Although most sedimentary rocks possess significant porosity and permeability when freshly deposited, the rate at which these vital properties undergo reduction with age is principally due to compaction. The compactability of a sedimentary rock depends more on the texture than it does on composition. The textures of many sedimentary rocks such as shale, siltstones, evaporites (rock salt), and dense limestones as well as igneous and metamorphic rocks are not effective reservoir materials because of compaction, unless they have been extensively fractured after they were formed. Fracturing produces secondary permeability meaning it was created AFTER the rock in question was formed. The fractures create new void spaces in the rocks which allow fluids to move into the voids. Primary permeability occurs DURING the compaction process when the original materials are forming the rock. The word "permeability" as it relates to this discussion of reservoir rock is used to mean both primary and secondary types.