Page 164 - Geochemistry of Oil Field Waters

ZINC 151

division for mineral waters based on the presence of heavy metals in waters
associated with joints and faults caused by tectonic movements of different
geological ages. He placed copper as the predominant heavy metal in the
Caledonian Group of the Orogenic Epoch (post-Silurian). Biochemical pro-
cesses are known to be responsible for enriching a deposit in metals such as
uranium, copper, and vanadium; therefore, this classification is restricted to
waters of igneous origin.
Most shales and carbonates contain about 45 and 4 ppm, respectively, of
copper, with sandstones containing less than 1 ppm (Mason, 1966). Sea
water contains about 0.003 mg/l, and most subsurface brines analyzed in this
laboratory contained from less than 0.5 mg/l up to about 3 mg/l. The solu-
bility of copper generally decreases with decreasing redox potential and
increases with increasing redox potential if reduced sulfur is present. Most
subsurface oilfield brines have relatively low redox potentials.

zinc

Zinc is a member of the I1 B group of elements and is predominantly
thiophile. Its abundance in the crust of the earth is about 0.013 wt.%
(Fleischer, 1962). Its geochemistry results from the similarity of its divalent
ionic radius and the radii of Mg+’, Ni+?, Co+’, Fe+’, and Mn+’
(Goldschmidt, 1958).
Zinc is dissolved readily as sulfate or chloride from acid rocks, such as
granite, during weathering. Conversely, zinc is not dissolved easily from
limestone with which it is deposited. Most alkaline waters do not extract
zinc; however, a solution of NH,, NH,NO,, and NaC10, can extract and
hold small quantities of zinc; the more acidic the water, the greater the
amount of zinc extracted. Zinc is precipitated as the sulfide, oxide, carbon-
ate, or silicate. Traces of zinc are found in sea water, but eventually zinc is
deposited in carbonated sediments or in bottom muds or sapropels as sulfide.
Shales, sandstones, and carbonates contain about 95, 16, and 20 ppm,
respectively, of zinc (Mason, 1966). Sea water contains about 0.01 mg/l, and
subsurface brines contain traces to more than 500 mg/l of zinc.
Mercury

Mercury is a member of the I1 B group of elements, which also includes
zinc and cadmium. It is relatively abundant for a heavy element, but still
must be considered scarce, with an abundance of about 4 x lo-’ wt.% of
the crust of the earth (Fleischer, 1962). Most commercial deposits of
mercury are of hydrothermal origin and are related to magmatic rocks; the
commercial ore is cinnabar, HgS, or the liquid metal itself (Goldschmidt,
1958). Mercury is predominantly thiophile, and its geochemistry is control-
led by the fact that it is volatile, with a boiling point of 357”C, and can be
reduced to the metal by ferrous iron. Therefore, in a magmatic environment