There are no criteria denning a critical water saturation above which oil 

 production cannot occur. A shaly sand or one containing lentils or nodules of 

 shale can have a much higher critical water saturation than a clean quartzitic 

 sandstone; a coarse, permeable sand will have a lower critical water saturation 

 than a fine-grained, tight, silty sandstone, but may actually contain a higher 

 percentage of water due to mud filtrate loss to the formation. 



The permeability factor is a useful criterion for water saturation; a higher 

 water percentage can be tolerated with decreasing permeability, and oil produc- 

 tion can be forecast from rock of low permeability with comparatively high 

 saturations. A very porous, permeable sand will soak up mud filtrate like a 

 sponge, and a high water saturation need not condemn the well. 



The oil saturation figure is acutally more revealing than the water. A 

 marked decrease in oil saturation with but little upward change in water content 

 usually indicates a transition zone or the actual water table. This applies only 

 to cores taken with water-base mud. 



Residual oil saturations in oil-producing rock will range from 5 to 10 

 percent for high-gravity, volatile crudes; 14 to 24 percent for average crudes; 

 and 30 to 45 percent for heavy, viscous crudes. 



A low to a trace of oil saturation with normal water content usually indi- 

 cates gas production. Fractures extending into the water table, regardless of the 

 oil-productive possibilities of the reservoir, will almost invariably result in water 

 production. 



Gas-oil and oil-water contacts are illustrated by the core analysis shown in 

 Table 12-111. The gas-productive portion of the reservoir is predicted by the nor- 

 mal water saturation figures and zero to trace of oil down to sample 5. At this 

 point the oil saturations make a decided upward increase with little change in 



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