ADVANCES IN OIL PROSPECTING 155 
every minute detail. All other formations except the shales exhibit 
rather irregular characteristics which makes it possible to correlate 
definitely the shale horizons in the two wells. Water sands are char- 
acterized by very low resistivity; the resistivity of the water sand at 
the bottom of the holes is almost nil. The Tonkawa limestone shows 
by a pronounced and characteristic resistivity peak. 
II. DIRECT LOCATION OF OIL 
There are two possibilities of locating oil directly or of distinguish- 
ing oil-bearing formations from barren strata: first, by resistivity 
measurements in wells; and second, by resistivity measurement at the 
surface. The former method has proved its merit almost everywhere 
it has been applied, but the second possibility has not yet been estab- 
lished as a commercial method, and only with exceptionally favorable 
conditions have direct indications been obtained from oil-bearing 
formations where they were known to exist at shallow depths. 
The possibility of obtaining a direct indication from an oil-bearing 
horizon in a well does not only mean the possibility of tracing this 
same oil formation in another well; but, as experience has shown, it is 
possible to determine where this oil formation becomes barren or 
changes into salt water in other wells (which is of practical importance 
in deciding the depth of water shut-off), as well as how thick and 
how prolific an oil sand is, the information on the productivity being 
a valuable guide which can not be obtained from the drill record. 
I. RESISTIVITY MEASUREMENTS IN WELLS 
Figure 37 illustrates without further comment how much more 
resistant oil-producing formations are than non-productive forma- 
tions. This figure is an example of a resistivity log taken in the Semi- 
nole area, the two sands being saturated with heavy oil. It is seen that 
the resistivities measured are much higher for oil-bearing beds than 
for highly resistant, compact sedimentary formations. In case of 
doubt the ambiguity can be eliminated by simultaneous records of 
porosities, the technique of which is described in Schlumberger’s 
paper on electrical coring (ref. list I;:). 
Figure 38, representing several electrical logs taken in the Mara- 
caibo field, not only illustrates very well the possibility of resistivity 
correlations of the oil beds directly in adjacent wells, but also shows 
how the resistivity of an oil bed changes if it passes from the pro- 
ductive to the non-productive stage. The upper tar sand and the first 
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