Page 274 - Geochemistry of Oil Field Waters

258 CLASSIFICATION OF OILFIELD WATERS

salinity and the sodium bicarbonate-calcium stage. No sodium bicarbonate is
present in sulfate-sodium, chloride-magnesium, or chloride-calcium types of
water; therefore, these types are classified as follows:
(1) Class A2 : secondary alkalinity predominates (alkaline earth carbonates
and bicarbonates).
(2) Class S2 : secondary salinity predominates (alkaline earth sulfates and
chlorides).
(3) Class S, : primary salinity predominates (alkali sulfates and chlorides).
(4) Class S,: tertiary salinity predominates (iron and aluminum sulfates
and chlorides and free strong acids).
Bicarbonate-sodium type waters contain sodium bicarbonate and are
classified as follows:
(5) Class A2 : secondary alkalinity predominates (alkaline earth carbonates
and bicarbonates).
(6) Class A, : primary alkalinity predominates (alkali carbonates and bicar-
bonates).
(7) Class S, : primary salinity predominates (alkali chlorides and sulfates).
(8) Class A, : tertiary alkalinity predominates (iron and aluminum carbon-
ates and bicarbonates).
The water classification is expressed by use of a formula representing
decreasing values of the Palmer characteristics. For example, S, S2 A2 indi-
cates that primary salinity is predominant and is followed by secondary
salinity and secondary alkalinity. Therefore, the classes are subdivided into
subclasses, and class S1 can include the subclass S, S2 A2, S1 A2 S2, S1 S2,
and S, . Table 8.1 outlines Sulin’s method of water characterization. Table
8.11 briefly outlines the relative values of the coefficients which determine
the four genetic types of waters.
The Palmer characteristics do not account for the interrelations between
chloride and sulfate and between calcium and magnesium. Therefore, Sulin
calculated the ratio SO4 /C1 and Ca/Mg to establish additional subgroups. The
complete water characterization included the following: (a) water formula
given in Palmer characteristics; (b) coefficients in percent equivalents for
S04/C1 and Ca/Mg; (c) sum of the milligram equivalents per 100 g of water
(Z r) to illustrate the degree of water mineralization; and (d) the genetic
coefficients (Na - C1)/ SO4 and (Ca - Na)/Mg to determine the water type,
and Na/C1 to determine related genetic types of water.

Hydrochemical indicators of hydrocarbons
Sulin (1946) noted that certain properties of subsurface waters were
favorable indicators of hydrocarbon accumulations. The bicarbonate-sodium
and chloride-calcium types of waters are widely found in oilfields. However,
the chloride-calcium type is the more favorable indicator if it has the most
characteristic composition plus certain minor or micro constituents. In gen-
eral, he determined that hydrocarbon accumulations are most commonly