Page 162 - Geochemistry of Oil Field Waters

MANGANESE 149

potassium, chloride, bromide, and iodide ions. Many analyses performed by
wet chemical methods indicate rather high concentrations of barium in some
subsurface brines. Some of these high results probably should be attributed
to strontium plus barium rather than barium only, because satisfactory
separation of the two in wet chemical methods is very difficult to accom-
plish.
Manganese

Manganese is a member of the VII B group of elements and is well known
for its multiplicity of oxidation states. Essentially it is cationic, and the
Mn+4 oxidation state usually is found in sediments. Its (+2) ionic radius is
0.80 8, while the ferric iron radius is 0.76 8 (see Table 5.IV); reasonable
amounts of interchange in crystal lattices between these two ions are possi-
ble. The abundance of manganese is about 0.1 wt.% of the earth’s crust
(Fleischer, 1962).
Manganese is present in many oilfield brines because it is readily dissolved
by waters containing carbon dioxide and sulfate. Except for titanium, man-
ganese is the most abundant trace element in igneous rocks. Nearly all
mineral groups of petrological importance contain manganese. During
weathering, manganese is dissolved mainly as the bicarbonate. Decomposition
of the bicarbonate leads to the formation of Md4 compounds. In a reducing
type of environment Mn+ compounds migrate in aqueous solutions. Mn+
compounds are less mobile, and Mn+4 compounds precipitate from aqueous
solutions. In general, manganese remains in solution at a low redox potential
and precipitates at a high redox potential.
According to Goldberg (1963), manganese oxide nodules on the ocean
bottom occur in both shallow water and deep water environments. He attri-
butes these deposits to slow oxidation of dissolved manganese in areas where
the waters contact an oxide surface. In most subsurface brines, the manga-
nese is in the reduced form (Mn+*) because the redox potential is low and
the pH is less than 7.0. Any in subsurface brines probably would be
suspended with particulate matter or complexed by organic compounds,
rather than in ionic solution.
Shales and carbonates contain about 850 ppm and 1,100 ppm, respec-
tively, of manganese (Mason, 1966). Sea water contains about 0.002 mg/l,
and many subsurface brines contain 1.0 to 6.0 mg/l of manganese.

Iron

Iron is a member of the VIII group of elements and is predominantly
siderophile. However, because it has an affinity for sulfur, it is also thiophile;
and because it commonly enters into silicate minerals, it is lithophile as well.
It is an ubiquitous element, with an abundance of about 5 wt.% of the
earth’s crust (Fleischer, 1962).