Pore-water squeezing from indurated shales

Abstract

High-pressure mechanical squeezing was applied to sample pore waters from a sequence of highly indurated and overconsolidated sedimentary rocks in a drillcore from a deep borehole in NE Switzerland. The rocks are generally rich in clay minerals (28–71wt.%), with low water contents of 3.5–5.6wt.%, resulting in extremely low hydraulic conductivities of 10−14–10−13m/s. First pore-water samples could generally be taken at 200MPa, and further aliquots were obtained at 300, 400 and 500MPa. Chemical and isotopic compositions of squeezed waters evolve with increasing pressure. Decreasing concentrations of Cl−, Br−, Na+ and K+ are explained by ion filtration due to the collapse of the pore space during squeezing. Increasing concentrations of Ca2+ and Mg2+ are considered to be a consequence of pressure-dependent solubilities of carbonate minerals in combination with sorption/desorption reactions. The pressure dependence was studied by model calculations considering equilibrium with carbonate minerals and the exchanger population on clay surfaces, and the trends observed in the experiments could be confirmed. The compositions of the squeezed waters were compared with results of independent methods, such as aqueous extraction and in-situ sampling of ground and pore waters. On this basis, it is concluded that the chemical and isotopic composition of pore water squeezed at the lowest pressure of 200MPa closely represents that of the in-situ pore water. The feasibility of sampling pore waters with water contents down to 3.5wt.% and possibly less opens new perspectives for studies targeted at palaeo-hydrogeological investigations using pore-water compositions in aquitards as geochemical archives.