PERVASIVE PRESSURE SOLUTION TRANSFER IN A QUARTZ SAND

Abstract

The kinetics of deformation of a quartz aggregate by pervasive pressure solution can be, under certain conditions of temperature and grain size, strongly dependent upon the diffusivity of silica into the grain-to-grain contacts. An analysis of the factors affecting this key parameter (and less well constrained in the analysis of the problem of rock deformation by pressure solution) is presented. This analysis is based on recent advances on studying silica surfaces and particularly on the existence of a silica gel layer on the silica surfaces undergoing dissolution. By reinvestigating the electroviscous effect occurring at the grain-to-grain contact the present analysis shows that the diffusivity of silica at the grain-to-grain contacts is likely to be relatively similar to that in the bulk pore water (maybe 1 order of magnitude smaller but not more). This contradicts the previous work by Rutter [1976], which has been the key reference used in many subsequent papers to justify an extremely low value for the diffusivity of silica at the grain-to-grain contacts (5 orders of magnitude smaller than the diffusivity of silica in free water). This finding has dramatic implications concerning the deformation rate of quartz sands and sandstones by pressure solution in sedimentary basins with regard to (1) the limiting step affecting the kinetics of the process (diffusion of the solute or dissolution/precipitation chemistry) and (2) the existence of a thermodynamic equilibrium state when deformation by pressure solution occurs over geological timescales. A poroviscoplastic model is used to describe deformation associated with pervasive pressure solution transfer in quartz sands. This model is shown to be consistent with the current state of knowledge of the surface chemistry of silica. In addition, the comparison between this model and both laboratory and field data is rather good.