Rock Classification from the Oil-driller's Standpoint

The ordinary well log is subjected to a great deal of criticism, much ofwhich is well founded. Sometimes, though, the difficulty in interpreting thelog is due to the fact that the geologist or engineer using the logs does notknow the limitations of the drilling method used. The rotary drill, especially, has inherent limitations that make it difficult to secure definite informationat all times. The identification of well-defined key beds is about all that canbe expected from the rotary log. The formation in a drilled hole, as reportedby the driller, has a direct relation to the speed with which the drill makesthe hole or to the reaction of the various strata on the bit, called the ?feelof the bit.? When this is not thoroughly understood by the geologist orengineer endeavoring to interpret the log, the result is an erroneouscorrelation with other wells or a discarding of the log as worthless. General Terms Hard and Soft - Hard and soft are relative terms. In the case of well logs, they are very misleading as they are used in connection with both resistance toabrasion and resistance to percussion. In technical rock classification, hardness is relative resistance to abrasion. The term brittleness is used inconnection with resistance to blows. These terms are misleading to thegeologist or engineer who is not familiar with both the cable-tool, or standardtool, method of drilling and the rotary method. In the case of the standardtools, the driller's report of the hardness of the formation is in terms of itsresistance to blows. For instance, a cable-tool driller might be able to makefrom 30 to 50 ft. a tour in a brittle limestone, which he would call soft andat the same time he might call a relatively soft (from a purely mineralogicalstandpoint) gypsum hard, because it is somewhat elastic and is not readilybroken by blows. The rotary driller would reverse the terms. The limestone ishard in that it resists the abrasive action of the bit, while the gypsum mightbe soft in that it is readily cut by the rotary bit. It is rare that wellsdrilled by the standard tools are correlated with those drilled by the rotary, but the technologist who has worked with well logs from one system might bemisled when working with the other. AIME 065–40