Water and argon permeability of phyllosilicate powders under medium to high pressure

摘要

Phyllosilicates are abundant throughout the upper crust. They are commonly found in upper crustal faults. Influence of the mineralogy on the low permeability of phyllosilicate-rich rocks is poorly characterized. We measured water and argon permeability of eight phyllosilicate powders at room temperature (talc, pyrophyllite, kaolinite, illite, montmorillonite (particle size < 2 μm), and muscovite, phlogopite, and biotite (particle size < 20 μm). Tests were performed under hydrostatic conditions, with a constant pore pressure of 10 MPa (water or argon) and varying confining pressure up to 160 MPa. Results show phyllosilicate powders have very low permeabilities (10~(-19) m~2 to 10~(-24) m~2), depending on mineral, pore fluid, and effective pressure. Lowest permeabilities were found for montmorillonite and talc, while muscovite was the most permeable. Permeability decreases with increased compaction of powders, and for fully compacted powders (after 10 pressure cycles), pressure dependency of permeability follows an exponential relationship with a pressure sensitive gradient of 0.004 MPa~(-1) to 0.015 MPa~(-1). Water permeability is always lower (up to 1.8 orders of magnitude) than for argon. The difference in argon-water permeability can be related to the hydrophobicity and hydrogen-bonding surface properties of minerals tested. Talc, the most hydrophobic mineral tested, showed the smallest difference between argon and water permeability, while montmorillonite showed the largest. Permeability might also be affected by packing controlled by frictional strength of particles, as well as by particle size and shape. Results indicate that type of phyllosilicate mineral, pore water chemistry, and grain size distribution will strongly affect permeability in phyllosilicate-rich regions of the upper crust such as faults.