Effects of Texture and Composition on Mechanical Behavior of Experimentally Deformed Carbonate Rocks

Abstract

Dry, low-porosity carbonate rocks with a wide range of texture and dolomite content were experimentally deformed at room temperature, strain rate of 10-4 sec-1, and at confining pressures of 0, 50, and 100 MPa (1 MPa = 10 bars = 145 psi) to evaluate the effects of texture and composition on ultimate strength and ductility. Of all the factors considered, weighted mean grain size and microcrystalline carbonate (micrite) content have the highest linear correlation coefficients with ultimate strength. Grain size is, therefore, the dominant intrinsic rock property that affects ultimate strength in low-porosity carbonate rocks. Systematic increases in total dolomite content (determined by X-ray diffraction) do not correlate significantly with ultimate stre gth when all of the variously textured rocks are compared. However, ultimate strength does increase with increasing dolomite content in rocks of similar texture. The approximate ultimate strengths of carbonate rocks with intermediate dolomite and/or micrite content can be predicted by the best-fit plane to the ultimate strengths of pure end members (Yule marble, Solenhofen limestone, Hasmark dolomite, Blair dolomite). For experiments at 100 MPa confining pressure, this plane is defined by the equation: 0.90 D + 2.07 M + 269 = ^sgru, where D is dolomite content in percent, M is microcrystalline carbonate content in percent, and ^sgru is ultimate strength in MPa. The equation of the plane defined by ultimate strengths of rocks of intermediate composition and texture is: 1.07 D + 2.29 M + 258 = ^sgru. The mean difference between the strengths predicted by these two planes is only 9 MPa. Most precursive microfractures in nonmicritic elements are intragranular. Although the density of intragranular microfractures increases with increasing confining pressure, grain boundary cracks are suppressed by confining pressure. Very few microfractures occur in the micrite itself. At a given confining pressure, highly micritic rocks exhibit much less abundant precursive microfractures than coarser grained rocks, and hence they are stronger.