ADVANCES IN OIL PROSPECTING 97 
(b) Mineral waters with varying concentrations. Such waters are ordi- 
narily locally limited, and are not commonly encountered on the sur- 
face of oil-bearing structures; they are not of as great importance 
for our problem as those previously mentioned, and the deep wat- 
ers to be discussed now. 
B. Deep waters. 
(a) Connate waters. 
These waters are ordinarily encountered in oil fields. As the name indi- 
cates, their origin dates back to the time when the formations were 
laid down in sedimentary basins. They contain chlorides of sodium, 
potassium, magnesium, and calcium. They differ from sea water, 
however, by the abundance of calcium chloride and by the absence 
of sulphates. They are distinguished by a very wide range of concen- 
trations, from dilute to concentrated solutions. Consequently, their 
resistance also varies within wide limits, to as low as a few ohms cm. 
(b) Mine waters. 
These waters are ordinarily solutions of metal sulphates, but they 
also contain carbonates of sodium, calcium, and magnesium. They 
vary widely in resistivity which may be as low as 30 ohms cm %. 
The most important chemical constituent of waters of all kinds 
which determines their conductivity is the chlorine content. Figure 
1 6 illustrates the relation between resistivity and chlorine content for 
three ranges of concentration. 


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Fic. 1.—Factors controlling resistivity of rock waters (after Sundberg). 
a: Relation between resistance factor P=p,/p and percentual volume »v of water in 
rocks. (p,=resistivity considering grain arrangement; p=resistivity not considering 
grain arrangement.) 
b: Relation between chlorine content of waters and their specific electric resistance. 
c: Relation between temperature and specific electric resistance for electrolytes. 
The next outstanding factor determining the resistivity of rocks 
and formations is the pore volume. The pore volume depends on the 
size and shape of the grains, and their mutual arrangement. The 
427 
