time as a second well is drilled and correlation is desired. The samples need not 

 be large. As little as two or three grams will suffice. They may be shipped air 

 mail in small envelopes quickly and inexpensively. 



Speed of Determination 



With a twin-furnace arrangement and multiple-point recording, DTA can 

 proceed at a rate of about 100 feet per hour, assuming normal 20-foot sample 

 intervals. This speed will keep up with usual drilling rates. Where drilling is 

 faster, a wider spacing of samples can be used with subsequent fill-in if desired 

 for further delineation of correlative breaks. 



Low Cost 



All other correlative techniques comparable to DTA in potentiality are more 

 expensive. In-the-hole logging involves idle rig time; paleontology requires ex- 

 tensive sample preparation and thoroughly trained observers; but, DTA can be 

 handled by semi-skilled personnel. 



Source of Additional Information 



DTA provides other information than correlative. It may give semiquanti- 

 tative analysis of mineral composition for aid in determining the geologic his- 

 tory of a stratigraphic section; source rock, possible corrosive effects on pipe, 

 drilling-mud contamination, cement contamination, and effects on in-the-hole 

 logs. 



One of the most rewarding avenues of research in the evaluation of sub- 

 surface minerals by DTA is the study of carbonates, particularly the dolomites. 

 DTA techniques can differentiate easily between types of carbonates and are suf- 

 ficiently sensitive to detect effects of minute amounts of dissolved salts upon the 

 decomposition reactions. For instance, in a mixture of calcite (CaC0 3 ), dolo- 

 mite (MgC0 3 • CaC0 3 ), and magnesite (MgC0 3 ), not just two but four decom- 

 position peaks result, with the double peak of dolomite remaining independent 

 of the separate decomposition reactions of calcite and magnesite. The effect of 

 small amounts of soluble salts — particularly the chlorides — in altering the shape 

 and characteristics of the dolomite decomposition has been studied widely by 

 DTA and is providing much new information relating to the environmental con- 

 ditions of source rock and the geologic age of these formations (Berg, 1943; 

 Graf, 1952). Other evaluation techniques applied to dolomites and other car- 

 bonates are available in the literature. These include studies of hydrothermal 

 solution sources of dolomite (Faust, 1949), distribution of oil in relation to 

 source rock (Hunt and Jamieson, 1956), correlation by thermoluminescence 

 (Bergstrom, 1956; Pitrat, 1956), grain size of carbonates vs environment of 

 deposition (Ginsburg, 1956), Ca/Mg ratio in relation to porosity (Chilingar, 



137 



