Mechanical compaction and the brittle—ductile transition in porous sandstones

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Abstract
Grain scale brittle fracture is an important mechanism for porosity reduction and tectonic deformation in rocks under diagenetic and low grade metamorphic conditions. Microstructural observations and acoustic emission measurements were used to elucidate the micromechanical processes involved (1) in pressure-induced grain crushing, and (2) in the brittle-ductile transition in porous sandstones. Experimental observations show that the critical effective pressure for the onset of grain crushing decreases as a function of increasing grain size and of increasing porosity. The correlation among grain crushing pressure, porosity and grain size can be interpreted using a Hertzian fracture mechanics model. Experimental observations show that both porosity and effective pressure have profound influence on the inelasticity and failure mode. Based on such observations, a brittle-ductile transition map for porous rocks can be constructed in the effective pressure-porosity space. The theoretical predictions of a fracture mechanics model of a pore-emanated crack are in qualitative agreement with experimental observations on the effect of pressure and porosity on inelastic behaviour and failure mode. However, such a model overestimates the crack stabilization effect due to confining stresses since it neglects grain rotation and pore collapse which are important deformation mechanisms in porous rocks. Implications of the experimental and theoretical results on depth — porosity relation, aseismicity at the shallow region of subduction zones, and faulting in sandstones are discussed.

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