Page 165 - Geochemistry of Oil Field Waters

152 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

the temperature and the redox potential control its occurrence. It is trans-
ported in hot springs (White et al., 1963).
Shales, sandstones, and carbonates contain about 0.4, 0.03, and 0.04 ppm,
respectively, of mercury. Sea water contains 3 x lo-’ mg/l, and subsurface
oilfield brines contain 0-0.15 mg/l. The samples containing 0.15 mg/l of
mercury were found in relatively dilute brines taken from the Cymric and
the Rio Bravo oilfields in California. Free mercury is found in the oils
produced from these fields, and the ages of the producing formations range
from Eocene to Pleistocene.
The mercury content of natural waters has been used to locate cinnabar
deposits (Dall’Aglio, 1968). The amounts of mercury in waters appear to
increase with increasing bicarbonate concentration. Karasik et al. (1965)
found that saline waters containing 200,000 mg/l of chloride contain very
small amounts of mercury, which suggests that anionic complexes such as
HgC14-* may not be important transporters of mercury. Brackish waters con-
taining up to 3,000 mg/l dissolved solids, up to 400 mg/l of bicarbonate, and
the iodide ion sometimes contain up to 10 ppb of mercury, while stronger
brines contain <0.1 ppb of mercury, which suggests that mercury may be
transported as Hg14-* in brackish waters.

Lead

Lead is a member of the IV A group of elements; it is ubiquitous in the
earth, but its abundance in the crust is only about 0.002 wt.% (Fleischer,
1962). It is extracted from its minerals during weathering and migrates in the
form of soluble-stable compounds. It is particularly soluble in acetic and
other acids. Because the bicarbonate form is more soluble than the carbon-
ate, lead can be transported as the bicarbonate. Most of the lead is
precipitated from waters before they reach the sea. Hemley (1953) studied
lead sulfide solubility related to ore deposition from saline waters. He con-
cluded that lead-complex concentrations increase with increasing concen-
trations of bivalent sulfur and decrease at pH values above 7. The solubility
of lead is limited primarily by the solubility restrictions of its sulfide and
sulfate in reducing and oxidizing systems. How its solubility is influenced by
many other ions, such as those found in a brine, has not been sufficiently
studied.
Shales, sandstones, and carbonates contain about 20, 7, and 9 ppm of
lead, respectively. Sea water contains about 0.003 mg/l, and subsurface
brines contain trace amounts to more than 100 mg/l of lead.

Cadmium
Cadmium is a member of the I1 B group of elements and may be consid-
ered one of the rarer elements; its abundance is about 3 x lo-’ wt.% of the
earth’s crust (Fleischer, 1962). It is strongly thiophile, but its chemistry