310 Subsurface Geologic Methods 



that of the drilling fluid; if not, the salinity will be normal or comparable 

 to that of cores from the oil column. 



4. Characteristic gas odor when fresh cores are broken. It is im- 

 portant to note that some gases have no apparent odor particularly when 

 composed primarily of methane and ethane, so that the lack of odor is 

 not always conclusive. 



5. An inspection of the mud sheath on the core may show evidence 

 of gas breaking from the pores of the sand. This is particularly true of 

 sands with low permeability. 



6. Connate-water saturations are usually less than forty percent of 

 the pore space. 



It is particularly difficult to differentiate between water- and gas- 

 productive sands when the reservoir pressure is very low. Low-pressure 

 gas sands may show total water saturations approaching 80 percent of 

 the pore space and still produce dry gas. 



Condensate Sands 



Gas-condensate or distillate sands from high-pressure reservoirs may 

 be recognized by characteristics similar to those for dry-gas sands, but 

 with the following differences: 



1. The high gravity of residual liquid hydrocarbons, usually above 

 50° A.P.I. 



2. Characteristic water- white color of the residual hydrocarbons; 

 saturation will vary between two and five percent of the pore space. 



Experience is the critical factor in detecting gas-condensate sands, 

 because the fluids retorted from sands with a high-shale content will 

 sometimes show a light-colored meniscus at the top of the water, which 

 may be taken as a condensate residual. 



Oil Sands 



Oil sands are not usually flushed by drilling fluids to so great an 

 extent as gas or water sands of comparable permeability, owing to the 

 influence of interfacial forces and the difference in the viscosities of the 

 fluids. Some low-pressure sands with very little solution gas and satu- 

 rated with oil of very low gravity may show very little flushing by drilling 

 fluid. 



The residual-oil saturation of an oil-productive sand will vary with 

 the factors discussed earlier in this paper under the heading "Residual 

 Oil and Condensate." 



Table 17 presents a relationship between the residual-oil gravity and 

 the normal saturation for oil sands having a reservoir pressure between 

 2,000 and 3,500 p.s.i. An inverse relationship is evident from table 17; 

 the higher the oil gravity, the lower the minimum residual oil content for 

 water-free production. For shallower sands with lower formation pressure, 

 the minimum saturation limits will be higher than those shown in the table. 



