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 section 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 connection with 

 coring operations. Careful recording aids in obtaining accuracy in well measure- 

 ments, particularly where continuous coring is done over a long section. Drilling- 

 time often makes it possible to interpret 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 formations of south-central Louisana, 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 interpretation 

 of elecric logs. It is 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 questionable 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 questionable beds sufficient to identify them as calcareous or arenaceous. 

 This use of drilling-time logs reduces the cost of sidewall 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 re- 

 serves of oil and gas often fail to represent the actual amount of eventual re- 

 covery. Although great progress has been made in understanding physical and 

 chemical reservoir conditions and factors relating to the recovery of petroleum 

 resources, the amount of reserves in place is given only as an estimate. Some 

 even discount the value of making estimates of this character because of the lack 

 of knowledge or possession of empirical data necessary to arrive at reliable con- 

 clusions. Efforts are continuously being made to increase the accuracy of reserves 

 estimates. The oil or gas content of a reservoir bed is generally given in barrels 

 of oil or mcf. of gas per acre foot. Lack of adequate knowledge pertaining to the 



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