POROS 

 ITY 



K alr 



RESIS- 

 TIVITY 



O.T.A. 

 CHARACTERISTICS 



Strong dolomite 



Strong dolomite 



Only trace of carbonate 

 More shale 



Same as 8249 



Strong calcite 



Trace of pyrite 



Strong quartz 

 Weak shale 



Same as 8255 



No carbonate 



Figure 7-13. Differential-thermal analysis vs. core analysis. 



discovery such as this can conceivably change the whole program of drilling and 

 production and perhaps be of economic value far beyond that of its ordinary 

 correlative use. 



DTA may indicate the presence of valuable ores contacted in drilling which 

 otherwise might be entirely overlooked. At times these ores are so finely divided 

 that they escape recognition by the ordinary visual or microscopic observations. 

 Regardless of their particle size, form, or association, if they are active thermally, 

 they will appear on the DTA log. 



CONCLUSION As with all methods of subsurface geologic 



analysis, DTA has limitations. In common 

 with other types of cuttings analysis, sampling must be controlled carefully. If 

 drilling-fluid servicing is lax, contamination may occur. Certain types of for- 

 mations are inert thermally and are not detected except as dilution of active 

 constituents is observed. However, these limitations are far outweighed by the 

 potential discussed herein for inexpensive correlation of subsurface formations, 

 corroboration and clarification of other logging methods, and analysis of the 

 mineral environment encountered during drilling. Although DTA has been used 

 only a few years in the petroleum industry, it has already proved of enormous 

 value to geologists and engineers. 



139 



