Chap. 4] GEOPHYSICAL METHODS IN OIL EXPLORATION 47 



detailing formations above and on the flanks of domes. The same applies 

 to the salt dome or salt anticline regions of Tehuantepec in Mexico, the 

 northern and north central regions of Germany, Rumania, and other areas 

 of similar geologic structure. 



For the buried-ridge type of oil structure, magnetic methods are still 

 first choice for reconnaissance surveying. However, as pointed out before, 

 care must be exercised in the interpretation of the results. The magnetic 

 methjod has been quite successful in general reconnaissance of the Amarillo 

 buried mountains, the Nemaha granite ridge in Kansas and its extension 

 into Oklahoma and Texas, and similar buried-ridge type structures. 

 Pendulum, gravimeter, and reflection-seismic methods are second choice 

 in reconnaissance for this type. For detail, torsion balance ranks first, 

 reflection-seismic methods second. Extensive torsion-balance, pendulum, 

 and gravimeter work has been done in the buried-ridge areas mentioned 

 above in connection with magnetic surveys. Very little has been accom- 

 plished by geophysical methods in location of oil pools controlled by 

 porosity variations and not by structure, that is, the lensing type of 

 deposit. Magnetic work has been reported to be somewhat successful; 

 resistivity methods also appear to be applicable. 



As far as the direct location of oil is concerned, it has not been demon- 

 strated beyond all doubt that this is possible by use of the methods avail- 

 able at present. Indications by the "Eltran" method appear in some 

 cases to be related to shallow stratigraphic conditions above deeper oil 

 accumulations. Eltran anomalies frequently coincide with hydrocarbon 

 halos revealed by soil analysis methods. To what extent these methods 

 can be depended upon to furnish reliable oil indications in completely new 

 areas remains to be seen. 



The only place where indications can be obtained from the oil itself is 

 underground, that is, by using the electrical logging method in wells. 

 In fact, the resistivities recorded have in some cases been correlated with 

 the productivity of formations. The advantages of electrical logging are: 

 (1) the increase in drilling speed as mechanical coring is eliminated or re- 

 duced to a minimum; (2) correlation of formations which may not have 

 distinct petrographic or paleontological characteristics; (3) the possibility 

 of locating water horizons; (4) continuity of the records; and (5) the ease 

 with which electrical logging may be combined with other procedures, 

 giving important physical data in a drill hole — such as measurement of 

 temperature, side-wall sampling by bullets, and the like. 



In geophysical oil exploration it is often impossible to select the most 

 suitable method, since other factors— cost, time, terrain, permits, and 

 secrecy — in addition to the geologic factors, must be considered. Cost is 

 of principal importance. It determines whether the work can be done 



