and below, quartz has disappeared and pyrite is present in amounts of 10 to 15 

 percent. Clay and calcite remain, but the organic matter is changed. Though 

 closer identification of mineral content of these two formations would be dif- 

 ficult without more involved testing procedures, it is not necessary for correla- 

 tion purposes. Anyone without experience in DTA can spot quickly the sharp 

 break between 3670 and 3690 feet. Contoured curves from another well show- 

 ing the same type of break would establish a possible correlative point, regard- 

 less of identification of all mineral components. 



A B 



Figure 7-6. Sections of DTA logs. 



Figure 7-6B is another example from an actual well log. In this instance the 

 upper curves (shallower formation) show a dolomitic sand containing very little 

 clay and carbonaceous material. Between 2265 and 2285 feet, a sharp change to 

 a pyritic clayey shale occurs. Dolomite (the double endothermic peak) at first 

 disappears at this break but comes in about 25 feet deeper and increases in con- 

 centration with depth. This point is a potential correlative marker that is ob- 

 vious even to the untrained eye. 



CORRELATION OF Wells cannot be correlated safely on the basis 



DTA LOGS of one or two DTA markers such as were 



pointed out in Figures 7-6A and 7-6B. Extend- 

 ed sections should be tested by DTA so that various trends, markers, maximum 

 and minimum concentrations of particular minerals, and definite formation 



130 



