Subsurface Logging Methods 455 



ally high pressure, where sloughing shale is encountered, and, in general, 

 where coring is difficult or impossible. In cases on record, hole trouble 

 developed so seriously that it was impossible to make other surveys, and 

 the mud and cuttings-analysis log was the principal, if not the only, source 

 of information on the lower section of the hole. An operator desiring to 

 quit a well because of severe hole trouble, such as junk, lost circulation, 

 or high pressures, may spend tens of thousands of dollars conditioning 

 the hole just to run the final electric log over perhaps only a few hundred 

 feet drilled since the last log was run. If a Baroid log had been obtained, 

 it would have provided reasonable assurance that nothing had been passed, 

 and the operator could have plugged and abandoned the hole as economic- 

 ally as possible. 



The method does not present a complete subsurface picture. It does 

 not give quantitative determinations of the amount of oil and gas occurring 

 in the formation, nor does it furnish quantitative information on the 

 productivity of the oil and gas horizons. A quantitative estimate is 

 prevented by the numerous factors which affect the concentration of 

 oil and gas in the mud and the cuttings. Some of these factors are 

 the ratio of the volume of formation drilled to the volume of mud used 

 to drill it; the flushing action of the drilling fluid, which in itself is 

 affected by the mud-filtration characteristics, drilling rate, speed of rota- 

 tion of the bit, differential pressure, and effective porosity and perme- 

 ability; and the amount of oil and gas which is recirculated, which depends 

 on the viscosity and gel properties of the drilling mud. However, the 

 method does give reliable qualitative information on the occurrence of 

 oil and gas, and the interpretation of the log should be made in the light 

 of all available information on that section of the hole logged. It is 

 extremely valuable in minimizing coring, as outlined before, in re- 

 moving the ever-present possibility that a production zone will be missed 

 and in rounding out the picture presented by other formation data. 



DRILLING-TIME LOGGING 



G. FREDERICK SHEPHERD 



Rate of penetration is considered here as the time required to rotary- 

 drill a linear unit of depth of a geologic formation of the earth's crust. 

 It is believed that drilling-time characteristics constitute a diagnostic prop- 

 erty of a rock resulting from its composition, mode of deposition, degree 

 of compaction, and other known physical features by definition of which 

 the rock is described. At present, drilling-time properties, measurable in 

 terms of rate of penetration, are qualitative in scope and relative in their 

 interpretation. That they may become quantitative and definable in fixed 

 values that may be significant in the determination of lithologic types 

 is anticipated. 



Rate of penetration may be measured in terms of drilling time and 



