integrated. In many instances, one method will give the desired results while 

 another will not. 



The wells J and K of Figure 7-8 were correlated by DTA. In this instance, 

 other available correlative tools were unsuccessful. Figure 7-9 shows sections 

 of electric logs of the same two wells. The electrical profile of well J did not pro- 

 vide good markers or zones and almost led to a false correlation. Regardless 

 of whether or not a correlation is valid, a best possible correlation can be 

 achieved between nearly any two sets of corresponding data obtained during the 

 drilling of two wells. The most dangerous pitfall in the use of DTA for correla- 

 tion is the same as for any other correlative tool — the tendency to correlate logs 

 from a single zone or limited stratigraphic interval. It is essential at all times 

 that a clear and realistic approach to correlation be maintained, no matter what 

 type of equipment or logging method is involved. 



Figure 7-10 illustrates good agreement between DTA and an electric log 

 through the same formations. The obvious sands indicated by the electric log 

 appear on DTA as more nearly straight lines with decided diminution of clay 

 and organic reactions and vast increase in quartz content. Furthermore, the 

 sands are not calcareous although the shales on either side are somewhat calcare- 

 ous. It frequently happens that otherwise unexplained resistivity development on 

 electric logs can be clarified by thermal analysis. High concentrations of dolo- 

 mite, for instance, will yield high resistivity in almost all instances, regardless 



WELL J 

 Self Potential Resistivity 



( [5 ' 



C* 



h- 



WELL K 

 Self Potential Resistivity 



Figure 7-9. Electric logs. 



133 



