A model for diagenetic mass transfer between adjacent sandstone and shale

Abstract

There are numerous suggestions that mass transfer between sandstone and shale is an important factor in the diagenesis of sandstone reservoirs. However, advective transport models for mass transport have not produced results that support these proposals. A model is developed that overcomes the problems of advective transport models by assuming that diffusive mass transfer is the principle transport mechanism. The process occurs when the rate of dissolution of reactive detrital components exceeds the local rate of diffusive transport. At that point in the burial history, units with greater amounts of a reactive component will generate more solute per unit time than units with less reactive component. The process forms chemical gradients that can rapidly transfer mass equivalent to several vol% of rock over distances of at least 5 m. The observed transport of potassium and silica between adjacent sandstone and shale in the North Sea during late-stage illitization was correctly predicted using this conceptual approach. The conceptual framework from the illite model can be extended to other diagenetic reactions, and suggests that the burial of adjacent sandstone–shale units could produce episodic diffusion gradients that transfer mass and form multiple generations of cement in sandstone reservoirs.