26o ALEX. W. McCOY 



by the minerals in, and surrounding, the water horizon. If the 

 water shows no connection between different lenses in a sedi- 

 mentary series, surely continuous sands or porous zones cannot 

 be expected and consequently the analyses check the impossibility 

 of migration from one lens to the other. 



The pressure on the oil as it exists in the porous stratum is 

 the same as that on the water which previously occupied the 

 space. A volume of water replaces approximately an equal 

 amount of oil, consequently the pressure in the reservoir remains 

 the same. When the relation of oil to advancing water is such 

 that the oil may be trapped and forced forward by capillary water, 

 considerable pressure may accumulate on the oil due entirely to 

 the capillary action of the water. Such conditions would be local 

 if ever present and of little importance in commercial oil pools. 



Pressures on fluid in an oil sand are often spoken of as "rock 

 pressure" or "gas pressure." Neither term exactly defines the 

 accumulated pressure on the fluid. The maximum static pres- 

 sure available in any porous zone is a function of the size of the 

 openings around that stratum. The determining factor is the 

 capillary resistance of the water in the adjoining small openings. 

 This factor depends upon the character of the sediment and the 

 amount of compactness of the same. As the beds are more deeply 

 buried, the shales become more compacted, and consequently the 

 maximum available pressure in a given reservoir would be 

 increased with depth. In this way there is a relation of maxi- 

 mum pressure to depth in the upper 10,000 feet of sedirrient. 

 However, pressures do not necessarily increase with depth as 

 would be indicated by the term "rock pressure." The amount 

 of water originally in the reservoir and the tightness of the sur- 

 rounding shales are other factors which may greatly change any 

 relation of accumulated pressures to depths. A buried sand 

 deposit may be completely filled with water and surrounded by a 

 very fine shale, so that a small amount of settling^ would increase 



^ In this paper it is assumed that settHng of the strata has caused the fluid pres- 

 sure in water sands of most of the mid-continent oil pools. With various degrees of 

 saturation in sands of irregular sedimentation, the resulting fluid pressures after 

 settling would be quite variable and irrespective of depth. Such is the case through- 



