Figure 1. The pore-structure evolution model for the three-stage compaction process [7]: 
(1) Stage-I: Sand or silt grains are suspended in fine grained material. 

(2) Stage-II: Either the sand/silt grains have come into contact with each other forming a framework supported pore-structure (IIa), or the fine grained material has been compacted to form a matrix supported porestructure (IIb).

(3) Stage-III: Pore-structure for diagenetically altered shales, with the dominant phases being cementation (IIIa) and dissolution (IIIb).