An Analysis of the Roles of Stress, Temperature, and pH in Chemical Compaction of Sandstones

Abstract

ABSTRACT Chemical compaction, also known as pressure solution, involves dissolution along grain contacts and transport of solute to the adjacent pore space by diffusion. The driving force for diffusion is a gradient in chemical potential, therefore chemical compaction requires persistent gradients in chemical potential along grain contacts, in order to drive diffusion continuously. Such gradients exist in porous rocks because of the heterogeneous distribution of stress over the grain surfaces. The rate of chemical compaction depends on stress, fluid pressure, temperature, and the composition of the pore fluid, which itself depends on various processes that add or remove solute. An alternative model of chemical compaction invokes the effect of sheet silicates on pH to account for localized dissolution of grains adjacent to stylolites, or where sheet silicates are present along grain contacts. In this model, the rate of chemical compaction is independent of stress and fluid pressure, depending only on temperature. This model is untenable because the proposed mechanism cannot give rise to the persistent gradient in chemical potential required to drive diffusion. However, sheet silicates may increase the rate of chemical compaction because of their influence on dissolution and diffusion rates. The development of anisotropic fabrics as a result of chemical compaction, and the widespread occurrence of stylolites in carbonates as well as sandstones, are inconsistent with the pH model and provide strong support for the role of stress in chemical compaction.