Shale Compaction

Lithification of shale occurs as a result of the compaction and cementation of wet mud.

Randomly oriented clay particles in the deposited sediment are reoriented as water is expelled during compaction. The compaction results from the increased load of newly deposited sediment. As the water content is reduced the pore-waters become more concentrated and cement is deposited from solution. Splitting surfaces form normal to the loading direction and parallel to the orientation of the platey clay minerals.

The process of volume reduction and water expulsion is called consolidation. If the fluid cannot be expelled the sediments remain unconsolidated. The rate of consolidation is controlled by the permeability of the sediment. Both porosity (volume of voids) and permeabilty (rate of fluid transmission) are drastically reduced by compaction and cementation.