The results from this experimental study show that minor amounts of clay mineral precipitating within a granular medium can drastically reduce permeability. In this study, de-ionized water equilibrating with a labradorite/quartz sand under variable P-T conditions results in the precipitation of a smectite, tentatively identified as Ca-beidellite. This secondary smectite precipitation causes permeability reduction often attaining an order of magnitude and in some cases, two orders of magnitude. The total permeability reduction observed over the course of four days is exponentially correlated to temperature and is also a function of stress, up to a limit set by the crushing strength of the material. Chemical analyses of Si and Al in the post experimental fluids exhibit a similar dependence on temperature and stress.

Using the acquired permeability and chemical data together with textural observations, a conceptual model has been developed, tested and confirmed. The basis of the model is that dissolution and precipitation is most intense during the early, undersaturated stages of each experiment, thus explaining why permeability reduction is most important early on. According to the model, Si concentration in the fluid is dominated by quartz dissolution and plays the most important role in determining the equilibrium state of the system. The presence of quartz causes the fluid to reach labradorite equilibrium more rapidly than if no quartz is present in the system, consequently resulting in less permeability reduction. This effect is observed in a labradorite-only experiment, which shows much greater permeability reduction than does the analogous quartz-labradorite experiment. The time scale for fluid equilibration is observed to be the same as that for permeability reduction. Si and Na increase to a plateau concentration while Ca and Al exhibit early concentration peaks which correlate to the onset of beidellite precipitation.

This study has shown that small amounts of secondary mineral precipitate can dramatically affect the flow of fluid through granular media. Unlike the experiments of this study, natural environments are most often open systems possessing thermal gradients. In such cases, equilibration of pore fluids will occur on time scales that are many orders of magnitude greater than was the case in this study. This then implies that the resulting drops in permeability would be much larger than the data of this study suggest. A full understanding of how these chemical changes affect permeability will critically improve the comprehension of fluid flow in sedimentary basins.

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