Thermalog 



Stlf Potential 



Resistivity 



Figure 7-11. Thermalog formation change vs. electric log. 



scription of cuttings, but even with very competent geologists, the human ele- 

 ment enters into the technique to such an extent that correlations are frequenly 

 impossible except over very short distances. Many variations not apparent to 

 the eye are recognized by the DTA. Figure 7-12 illustrates sections from a forma- 

 tion described merely as undifferentiated volcanics. The thermal curves show 

 weathering and alteration to an almost unbelievable extent and could provide 

 correlative clues in a zone where little other information would support a cor- 

 relation. 



ADVANTAGES OF DTA There are many advantages of DTA for cor- 



relation work which have not been covered 

 or which have only been mentioned briefly. The advantages given below should 

 be considered in any drilling program. 



Primary Nature of Property Measured 



DTA is a measurement of the basic primary composition of the formation 

 and is not affected by accompanying environmental conditions. In-the-hole log- 

 ging techniques are dependent on secondary effects, and the properties measured 

 are complex resultants due not only to the mineral assemblages but also to par- 

 ticle size, type of cementation, formation fluid, permeability, surface phenomena, 



135 



