Clay mineral diagenesis and quartz cementation in mudstones: The effects of smectite to illite reaction on rock properties

The Late Cretaceous to Early Tertiary sequence of the Vøring and Møre Basins from the Norwegian Sea has been examined with respect to mineralogy based on 319 cutting samples from five wells. A clear relationship between mineralogy and well log data is demonstrated. A significant change with respect to velocity, porosity and density occurs within the depth interval corresponding to 80–90 °C. At shallow depths/temperatures (<2.0 km/70 °C), compaction is mainly mechanical and the physical properties are similar to what has been measured by experimental compaction of mudstones. At greater depths, however, the log derived velocities and densities are higher than those produced by experimental compaction indicating significant chemical compaction. XRD analyses show a progressive alteration of smectite to illite (S–I) within this depth/temperature interval which results in the release of significant amounts of silica into solution. Detrital silt and fine-grained quartz showed no secondary quartz overgrowths. These grains are isolated within a clay matrix and surrounded by clay minerals, thus limiting the available surface area and pore space for quartz overgrowths. Chemical analyses (XRF) indicate that silica is conserved within this depth interval, and the amount released from S–I alteration was locally precipitated. Field emission gun-scanning electron microscopy (FEG-SEM) and cathode luminescence (CL) identified authigenic micro-crystalline quartz cement within the clay matrix at temperatures above ∼85 °C. This is accompanied by an increase in velocity and density indicating that the S–I reaction and the precipitation of authigenic quartz caused a significant change in the rock stiffness.