Mapping secondary mineral formation in porous media using heavy metal tracers

A new technique is presented which allows easy identification of secondary minerals, formed during experimental diagenesis. Reacting fluids are doped with Ba and Sr; heavy metals which substitute into the alkali‐bearing authigenic minerals that precipitate because of alteration of primary labradorite and quartz. The main secondary phases observed are a Ba‐rich zeolite mineral, strontianite, and a Ba‐rich calcite. When polished sections of postexperiment specimens are viewed using scanning electron microscope backscatter imaging, secondary phases appear as bright patches due to their higher density and average atomic number. The more difficult method of X‐ray elemental mapping confirms that these zones represent secondary minerals rich in Ba and Sr. This technique allows the spatial distribution of secondary minerals to be mapped out at the specimen scale in a short time frame. Image analysis shows that observed reductions in permeability are caused primarily by mineral precipitation in narrow pore throats and in areas containing an abundance of fine‐grained material. Secondary mineral formation is concentrated near the top of the sample and is interpreted to migrate through the sample as a reaction front. Mineral coverage is then quantified and used to explain the observed permeability evolution.