Rock Drilling time 



Minutes per foot 



Shale 4 



Sandstone 15 



Shale 6 



Sandstone 15 



Shale 12 



Sandstone 8 



It can be seen that the relative rates of drilling of shale and sandstone are 

 reversed in the top two and bottom two units. Nevertheless, the changes are 

 obvious, and these breaks aid the sample man in fixing boundaries on litho- 

 logic units. 



Factors other than the character of the rock also affect the rate of drilling. 

 Some of these factors are type of drilling fluid, speed of rotation, rate of circula- 

 tion of fluid, weight on the bit, and sharpness and kind of bit. A bit designed 

 for drilling shale would be poor for hard sandstones. When a new bit is run 

 into the hole, the rate of drilling is generally higher. However, the relative 

 rates in various lithic units are not greatly affected. 



Several oil-well service companies use devices which record the drilling 

 rates automatically. 



Electric, Radioactivity, and Other Logs 



The well-sample examiner must rely on electric, radioactivity, and other 

 mechanical logs for exact determination of lithologic boundaries. Certain con- 

 stituents of the rock column may be lost in the drilling fluid and thus be poorly 

 represented in the samples ; others, such as fissile shales or gypsum, may appear 

 in abundance by caving and thereby lead the microscopist astray. It is routine 

 practice for experienced sample men to work always with one of these physical 

 logs lying alongside the sample log strip and to check constantly the response 

 of the electric or radioactivity curves to lithologic changes observed in the cut- 

 tings. As mentioned earlier, these logs must be used judiciously, for radical 

 departures of the curves may have little or no bearing on lithologies ; for example, 

 a profound break in a resistivity curve may appear in a homogeneous sandstone 

 where oil or gas rest upon salt water. 



It is well to keep in mind that the electric and radioactivity logging devices 

 are geophysical instruments. The records (logs) obtained are geophysical 

 records, and they must be interpreted in a geologic or lithologic sense just as a 

 seismogram must be interpreted in terms of geologic conditions or phenomena. 

 It is always wise to study and compare the physical logs from nearby wells 

 where lithologies have been determined from cores. Armed with such data, 

 one can use the electric log empirically with some confidence. 



53 



