Subsurface Logging Methods 457 



Drilling Time: Diagnostic of Lithology 



The relationship between rate of penetration and lithology has been 

 understood for many years. The application of drilling time was recog- 

 nized at least seventy years ago,"^ and the interpretative value has been 

 appreciated for more than fifteen years. The use of drilling-time data 

 has widened continuously as mechanical devices for their recording have 

 become available and new applications of the data have been found. 



Drillers probably were the first to learn that a change in drilling 

 rate meant a change in the type of rock penetrated by the bit. Limestone, 

 cap rock, shale, or sandstone would be recognized prior to confirmation 

 by examination of the cuttings. Until recently the application of drilling- 

 time data was essentially one of qualitative significance, and methods of 

 observing rate of penetration were formerly far from exact. 



Regardless of the method of observation or the crudeness of technique 

 in measuring drilling time, one fact remains unchanged and should be 

 emphasized at the outset. Drilling-time characteristics are but one of 

 many diagnostic properties of a rock; therefore, the use of rate-of -pene- 

 tration data must be considered as corroborative of other ■ techniques by 

 which lithologic properties are recognized. Sample examination, coring, 

 electric and radioactivity logging, temperature and caliper surveying, 

 drilling-time logging, and other means of geologic observation must go 

 hand in hand to meet today's demand for more scientific methods for 

 finding oil and gas reserves. 



If, when drilling under uniform conditions, the bit's penetration 

 changes from a slow rate to a faster rate or vice versa, it is an indication 

 that a new type of lithology has been encountered. The obvious examples 

 are readily recognizable and well known. A driller could hardly fail to 

 know when he has encountered cap rock or a sandstone by "the way the 

 bit acts." Certainly a change from crystalline limestone to dense dolo- 

 mite or from hard shale to limestone is more difficult to observe, but any 

 change in the characteristics of lithology should cause a change in the 

 rate of penetration, provided all contributing factors remain constant. 



One might question whether rate of penetration is scientifically a 

 property of a rock because, at the present time at least, it is not capable 

 of being catalogued in quantitative terms. This is a weakness in technical 

 procedures, or the fault may be the inability to evaluate contributing 

 factors, but this does not alter the fact that rate of penetration is a petro- 

 graphic property. If there were no means of determining the identity 

 of mineral constituents of a rock, it would not be wrong to state that 

 mineral composition is a diagnostic property by means of which a specific 

 lithologic type could be identified. 



It can not be claimed that a certain sandstone having ninety percent 

 quartz, six percent feldspar, three percent femags, and one percent auxil- 

 iary minerals, for example, will drill at a rate of one foot in three minutes 



Carll, J. F., Discussion of Driving Time: Second Geological Survey of Pennsylvania, vol. 3, 1880. 

 By personal correspondence witli Dr. J. V. Howell. 



