478 



Subsurface Geologic Methods 



tions are now controlled by instruments, the driller has a better oppor- 

 tunity to record the relative hardness of formations by keeping a record 

 of footage drilled per unit of time. 



In the event that a geologist is assigned to watch or "sit on" a well, 

 it becomes his responsibility to adjust the driller's lithologic "call" and 

 suggest necessary modifications on the tour report before it is submitted 

 to the field or home office. A driller's log in conjunction with other log 

 data is useful in making final interpretative logs (fig. 221). 



DRILLING-TIME LOGGING 



P. B. NICHOLS 



Subsurface lithologic units, which may be identified by means of 

 sample-examination, electric-logging, or radioactivity methods, may also 

 be recognized and delineated by drilling-time logging. In rotary drill- 

 ing under normal operating conditions, major changes in the rate of pene- 

 tration are due to changes in the lithology or texture of the formation 

 being drilled. In other words, lithology governs the rate of penetration. 

 Mechanical factors such as weight on the bit, mud-pump volume and 

 pressure, and rotary r.p.m. are important in making hole, but they are 

 not responsible for the type of drilling-time "breaks" that stand out prom- 

 inently on mechanical well logs. 



Figure 222. "Geolograph" recording unit showing footage pen (left) and down-time 

 pen (right) ; depth meter and revolving indicator are on front. Actuating cable 

 passes through guide pipes in back. 



