The permeability of a sedimentary deposit is due to the small spaces between the particles that make up the deposit. These pores allow water to pass through the deposit.
As the deposits are buried under layers of deposits and the pressure on the layers lying right at the bottom increases, the particles are forced to come closer and the spaces between them decrease. An increase in pressure also leads to several transformations in terms of the physical and chemical properties of the particles. Many chemical reactions start to take place that increase the temperature and lead to a release of minerals that act like cement and help unite the small particulate matter. In many cases biological factors also play a part in the formation of sedimentary rocks.