Precipitation sealing and diagenesis 1. Experimental results

During burial and diagenesis of granular aggregates, significant permeability reduction may be induced by the formation of low-temperature, authigenic minerals. To quantitatively assess the importance of this process, we have conducted a series of hydrothermal flow-through experiments using deionized water and labradorite/quartz sand. All experiments were conducted in a modified triaxial apparatus, configured to allow continuous permeability measurements. Under most of the conditions tested, significant permeability reduction is observed with no concurrent decrease in porosity. The overall permeability reduction sometimes exceeds 1 order of magnitude over 4 days and is positively correlated to temperature and deviatoric stress. Scanning electron microscope observations together with data from additional experiments show that the observed permeability reduction is entirely a result of secondary mineral growth. Si and Al concentrations in the postexperiment fluids are also correlated to temperature and stress, confirming the link between the chemical state of the system and permeability behavior. In all experiments, permeability reduction is fastest early and levels off in the late stages. To explain the permeability behavior as a function of time, a conceptual model is developed in which precipitation of authigenic minerals is rapid at early times while dissolution of quartz and labradorite is most active. As the system approaches equilibrium, the components necessary for secondary mineral formation are liberated at a lower rate, thereby causing precipitation to slow. Although authigenic mineral formation does not reduce total pore space in these experiments, there is a reduction in effective porosity, which results in permeability reduction.