An integrated model for transitional pressure solution in sandstones

A key aspect of the deformation of sedimentary rocks during diagenesis is pressure solution. However, it is often not clear why some rocks of a given mineralogy, depth, and pressure exhibit a great deal of pressure solution while very similar rocks under similar conditions do not. To address such difficulties, which we believe arise from the natural, non-linear physico-chemical processes, a quantitative model for pressure solution applied to sandstones has been developed. This model subsumes various factors: (1) pressure, temperature, and burial rate; (2) grain size, shape, and packing of the mineral grains; (3) the variability of water film thickness and coefficient of diffusion between any two grains in contact as a function of stress across the contact, pore fluid pressure, salinity, pH, and temperature. In this study we distinguish several types of surface sites over quartz grains in a sandstone with corresponding differences in chemical potential, thus allowing the diffusive transfer of matter from one site to another. The contact between two grains is divided into two parts. The actual true contact between two grains is a site of 'water film diffusion', and the inclusions inside the contact, the free-face inclusion contacts, are sites of 'free-face pressure solution'. On the pore surface site, precipitation occurs. It is shown that a kinetics transition from a reaction-limited deformation to a diffusion-limited deformation occurs in the upper crust. A geometric transition occurs too due to the textural evolution of the rock with deformation. The main goal of this paper is to show the dynamics of the shifting importance of actual and free-face inclusion contacts using a unified model including both features. The result is a predictive model for porosity variations with depth due to pressure solution in sandstones. (C) 1999 Elsevier Science B.V. All rights reserved.