PRINCIPLES OF OIL ACCUMULATION 261 



the pressure on the water over that of a deeper sand, which might 

 not have been so. nearly saturated nor surrounded by so fine a 

 shale. 



The following estimate is made to show that the amount of 

 oil in any producing field could have been derived entirely from 

 shales immediately surrounding the oil sand. A series of shales 

 aggregating 10 feet of bituminous sediment, yielding 25 gallons 

 to the ton would furnish 17,000 barrels of oil per acre. Assuming 

 a 25 per cent extraction, the acre yield would be over 4,000 barrels. 

 The average acre yield in Oklahoma and Kansas ranges from 

 2,500 to 3,000 barrels. In case of 30,000-barrel-acre yield, which 

 is a rare exception, the total amount of oil, considering a 25 per 

 cent extraction, could be had from an acre bed of shale aggregating 

 50 feet of bituminous material yielding 40 gallons to the ton. 

 Large acre yields are probably due to concentration in the sand 

 along zones of exceptional openings, and the oil might have been 

 drained from an area of several acres. 



CONCLUSION 



The foregoing evidence leads to the following conclusions: 



1. Bituminous shales are in close relationship with the produ- 

 cing sand of an oil field. 



2. This bituminous material is in solid form and is only changed 

 to petroleum in local areas of differential movement. 



3. After such a change is made, the accumulation of oil into 

 commercial pools is accomplished by capillary water; and this 

 interchange only takes place in local areas where the oil-soaked 

 shale is in direct contact with the water of the reservoir rock. 

 Such conditions are explainable either by joints or faults. 



4. After the oil once reaches the sand, some adjustment takes 

 place until the oil has found the larger openings and then 

 it remains there indefinitely. 



out the Pennsylvanian rocks of Oklahoma, Kansas, and Texas. Moreover, in a 

 number of cases the. water may be reduced or exhausted by pumping the same as the 

 oil, and the decUne in production is according to the regular PV curve, showing that 

 the pool is not connected with a source of supply, but has a definite volume in a 

 definite space. 



