ADVANCES IN OIL PROSPECTING I4I 
on the other hand, a condition which is by no means representative of 
all oil reservoirs. 
A rather serious argument has been advanced lately by Hedstrom 
(ref. list No. IIIg) against the direct electrical prospecting for oil. 
This author claims that from theory and experiment it follows that a 
formation which is practically an insulator does not show in the 
electrical results any differently from what it would if its conductivity 
were only 10 times smaller than that of the surrounding formation; 
hence, a poorly conductive sand or limestone with a conductivity 
only about 10 times less as compared with that of the surrounding 
strata, would not be distinguishable from the effect of an oil forma- 
tion. 
This argument, however, does not preclude the possibility of 
direct location of oil in an area where, by correlation with geologic 
data, a poor conductor is known to be oil bearing, and where it would 
thus be possible to trace this oil-bearing horizon in various depths 
of structure, and to determine where the filling of the pores with oil 
ceases and where the oil is replaced by salt water. If the argument 
just advanced is correct, it would seldom be possible to distinguish 
between a sand filled with gas or a dry sand, and a sand filled with 
oil. 
One might finally say in commenting on Hedstrom’s argument, 
that with a greater perfection of technique of electrical prospecting 
we may some day be able to distinguish the surface effects of poor 
conductors with a greater precision and obtain an electrical log in 
which the formation resistivities could be represented by their true 
and not their apparent resistivity, so that an oil sand would show 
somewhat like the sands illustrated in the electrical logs demon- 
strated in Figure 37 and Figure 38. 
However, as already stated, it is beyond the scope of this paper 
to take sides in this argument, and we will merely confine ourselves 
to a discussion of the results thus far obtained in attempting to locate 
the oil directly at the end of the following section (F II). 
F. RESULTS OBTAINED WITH RESISTIVITY AND 
POTENTIAL-DrRop-RATIO METHODS 
I. IN STRUCTURAL WORK 
Several outstanding examples are here described which illustrate 
both the possibilities and the limitations of the resistivity and po- 
tential-drop-ratio methods in determining geologic structure. 
471 
