4. Conversely, those conditions that contribute to minimum shows of oil 

 and gas in the drilling mud are (a) abnormally low drilling rate for 

 permeable formations; (b) high mud weight or pressure differential 

 between the mud and the formations; (c) high permeability; and (d) 

 low porosity. 



Reliable shows are usually obtained in the cuttings, except in unconsol- 

 idated sands where the formation fluids mix with the mud stream or where 

 circulating rate or hole conditions and mud properties are such that the cuttings 

 are not transported readily to the surface, where they can be collected and 

 properly related to their depth of origin. 



Figure 17-6. Automatic recording gas detector chart. 



Relatively large shows of gas in the cuttings are obtained where the for- 

 mations have extremely low permeability and contain gas under high pressure. 

 To be sure of getting the indications of formation oil and gas under all con- 

 ditions, it is necessary to resort to both drilling-mud and cuttings analyses. 

 Zones of commercial importance will always be reflected in the drilling-mud 

 analysis. 



Where circumstances make it impractical to use the logging service de- 

 scribed above, mud and cuttings analysis may be obtained through the use of 

 an automatic gas detector (fig. 17-5). This detector utilizes the same gas- 

 detecting equipment but does not require an operator on duty 24 hours a day. 

 An alarm is set to signal an increase in gas when the instrument is left un- 

 attended. 



365 



