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An experimental investigation of textural controls on the brittle deformation of dolomite Austin, Nicholas J.

Abstract

The brittle deformation of dolomite, which is a common lithology in numerous thrust systems and hydrocarbon reservoirs around the world, is strongly related to its textural features. Generally for the purpose of characterizing rock deformation these diverse features are shoehorned under the headings of porosity and grain size, based on the assumption that the concentration and length of the initial flaws vary proportionally with either one or both of these properties. In dolomite there is often substantial textural variation that influences the nature of the initial flaws, yet does not relate to grain size and/or porosity. In order to investigate the role of texture on the brittle deformation of dolomite, a series of 23 triaxial deformation experiments were performed at confining pressures of 25, 50, and 100 MPa, dry, at room temperature, on dolomite from three texturally distinct sample suites: the Badshot Formation, the Niagara Formation, and the Rock Creek quarry. The variations in the mechanical response of these mineralogically and chemically similar dolomites, and the ensuing microstructures, indicate that differences in texture can promote the transition from brittle faulting to cataclastic flow, and can vary the peak strength. Grain boundary textures promote or inhibit the ability of grains to shear and rotate with respect to one another, whereas the presence of intragranular flaws, such as cleavage, that act as weaknesses, promote intragranular deformation. Both grain and grain boundary textures strongly influence the mechanics of deformation, including the peak strength, due to their influence on the length and concentration of the initial flaws. An empirical failure criterion for the peak strength of dolomite is formulated that includes only the effective Young's modulus, the confining pressure, and the empirically defined uniaxial compressive strength; when a grain size term is added, the quality of fit to the experimental data is significantly reduced. Porosity and the concentration and length of the initial flaws are embedded in the effective Young's modulus, which is measured from the differential stress-axial strain curve for each deformation experiment.

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