Concentration changes of pore solutions during compaction of clay sediments

ABSTRACT Pure montmorillonite and kaolinite clay muds with solutions of NaCl and CaCl2 have been compressed at pressures between 30 and 3200 atm. For a given clay the equilibrium porosity reached at a distinct pressure does not depend on electrolyte concentration. Compaction rate and clay mineral texture, on the other hand, are strongly influenced by salt concentration: the compaction rate is slow and the texture good if salt concentration is low, and vice versa. This behavior is explained by an aggregate structure of salt-containing clays. For pressures between 30 and 800 atm and with montmorillonite clay the concentration of electrolyte in the pore solution diminishes with increasing compaction. This can be explained by the electrochemical properties of base exchanging clays. At h gher pressures up to 3200 atm an increase of salt concentration within the remaining pore solution has been observed which may be caused by inclusion of small droplets of solution in the highly compressed clay acting as a barrier to ions. The experimental results give new evidence for the origin of some saltrich pore solutions in sedimentary rocks.