Miscellaneous Subsurface Methods 623 



Correlation Between Coring and Electric Logging 



In considering the value of core data, it must be remembered that the 

 examination of formation cores, either in the field or in the laboratory, 

 provides the only direct information concerning the physical properties 

 of the formation that the drill penetrates and that these core data are the 

 basis of electric and other log interpretations. In figure 324 are illustrated 

 graphic means of presenting electric-log and core-analysis data. An ex- 

 amination of formation cuttings and the use of other logging devices may 

 substantiate core-analysis data, but individually those methods are usually 

 subject to variable factors and broad interpretations. When the geologist 

 uses core data for correlation, he is generally certain that they are ac- 

 curate; and once the reaction of an electric log in a particular formation 

 in a certain area has been established or substantiated by core-data inter- 

 pretations, the log becomes a very useful tool. Likewise, formation tests 

 are necessary to determine electric-log characteristics in regard to the prob- 

 able type of fluid that a formation will produce. It has been found that 

 an interpretation of electric logs of certain holes is often very misleading; 

 the logs are very valuable, however, in determining the tops and thicknesses 

 of certain sections when they are used in conjunction with core data, es- 

 pecially when core recovery has been poor. 



Since electric logs are influenced sometimes by the type of drilling 

 mud and the mineral content of the section logged, core logs are often the 

 only means of formation interpretation. Sometimes a correlation between 

 an electric log and a core log on producing formations results in finding 

 measurement errors, which may cause subsequent difficulty. In using elec- 

 tric logs for purposes other than correlations, limitations must be recog- 

 nized, such as failure to register the presence of sands or other zones in a 

 producing horizon and the reverse reaction on producing sands in certain 

 areas. 



Limitation of Coring Techniques and Applications 

 TO Geologic Problems 



Even though coring is the best tool available for the correlation and 

 interpretation of geologic formations, it too has certain limitations that 

 must be recognized. Perhaps the greatest limitation of coring is that 

 samples of the complete section cored are rarely obtained for examination. 

 The amount of cored section recovered is often as low as sixty percent, 

 and sometimes there is no recovery. When full recovery is not obtained, 

 it is usually difficult to place the recovered section in the correct position 

 in the formation log and to assign physical values to the entire section 

 from those obtained by analysis of the recovered section. Then too, after 

 a core has been recovered, it is not possible to analyze the entire recovered 

 section; thus more limitations are introduced, because the core has to be 

 sampled and tests obtained only on the sampled portions. Work on gas- 

 oil and water-oil contacts from core-analysis data has certain limitations. 



