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.
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.