Page 277 - Geochemistry of Oil Field Waters

MODIFICATION OF SULIN’S SYSTEM 26 1

(Na-Cl)/SO, >l. As Sulin (1946) noted, if the ratio Na/Cl in epm is
greater than 1, the water contains more sodium than chloride and the excess
sodium can react with sulfate or bicarbonate ions. Therefore, such waters
belong to the bicarbonate-sodium or sulfate-sodium types. If the ratio
(Na - C1)/S04 is greater than 1, it indicates an excess of sodium with respect
to both chloride and sulfate.
(2) Waters of the sulfate-sodium type with (Na - Cl)/S04 < 1. This ratio,
if less than 1, indicates that all of the sodium will react with chloride or
sulfate.
(3) Waters of the chloride-magnesium type with (Cl- Na)/Mg < 1. A ratio
of this type indicates that all of the chloride will react with sodium and
magnesiun. Such a water is characteristic of the transition zone between a
hydrodynamic area which is becoming more hydrostatic in the deeper part
of the basin, and the amount of dissolved bromide increases directly with the
(Cl- Na)/Mg ratio.
(4) Waters of the chloride-calcium type with (Cl- Na)/Mg > 1. This ratio
indicates an excess of chloride with respect to sodium and magnesium, and
the excess will react with calcium. This type of water occurs in deeper zones
which are isolated from the influence of infiltration waters and are hydro-
static or almost hydrostatic.
Bojarski observed a large variation in the chemical composition in the
chloride-calcium type of water and subdivided this type as follows:
(a) The first class, chloride-calcium I with Na/Cl > 0.85 characterizes an
active hydrodynamic zone with considerable water movement. It is con-
sidered a zone of little prospect for the preservation of hydrocarbon
deposits.
(b) The second class, chloride-calcium I1 with Na/C1 = 0.85-0.75, charac-
terizes the transition zone between an active hydrodynamic zone and a more
stable hydrostatic zone of the sedimentation basin, which is generally con-
sidered a poor zone for hydrocarbon preservation.
(c) The third class, chloride-calcium I11 with Na/Cl = 0.75-0.65 (0.60),
characterizes favorable conditions for the preservation of hydrocarbon
deposits.It is designated as a fairly favorable environment for the preserva-
tion of hydrocarbons.
(d) The fourth class, chloride-calcium IV with Na/C1 = 0.654.50, is
characterized by complete isolation of the hydrocarbon accumulations as
well as by the presence of residual waters. It is considered a good zone for
the preservation of hydrocarbons.
(e) The fifth class, chloride-calcium V with Na/C1 < 0.50, is characterized
by the presence of ancient residual sea water which has been highly altered
since original deposition, both in the concentration of dissolved solids and in
the ratios of the dissolved constituents. Bojarski considers a zone of this type
to be one of the most likely areas where hydrocarbons are accumulated.
Additional characteristics of water associated with hydrocarbon accumula-
tions are as follows: (1) iodide > 1 mg/l; (2) bromide > 300 mg/l (increasing