the mud. In the cuttings, oil is detected both visually by use of the ultraviolet 

 light and instrumentally by the filaments that measure the heavier hydrocarbons. 

 Mass spectrometers, gas chromotographers, and other complicated equipment 

 have been used for a more complete breakdown of hydrocarbons. The addition- 

 al information thus obtained has not proved of value, however, in predicting the 

 commercial possibilities of a formation. 



The equipment necessary to relate shows to depth are the depth meter and 

 counters that register the actions of the mud pump and the circulation of the 

 mud. The depth meter used is an instrument that operates automatically, once 

 it is set with the depth of the bit at some point in the hole. It is used to measure 

 changes in depth from that point and does this job automatically and quite 

 accurately. For technical reasons, however, no attempt is made with this appa- 

 ratus to measure the depth of the hole from the surface. 



In relating the oil and gas shows to depth, it is neccessary to follow the 

 movement of the drilling mud and the cuttings, which it carries, from the time 

 the mud leaves the bit until they reach the surface. The use of clocks for making 

 this determination is based on the assumption that the circulation rate of the 

 mud is constant. Actually, this assumption is dependable in most instances, 

 but there are times when it is not valid. It is preferable, therefore, to consider 

 the pump as a displacement meter and to count the number of strokes it makes 

 as an indication of the volume of circulated mud. 



By using such tracers as cellophane strips, oats, or, if conditions allow, 

 carbide, which are placed in the drill pipe at the surface, one is able to deter- 

 mine the number of pump strokes required to circulate mud through the hole. 

 Because the volume of the drill pipe and the output of the pump are known, 

 the number of strokes required to pump mud from the surface to the bit can be 

 fairly accurately determined. This number of strokes is subtracted from the 

 number required for complete circulation of mud through the hole to give the 

 amount of pump action required to circulate mud from the bit to the surface. 

 By making accurate observations of the pump-counter readings at certain depth 

 intervals, one can determine the bit depth. 



Auxiliary equipment, which is not essential to the logging method but 

 which is used to reduce the burden of work on the operator and to give addi- 

 tional valuable information, consists of (1) an automatic recorder that indicates 

 the gas shows and the action of the pump, together with changes in depth; (2) 

 a pump-rate meter that is used in determining washouts of the drill pipe and in 

 some instances the development of a gas blowout; and (3) binocular micro- 

 scope for inspecting the lithology of the cuttings. 



From information obtained by the operator and described above, a log 

 is then prepared as illustrated in Figure 17-4. 



361 



