624 



Subsurface Geologic Methods 



Usually oil percentages, determined from core analysis, are much lower 

 above the gas-oil contact than below it, but this is not always true es- 

 pecially when the oil has a rather high gravity. Frequently a recovered 

 core near the water-oil contact appears to contain more oil than those 

 higher in the oil column. Past experience has indicated that cores usually 

 are subjected to' considerable flushing by filtrates from drilling muds 

 When water-base muds are used, oil is generally flushed from the core 

 and oil-saturation values are determined from core analyses may be too 

 low. Likewise, when oil-base mud is used, the water-saturation values de- 

 termined on some cores, especially those near or below a water-oil contact 

 may be erratic. It has also been found that some oil sections contain ar- 

 gillaceous materials, which may have various types of nonproducible 



7080 RTffeti 



„ °^80"230 20 40 



Ralenliol Mv Impedance. Ohms PERMEABILITY 



ELECTRICAL LOG 



Dorcys 



POROSITY 

 Pef cent 



Figure 324. Core-analysis data from cores obtained with 

 low-filtration drilling fluid in hole. 



50 100 



OL SATURATION 

 Ptr cent of 

 Pore Spoee 



