Thermalog 



Self Potential 



Resistivity 



Figure 7-10. Correlation of thermalog and electric log. 



of the formation fluids present. Likewise, naturally conductive solid minerals 

 will have the reverse effect. 



Another example to emphasize the necessity of using all available correlative 

 tools is shown in Figure 7-11. The section covered by DTA is the one previously 

 shown in Figure 7-6A in which a sharp major break separates two types of shales. 

 The electric log for this section, however, failed to distinguish appreciably be- 

 tween the shales because of lack of change in formation fluid content and perme- 

 ability. 



In comparison with paleontological correlation, DTA agreement has been 

 excellent (Mangold, 1955). Here again, the need for more than one correlation 

 approach is demonstrated. In many formations fossils are not present. In some 

 formations faunal migration has caused anomalies that tend to confuse and ob- 

 scure correlation. The wells represented by DTA in Figures 7-7 and 7-8 did not 

 yield fossils. The break in Figure 7-11, however, was accurately spotted by micro- 

 faunal change. 



In comparison with other techniques based on mineral analysis, DTA offers 

 decided benefits. For many correlation programs, thin-section analysis is ex- 

 pensive; — as is chemical analysis. Spectrographic analysis is feasible sometimes 

 and may supply some correlative data; but, like chemical analysis, it gives re- 

 sults in terms of elements rather than of compounds and mineral aggregations. 

 Attempts have been made to correlate wells by microscopic observation and de- 



134 



