464 Subsurface Geologic Methods 



local correlation is based on the succession of a series of beds predomin- 

 antly shale and sandstone. The stratigraphic position of any portion of 

 a drilling well can be established in advance of electric logging by observ- 

 ing the sequence of beds penetrated as revealed by a drilling-time log. 



It would be easy, for example, for the sequence of beds illustrated 

 in figure 216 to correspond to a similar sequence of beds above or below 

 the portion of the well shown. It would be difficult, if not beyond the 

 realm of possibility, for the full length of section drilled and logged, below 

 that depth at which drilling time becomes diagnostic of lithology, to 

 correspond and be correlated erroneously with the same stratigraphic sec- 

 tion of another well where such correlation could otherwise be established. 



The economic and geologic value of this use of drilling-time logs is 

 apparent. In the writer's experience many preliminary correlation runs 

 of electric logs have been unnecessary because the purpose for which 

 they would have been run was adequately served by drilling-time logs 

 plotted as the drilling took place. 



The most widely recognized geologic use of drilling time is in con- 

 nection with coring operations. Careful recording aids in obtaining ac- 

 curacy in well measurements, particularly where continuous coring is 

 done over a long section. Drilling time often makes it possible to inter- 

 pret the lithology of missing portions of cores recovered and identifies 

 the portion of section cored from which the recovered core came. 



Unless 100 percent of the core is recovered, it may be difficult to 

 determine the net thickness of productive formations, even with the aid 

 of an electric or radioactivity log. In areas where limestone streaks are 

 interbedded with saturated sandstone, as in the Oligocene formation of 

 south-central Louisiana, it is nearly impossible to interpret an electric 

 log correctly without corroborative data. Greatest accuracy may be 

 obtained in such problems by the use of electric logs, drilling-time logs, 

 and cores combined. 



Another important use of drilling-time data is their aid in the inter- 

 pretation of electric logs. It is a common practice in drilling wells in the 

 Gulf Coast area to rely on sidewall cores to check questionable shows of 

 saturation in beds not cored during the drilling. Some of these question- 

 able shows are thin calcareous beds which produce resistivity kicks on 

 electric logs that are not unlike those that might be caused by saturated 

 sandstones. The detailed examination of an electric log in conjunction 

 with an accurate drilling-time log may reveal information on these ques- 

 tionable beds sufficient to identify them as calcareous or arenaceous. This 

 use of drilling-time logs reduces the cost of side-wall coring and effects a 

 further saving of rig time. 



The use of drilling-time logs as an aid in the interpretation of electric 

 logs may be applied to the problem of reserves estimate. Estimates of 

 ultimate reserves of oil and gas often fail to represent the actual amount 

 of eventual recovery. Although great progress has been made in under- 



