APLIKACE TEXTURNÍ RTG-DIFRAKČNÍ ANALÝZY V TEKTONICE – KVANTIFIKACE PŘEDNOSTNÍ ORIENTACE KALCITU V KARBONÁTOVÝCH HORNINÁCH
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The goal of this work was to apply texture x-ray diffraction analysis to study naturally strained rocks, in which the quantification of main preferred orientation cannot be conducted by the optical methods. This method has mainly been developed for metallography and its application in geology has been very limited so far. Samples of the fine-grained limestone have been collected from an outcrop, in which the direction of tectonic movement has been known. Thus, the tectonic situation could be correlated with the data obtained by XRD texture analysis. Analyses have been done by two devices with different geometry of experiment. The first experiment (Schulz reflection geometry) needed correction for the gain data, because of tilting of the sample, which led to the misalignment of the sample from the x-ray beam direction. The second one (in-plane geometry) has been measured, when the sample has been fixed and rotated, thus the correction was not needed. The results in a form of pole figures reflect the mechanism of deformation. The orientation of cleavage planes of calcite parallel to foliation indicates a cataclastic flow. Thus, the method could be used to study deformation mechanisms. The asymmetry of the results can show sense of shear, but it could also reflect inhomogenities of the samples.Keywords:
Cataclastic rock
Outcrop
Pole figure
Sample (material)
Cataclastic rock
Deformation bands
Dilatant
Overburden pressure
Micromechanics
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Cataclastic rock
Brittleness
Ductility (Earth science)
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Abstract Cataclastic bands in high-porosity sandstones significantly influence fluid flow, thus impacting the exploration and development of oil and gas. However, little experimental research has been conducted on the main factors controlling the formation, evolution, and physical properties of cataclastic bands. Moreover, it is difficult to use field surveys to discern variations and trends in the structural and physical properties of cataclastic bands formed during different deformation processes. In this study, we used a high-pressure and low-velocity ring-shear apparatus to analyze high-porosity, pure sandstone. Multiple sets of ring-shear experiments were carried out using the effective normal stress or shear displacement as a single variable. The experimental samples were analyzed based on physical property tests and thin sections. Our results indicate that the particles in the cataclastic bands generally have better roundness and are smaller (by at least two to three orders of magnitude) than the host rock. The porosity and permeability of the cataclastic bands are ~70% lower and two to three orders of magnitude lower than those of the host rock, respectively. The characteristics of the cataclastic bands are controlled by two main factors, namely, the effective normal stress and shear displacement. The effective normal stress controls the intensity of the cataclasis, and the shear displacement controls the physical properties of the grains and indirectly controls the evolutionary stage, which corresponds to the intensity of cataclasis. As the effective normal stress or shear displacement increases, the cataclasis in the cataclastic bands intensifies, and the grain size decreases; then, the decrease in the porosity gradually declines, and the permeability decrease and thickness increase and then plateau. The results of this study reveal the evolutionary mechanisms of the structural and physical properties of cataclastic bands in high-porosity sandstones and lay a theoretical foundation for determining the effect of these bands on fluid flow in oil and gas reservoirs.
Cataclastic rock
Deformation bands
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Cataclastic rock
Petrophysics
Deformation bands
Mylonite
Deformation mechanism
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A new quantitative method for the characterization of weak cataclastic fault rocks (e.g. fault breccia, gouges) is presented. Applied to find correlations with geomechanical laboratory tests, the thin section and X-ray diffraction analyses used for the method are on a similar scale. The method is shown to be appropriate for the characterization of a large range of different types of weak cataclastic fault rocks (kakirites) and involves appraising their mineralogical composition and microstructural characteristics. The correlation of the geological and geomechanical properties of kakirites based on this approach has shown promising results. The method could provide, in the future, the required geological input data for a mathematical modelling of the geomechanical behaviour of cataclastic fault zones.
Cataclastic rock
Characterization
Breccia
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