Ch. 32] CLAY CONTENT 607 



theory, the permeability of natural sands cannot be calculated from 

 the statistical parameters derived from the grain-size-distribution 

 curve. Most natural sands have smaller and variable porosity and 

 contain varying amounts of clay and interstitial cement, which have a 

 profound effect on permeability. 



Packing 



As pointed out by Slichter, the manner of packing of the grains af- 

 fects permeability and porosity, but very little experimental work has 

 been done to evaluate it. Fraser (1935) found that natural beach 

 sands have porosities ranging from 38 to 46 percent. The variation is 

 due, at least in part, to differences in the manner of deposition. The 

 manner of packing is changed by the compaction resulting from the 

 pressure of overlying strata. Tickell, Mechem, and McCurdy (1933) 

 reduced porosity about 3 percent by compaction under pressures up 

 to about 400 pounds per square inch. 



In general, sands in regions where the rocks are highly consolidated 

 show lower porosity than those in areas where the section has suffered 

 less compaction. Thus in the Appalachian Basin the average porosity 

 of oil-producing sands is between 15 and 20 percent, in the Mid-Con- 

 tinent area between 20 and 25 percent, and in the Gulf Coast area 

 between 25 and 30 percent. However, this reduction in porosity with 

 age and depth of burial is probably due much less to the physical re- 

 arrangement of grains under overburden pressure than to secondary 

 cementation. At any one locality there appears to be no regular and 

 consistent decrease in porosity with depth in sandstones, although there 

 is such a relation for shales that are more susceptible to compaction. 



Clay Content 



The porosity and permeability of sandstones are both greatly af- 

 fected by the amount of clay deposited with the sand grains. Very 

 little analytical work has been done to determine the effect of clay 

 content on porosity, permeability and capillary properties, but the wide 

 variations of these properties are more likely due to the presence of 

 clay and cement than to the grain size and shape, whose relations have 

 been reported. 



The porosities of oil-producing sands generally range between 10 and 

 30 percent. In any particular pool, the porosity ranges from a maxi- 

 mum down to a low value in the shaly layers. The clay may be present 

 as thin laminae of shale, or interspersed generally between the sand 

 grains. Much of the clay firmly adheres to the sand, so that ordinary 

 methods of mechanical analysis may report less clay than is actually 



