distillation methods. Retorting, however, cannot be applied because the tempera- 

 tures involved would nullify completely much of the subsequent DTA reactions. 

 After the samples are washed and cleaned, they must be dried at low temperature 

 and then pulverized to a 60- to 100-mesh size, after which they are ready for 

 thermal analysis. 



MECHANICS OF DTA The necessary differential -thermal analysis ap- 



paratus consists of a furnace (or preferably 

 a twin-furnace arrangement) , a multiple sample holder supported in the furnace 

 or over which the furnace may be lowered, an automatic temperature controller 

 to provide uniform heating of the furnace, an automatic multipoint recorder for 

 translating thermocouple millivoltage into thermal curves, and the necessary ther- 

 mocouples and electrical circuits. Differential thermocouples are used; one junc- 

 tion extends into a sample cavity and the other into a similar cavity in the sample 

 holder reserved for the inert alumina. Optimum rate of temperature rise in the 

 furnace generally is considered to be from 600 to 900C per hour, with the rate 

 maintained as nearly constant as the controller can provide. Range of tempera- 

 ture covered should be from room temperature to 1000C. Higher temperatures 

 sometimes are desirable for identification of certain minerals but are not prac- 

 tical for most correlation works. Chromel-alumel thermocouples have been 

 found most satisfactory because of their reasonable cost, high sensitivity, uni- 

 formity, and their nearly straight-line millivoltage-temperature relationship. A 

 more complete description of DTA equipment adaptable to subsurface correla- 

 tion work has been presented by Kerr, Kulp, and Hamilton (1949) ; and by Kerr 

 andKulp (1951). 



DTA LOGS Thermal curves, determined by means of the 



apparatus just described, must be corrected 

 as necessary to delete any drift of the base lines due to slight thermal nonuni- 

 formity in the sample holder and furnace, and to minute differences in the 

 thermocouple- junction welds. Such drift can be determined periodically by mak- 

 ing blank runs with alumina in all sample cavities. After correcting, the curves 

 are reduced to the desired size by pantograph or photography, and plotted on 

 the log so that each base line coincides with the correct corresponding depth. 

 The scale usually employed is the same as that of electric and radioactivity logs, 

 i.e., 1 inch = 50 feet. This scale standardization makes comparison easy. 



Figure 7-6 shows reproductions of small sections of actual DTA logs. Figure 

 7-6A illustrates an obvious break between two rather complex mineral assem- 

 blages. The formation down through 3670 feet is a shale containing consider- 

 able organic matter, a low percentage of calcite, and some quartz. At 3690 feet 



129 



