Ch. 32] POROSITY OF SHALES 611 



flow in a lateral direction toward areas of less rapid sedimentation. 

 Occasionally pools are found in which the pressure in the fluids may be 

 considerably above the normal hydrostatic pressure at their depth, as 

 in certain reservoirs in the Texas Gulf Coast. It is possible that the 

 rate of sedimentation was especially fast, and that the clays were es- 

 pecially impermeable, so that it has taken the water a long time to 

 be expelled. 



When the pressure in the fluids in a sand reservoir is lowered to much 

 less than normal hydrostatic, a pressure gradient between the reservoir 

 and the adjacent shales is set up. This gradient causes a slow migra- 

 tion of water from the shale to the sand. A few oil pools, notably 

 Goose Creek, Texas, have shown subsidence of the surface. It has 

 been suggested (Pratt, 1927) that the subsidence is not due to the 

 withdrawal of solid sand particles, but to the local compaction of the 

 shales adjacent to the reservoir resulting from the migration of their 

 interstitial water into the sand. 



Although the compaction of shales is most rapid during the early 

 stages, it continues to great depths of burial. Athy (1930) determined 

 the density (which varies inversely with the porosity) of a large num- 

 ber of samples of Pennsylvanian and Permian shales from Oklahoma. 

 The density of these samples plotted against depth is shown in Fig. 12. 

 They show a diminishing rate of increase of density with depth from 

 about 2.2 at 600 feet to 2.6 at 5,000 feet. By extrapolation, it was 

 estimated that a density of 2.2 indicated burial to about 2,000 feet, 

 so that about 1,400 feet of overburden had been removed by erosion in 

 this area. If the density of the mineral grains is 2.7, a dry density of 

 2.2 indicates a porosity of about 19 percent, and a density of 2.6 in- 

 dicates a porosity of 4 percent. During burial from 2,000 to 7,000 

 feet, therefore, more than 1,000 barrels of water was expelled from 

 each acre-foot of shale. During the early stages of compaction much 

 greater quantities are expelled. 



The lateral migration of water may be an effective agent in the ac- 

 cumulation of oil. The chemical constitution of the organic matter in 

 the shales is unknown. It is largely insoluble in organic solvents and 

 probably contains oxygen and nitrogen, so that it does not resemble 

 hydrocarbons. If, however, hydrocarbons are formed from this mate- 

 rial by catalysis, radioactive bombardment, hydrogenation, or some 

 other mechanism, the moving water may sweep it along (McCoy and 

 Keyte, 1934). The oil droplets will tend to coalesce and segregate in 

 the coarser pore openings in the sands. If the oil is carried along as 

 tiny droplets in a current of water, it will be filtered out at any point 



