610 russell-dickey. POROSITY AND PERMEABILITY [Ch. 32 



and the permeability only slightly by comparison, because the finer and 

 less permeable pores are filled in preference to the larger and more 

 permeable ones. This contrasts with the effect of clay, where it is ex- 

 pected (but also not demonstrated) that the permeability may be re- 

 duced more than the porosity. The coalescence of the grains with the 

 retention of the larger pore openings may result in a hard quartzite 

 with considerable permeability. The texture and pore pattern of such 

 a rock bears little resemblance to those of packed spheres, or even 

 to those of a clean unconsolidated sand. 



POROSITY OF SHALES 



Fine silts and clays settled out of still water have a porosity greater 

 than 75 percent (Hedberg, 1926), the porosity increasing as the grain 

 size decreases (Trask, 1931) . The fine and platy character of the clay 

 particles causes them to settle into a sort of open lattice or cellular 

 structure. As the pressure of the superincumbent sediments increases 

 with continued deposition, compaction occurs by the collapsing of the 

 cells and the expulsion of the interstitial water. There is thus a con- 

 tinual decrease in the volume and porosity of argillaceous sediments as 

 the weight of the overburden increases. 



Soft clays have a high porosity but very low permeability, so that 

 the water is expelled slowly. The addition of an increment of over- 

 burden results in the partial collapse in the structure of the solids. 

 As the interstitial water cannot immediately escape, it assumes part 

 of the load, which causes an excess pressure in the water above the 

 normal hydrostatic pressure for its depth (Terzaghi, 1943). A pressure 

 gradient then exists which causes the water to filter slowly to regions of 

 lower excess pressure. During the early stages of compaction most of 

 the water is expelled upward. As compaction proceeds, however, the 

 permeability in a vertical direction is reduced more than that in a 

 horizontal direction. This is due partly to the flattening down of the 

 platy clay and micaceous particles and partly to the existence of more 

 permeable sandy layers that may extend considerable distances in a 

 horizontal direction. There are thus numerous paths of migration, with 

 different lengths and permeabilities, by which the water may flow to 

 zones of lower excess pressure. The water will distribute its flow in 

 proportion to the relative resistance of the various paths. Some of it 

 will therefore move long distances parallel to the bedding, and some 

 may actually move downward a short distance to reach a permeable 

 layer. During compaction, therefore, there exists a component of water 



