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, anhydrite, 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. 

 Drilling-time characteristics are but one of many diagnostic properties of a 

 rock; therefore, the use of rate-of-penetration data must be considered as cor- 

 roborative 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 accommodate 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 anhydrite or a sandstone by "the way the bit acts." Certainly 

 a change from crystalline limestone to dense dolomite 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 

 other 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 cata- 

 logued 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 petrographic 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 90 percent quartz, 

 6 percent feldspar, 3 percent mafics, and 1 percent auxiliary minerals, for 

 example, will drill at a rate of 1 foot in 3 minutes and 15 seconds; nor, 

 conversely, that any rock which drills at that rate is necessarily that particular 

 type of sandstone. Under one set of conditions it may drill in exactly that 

 amount of time, and with other drilling conditions it may require much less 

 or much more time. Nevertheless, we are defining rate of penetration as a 

 fixed lithologic property, comparable to electric, radioactive, or mineralogic 

 properties, and the hypothetical fixed time required to drill 1 foot of sandstone 

 such as that described above could be defined as diagnostic of that rock. 



369 



