain information on textures. The analysis reveals that thecaxes of the trigonal crystallites are preferentially oriented perpendicular to the fracture surfaces. By averaging single-crystal elastic properties over the orientation distribution, it is possible to estimate the elastic anisotropy of ettringite aggregates.

Texture Analysis of Earth Materials. Comparison of EBSD With Other Diffraction Techniques

1999 ◽  
Vol 5 (S2) ◽  
pp. 228-229
Author(s):  
H.-R. Wenk
Keyword(s):  
Orientation Distribution ◽  
Two Dimensions ◽  
Polycrystalline Materials ◽  
X Ray Diffraction ◽  
Electron Backscattered Diffraction ◽  
Crystallographic Preferred Orientation ◽  
Orientation Imaging ◽  
New Information ◽  
Overlapping Peaks ◽  
Diffraction Patterns

An important feature of polycrystalline materials is the orientation distribution of crystallites also known as crystallographic preferred orientation or texture [1]. Conventionally it is measured by x-ray diffraction, averaging over sample surfaces. With demands for more quantitative material characterization, both in engineering and earth sciences, new methods have been developed. Two dimensions are of interest: Averages over larger sample volumes give a better representation to estimate bulk physical properties. Here neutron diffraction is advantageous. Because of minimal absorption large sample volumes (1-50 cm3), rather than surfaces can be analyzed [2]. If textures are locally heterogeneous, it may be of importance to analyze small regions. With a synchrotron microfocus beam volumes as small as 5 μm3 can be characterized [3]. These methods have been quantified and are extensively applied in metallurgy and geology. They provide good statistics for appropriate sample grain size but they analyze bulk textures and contain no information on local orientation correlations. Furthermore, for geological samples with low crystal symmetry, the diffraction patterns are often very complex with many overlapping peaks, making identification difficult. In such cases orientation imaging, using electron backscattered diffraction patterns (EBSD) in the SEM is useful [4]. Though grain statistics are generally much inferior, new information can be gained.

Elastic anisotropy of oceanic serpentinites – influence of CPO and microstructure

2021 ◽  
Author(s):  
Rebecca Kühn ◽  
Jan Behrmann ◽  
Rüdiger Kilian ◽  
Bernd Leiss ◽  
Michael Stipp
Keyword(s):  
Physical Properties ◽  
Elastic Properties ◽  
Seismic Data ◽  
Elastic Anisotropy ◽  
Rietveld Method ◽  
High Energy ◽  
Low Grade ◽  
Crystallographic Preferred Orientation ◽  
X Ray ◽  
The Impact

<p>Physical properties of rocks are mainly controlled by the modal composition, crystallographic preferred orientation (CPO) and microstructure of a rock. One of the most relevant physical properties related to the interpretation of seismic data are the elastic properties of a mineral aggregate. Changes of elastic properties - and hence changes in our interpretation of the tectonic architecture of certain regions - can be related to mineral reactions and deformation.</p><p>In order to explore the impact of mineral reaction and deformation on elastic anisotropy, we study oceanic serpentinites formed at low-grade metamorphic conditions by hydration of peridotites. Samples are obtained from the Atlantis Massif, which is an Oceanic Core Complex located at 30°N, Mid-Atlantic Ridge. During IODP Expedition 357, oceanic serpentinites were recovered from drill cores along the southern wall of the Massif. Fully serpentinized samples displaying variable microstructures were analyzed regarding the influence of microstructure and CPO on the overall elastic anisotropy. Microstructure analysis was based on optical microscopy and large area micro X-ray fluorescence mapping. For CPO analysis synchrotron high energy X-ray diffraction in combination with the Rietveld method was applied and the derived CPO was used to compute seismic properties.</p><p>Serpentinites with a typical mesh microstructure are interpreted to represent undeformed samples and show a close to uniform CPO. The increase in fabric anisotropy of vein-like magnetite aggregates is interpreted as an increase in deformation. Samples show a single c-axis-maximum and enhanced CPO. Calculated seismic anisotropies show up to >5% anisotropy for compressional waves (Vp) and shear wave splitting up to 0.15 km/s in the deformed samples. Hence, such an anisotropy can be used to differentiate deformed from undeformed zones in seismic data sets using the elastic anisotropy data.</p>

scholarly journals Linking preferred orientations to elastic anisotropy in Muderong Shale, Australia

Geophysics ◽  
2015 ◽  
Vol 80 (1) ◽  
pp. C9-C19 ◽  
Author(s):  
Waruntorn Kanitpanyacharoen ◽  
Roman Vasin ◽  
Hans-Rudolf Wenk ◽  
David. N. Dewhurst
Keyword(s):  
Aspect Ratio ◽  
Volume Fraction ◽  
Elastic Anisotropy ◽  
Ambient Pressure ◽  
Preferred Orientation ◽  
Validity And Reliability ◽  
Crystallographic Preferred Orientation ◽  
Aspect Ratios ◽  
Average Aspect Ratio ◽  
Self Consistent

The significance of shales for unconventional hydrocarbon reservoirs, nuclear waste repositories, and geologic carbon storage has opened new research frontiers in geophysics. Among many of its unique physical properties, elastic anisotropy had long been investigated by experimental and computational approaches. Here, we calculated elastic properties of Cretaceous Muderong Shale from Australia with a self-consistent averaging method based on microstructural information. The volume fraction and crystallographic preferred orientation distributions of constituent minerals were based on synchrotron x-ray diffraction experiments. Aspect ratios of minerals and pores, determined from scanning electron microscopy, were introduced in the self-consistent averaging. Our analysis suggested that phyllosilicates (i.e., illite-mica, illite-smectite, kaolinite, and chlorite) were dominant with [Formula: see text]. The shape of clay platelets displayed an average aspect ratio of 0.05. These platelets were aligned parallel to the bedding plane with a high degree of preferred orientation. The estimated porosity at ambient pressure was [Formula: see text] and was divided into equiaxial pores and flat pores with an average aspect ratio of 0.01. Our model gave results that compared satisfactorily with values derived from ultrasonic velocity measurements, confirming the validity and reliability of our approximations and averaging approach.

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