Experimental mechanical compaction of clay mineral aggregates—Changes in physical properties of mudstones during burial

Dry and brine-saturated clay aggregates ranging from pure smectite to pure kaolinite were compacted in the laboratory. Experiments were conducted by increasing vertical effective stress up to 50 MPa to study the changes in physical properties of mudstones during burial. The results suggest that the physical properties (porosity, density, acoustic velocity, etc.) of mudstones vary greatly with increasing effective stress, clay mineralogy and fluid content. Kaolinite aggregates are much more compressible than those composed of smectite. Brine-saturated clay mixtures are much more compressible than dry clay mixtures. Brine may soften and lubricate the clay matrix compared to dry clays, resulting in higher compressibility in the brine-saturated state. The lesser degree of compaction of dry smectite compared to dry kaolinite aggregates can be explained by the differences in grain size as kaolinitic clays have much larger grains than smectitic clays. The extremely fine-grained nature of smectite implies that the imposed vertical stress is distributed over a very large number of grain contacts so that the force per contact area is very low in smectite compared to that in coarse-grained kaolinite. At 20 MPa effective stress, corresponding to about 2 km burial depth with hydrostatic pore pressure, brine-saturated pure kaolinite compacted to 20% porosity, while pure smectite retained 41% porosity. Brine-saturated clay mixtures show higher Vp and lower Vs than dry clay mixtures. A pronounced difference in Vs was observed among the brine-saturated clay mixtures. The lowest Vs as a function of vertical effective stress was found for pure smectite. For a given porosity value, smectitic clays have higher velocities and elastic moduli than the kaolinitic clays. The Vp/Vs ratio is significantly higher in smectite than in kaolinite aggregates. Treating mudstones as a uniform class of rocks introduces significant errors in estimates of physical properties, which are strongly affected by the clay mineralogy, grain size, the amount of other minerals present and fluid content. Our results have implications for well log interpretation and mudstone and shale property determinations from seismic data at shallow burial depth (<2 km, 80 °C), above depths where significant chemical compaction takes place.