3600 



3625 



,650 



3675 



3700 



3725 



lOmin. 



3020 



3030 



5min. 



x- Connection 



3040 



3050 



3060 



3070 



3080 



3090 



Figure 18-2. Two curves plotted from data on drilling-time chart of Figure 18-1. 



quired and where difficult correlation problems may be solved only in the study 

 of minor features that are best disclosed in the pattern of a log plotted from 

 drilling-time data. The latter method requires less time for recording original 

 data and for plotting and is useful where rapid interpretations are required and 

 where it is used in conjunction with other methods that are based on average 

 data as well, such as sample-examination logs. 



As will be shown further in this section the pattern revealed by a drilling- 

 time log has characteristics very similar to that of an electric-potential curve. 

 The differences between drilling-time and drilling-rate logs may be illustrated by 

 the electric log of a well drilled and the original drilling-time record of the 

 same well. In Figure 18-1 the porous sands are indicated by the relatively close 

 spacing of the foot marks on the drilling-time chart and are confirmed by the 

 electric log. It will be noted that the streak 3669-3673 feet did not drill as fast as 

 the upper and lower sands and may be interpreted as a shaly sand. This inter- 

 pretation is supported by the potential curve of the electric log. 



In Figure 18-2 are two curves plotted from the data on the drilling-time 

 chart of Figure 18-1, using the normal scale for correlation on the left and the 

 enlarged detail scale for lithologic interpretation on the right. A close compari- 

 son of the detailed curve with the potential curve in Figure 18-1 reveals not only 



372 



