A micromechanical investigation of diagenetically-induced changes to the anisotropic elastic properties of calcareous mudstones

Abstract

AbstractMicroscale diagenetic changes that occur during burial exert a profound influence on the elastic and mechanical properties of sediments—but are poorly quantified. The focus here is on how diagenesis influences the elastic properties of carbonate-rich mudstones, which are subject to a wide range of physical and chemical changes. Nanoindentation data for gas-window (180 $$^\circ$$ ∘ C) Eagle Ford formation samples give intrinsic indentation moduli of the clay-sized calcite matrix of $$M =$$ M = 40–50 GPa, which contrasts with $$M =$$ M = 60–77 GPa for diagenetic calcite fills of foraminifera tests, closer to values for highly crystalline calcite. The matrix calcite is weakly anisotropic. Inverse analysis of immature (< 70 $$^\circ$$ ∘ C) organic-rich chalks gives much lower intrinsic indentation moduli for biogenic calcite, between 17 and 30 GPa; the calcite is also more anisotropic, with values of $$M_1/M_3 \sim$$ M 1 / M 3 ∼ 1.3. Diagenesis, which includes recrystallisation and pore-filling cementation, results in calcite becoming elastically stiffer and behaving in an increasingly isotropic manner, in agreement with grain scale studies using atomic force microscopy. The results demonstrate that nanoindentation can resolve diagenetic contributions to the mechanical response of mudstones, and suggest intrinsic structural changes to calcite, in addition to diagenetic cementation, need to be accounted for in rock-physics models of mud-rich sediments.