A review and discussion on the influences of grain-coating clay minerals on water-rock interactions in sandstones

This work reviews the current understanding and challenges in evaluating the influences of grain-coating clay minerals on water-rock interactions in sandstones. Subsequently, mathematical formulations and reactive transport modelling were employed to advance the understanding of clay coats. Firstly, clay coats can reduce the accessible surface area of detrital grains, which is collectively controlled by the coat coverage, clay platelet thickness, and the width of gap between neighbouring clay platelets. Nevertheless, an extensive coat coverage does not necessarily lead to a significant decrease in the accessible surface area of detrital grains. Secondly, clay coats can retard the diffusion of ions and molecules passing through the coating layer, which is jointly controlled by the microporosity, tortuosity, and thickness of the coating layer. These two effects may significantly decelerate the dissolution rate of detrital reactive grains being coated (e.g., feldspars) and inhibit the precipitation of secondary quartz. Furthermore, the inhibition of secondary quartz by clay coats is also significantly controlled by the width of gap between neighbouring clay platelets and the solid barrier created by clay platelets themselves. Moreover, although the identification and quantification remain challenging, clay coats may undergo a significant dissolution. As a result, the reactivity and compositions of clay coats may significantly impact mineral dissolution and precipitation and sandstone porosity. This study, for the first time, integrates the multiple roles of clay coats in water-rock interactions in sandstones and may serve as a reference for analyzing the influences of clay coats on sandstone diagenesis and geological gas storage.