Compaction and lithification of muds (composed of normal terrestrial weathering minerals) into shales involves a major mineralogic change that has been followed in many sedimentary basins throughout the world. Unstable mineral components, such as potassium feldspar and mica, begin to decompose at temperatures around 60°C and the released chemical components react with dioctahedral smectite to produce mixed-layer illite/smectite, chlorite, and quartz. The extent of the reaction is highly dependent on temperature and time.

This diagenetic reaction may influence the rate at which liquid hydrocarbons are generated because the surface electrical charge of smectite--and therefore its efficiency as a catalyst--increases during the reaction. In addition, dehydration of smectite on its conversion to illite can lead to overpressuring of pore fluids that may be involved in migration of liquid hydrocarbons from the shale to reservoir rocks. The timing of liquid hydrocarbon and new pore-water generation, and overpressuring caused by smectite dehydration may be critical to the production potential of a sedimentary basin.

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