Page 244 - Geochemistry of Oil Field Waters

228 ORIGIN OF OILFIELD WATERS

TABLE 7 .XV
Bioconcentrated bromide and iodide in seaweeds and corals

Bromide (ppm) Iodide (ppm)
Seaweed
Laminaria digitata (dry matter) 1,380 5 10-8,000
Laminaria saccharina (dry matter) 340 2,000
Desmaresta (ash) 6,800 5,200

Corals*
Gorgonia uerrucosa 16,200 69,200
Gorgonellidae 19,800 22,100
Isididae 7,400 20,300

* After Vinogradov (1953).

However, with each crystallization, more bromide is left in solution than is
entrained in the precipitate.
Sylvite (KC1) begins to precipitate when the chloride concentration is
about 360,000 mg/l (Table 7.111), followed by carnallite (MgC12 *KCI *6H2 0)
and bischoffite (MgC12 *6H, 0). During evaporation the concentration rate of
bromide in solution increases. The change in the slope of the curve in Fig.
7.4 illustrates this approximately.
Other concentration mechanisms operate to account for the high bromide
concentrations (6,000 mg/l and up) found in some brines. One of the
mechanisms is related to bioconcentrators such as seaweeds and corals. The
seaweeds and corals concentrate the bromide, they die, and are buried with
the sediments. Later the bromide is leached by the surrounding waters. Table
7.XV illustrates some of the concentrations of bromide and iodide that
Vinogradov (1953) found in various seaweeds and corals.
Laboratory experiments have demonstrated that bromide is accommo-
dated in the halite crystal lattice and replaces chloride in solid solution
(Borchert and Muir, L964). The weight percentage of bromide in solid solu-
tion in the halite lattice is related to its weight percentage in the parent brine
as :
wt.% Br (in halite)
C=
wt.% Br (in solution)
where C = the pa@ition coefficient.
In most natural environments C = 0.14 (Braitsch and Herrmann, 1964).
In a marine salt sequence the wt.% Br/NaCl rises from about 0.007 wt.% at
the bottom to 0.02 wt.% at the beginning of potassium precipitation. How-
ever, the bromide concentration with a given natural halite sequence may
vary considerably, even though theoretically it should increase continuously