Abstract

Relative to the time-dependent compaction behavior of clean quartz sands, little experimental work has been carried out on phyllosilicate-rich siliciclastic rocks (mudstones). Compaction is expected to be produced by intracrystalline plastic deformation of the phyllosilicate matrix, coupled with pressure solution of embedded quartz grains and with silica reprecipitation in the adjacent pore spaces or loss from the system via an advective flux of pore water. Time dependence of compaction also arises in nature owing to the generation of excess pore pressure, which lowers effective stress and hence compaction rate until such pressure excess becomes dissipated by permeation.

A suite of experiments was performed on synthetic mixtures of quartz sand in a matrix of fine grained illite + muscovite, hydrostatically compacted at a range of effective pressures between 10 MPa and 210 MPa, with a constant pore water pressure of 70 MPa and at temperatures ranging between 300 and 450°C. A small adjacent volume of porous quartz sand was provided as a precipitation sink for silica dissolved from the phyllosilicate + quartz mixture. Volume strains were measured by (a) direct measurement of sample length and (b) measurement of amount of pore water expelled at constant pore pressure, and interpreted with the aid of quantitative petrography on compacted samples.

Control experiments were performed on (i) dry, clean sand, (ii) dry sand + phyllosilicate, and (iii) wet, clean sand. Dry, clean sand showed almost no compaction. Wet, clean sand showed a degree of compaction similar to dry phyllosilicate + sand, but wet phyllosilicate + sand showed dramatically more compaction. Compaction rate φ for the wet, mixed phyllosilicate/sand samples is described by  
formula
in which σ is effective stress (MPa), φ is porosity (%), R is the gas constant, and T is temperature (K). Petrographic study showed interpenetration of grain contacts, formation of quartz overgrowths, a decrease in the amount of quartz embedded in the phyllosilicate matrix, and an increase in mean grain size of quartz with progressive compaction. These effects are attributed to solution and redeposition of quartz.

The empirical compaction law was integrated over a "natural" burial history. The porosity evolution is extremely sensitive to the evolution of pore fluid pressure and relatively less to temperature gradient and burial rate. There is reasonable qualitative agreement when compared with porosity/depth relations reported in the literature.

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