Page 166 - Geochemistry of Oil Field Waters

BORON 153

differs from that of zinc in that it will precipitate from a strong acid solu-
tion, whereas zinc will not. There are few independent cadmium minerals,
and its distribution is mainly that of a “guest” atom or ion in minerals. It
frequently is present in lead-zinc deposits and occurs in solid solution in
hypogene sulfides. A main carrier of cadmium is sphalerite, and oxidation of
sphalerite or other sulfides containing cadmium will release the soluble
cadmium sulfate.
Shales and carbonates contain about 0.3 and 0.035 ppm of cadmium,
respectively, and sandstones contain less than 0.01 ppm (Mason, 1966). Sea
water contains about 0.0001 mg/l, and the subsurface oilfield brines may
contain from 0 to about 0.001 mg/l of cadmium. Subsurface brines of the
sulfate type in contact with lead-zinc deposits probably contain higher
concentrations of cadmium.

Boron

Boron is a member of the I11 A group of elements, and it is an oxyphile
and lithophile element. Its abundance in the crust of the earth is about 0.001
wt.% (Fleischer, 1962). It has small atomic and ionic radii.
Knowledge of the presence of boron compounds in oilfield waters is im-
portant for several reasons. Boron is useful in identifying the sources of
brines intrusive to oil wells, or in fresh-water lakes or streams. In concen-
trations exceeding 100 mg/l, it affects electric log deflections. Boron is
present in oilfield brines as boric acid, inorganic borates, and organic borates.
When it is present as undissociated boric acid, it is an important buffer
mechanism, being second only to the carbonate system. It may be precipi-
tated as the relatively insoluble calcium and magnesium borates.
Kazmina (1951) calculated the borate-chloride coefficient of some
Russian oilfield waters. With a plot of the borate-chloride coefficient in
logarithmic coordinates as a function of chloride content, he distinguished
genetic groups of natural waters found in oil-bearing regions.
Mitgarts (1956) studied the significance of boron and other elements in
petroleum prospecting. In general, boron, together with bromine and iodine,
is always associated with waters accompanying petroleum. Like chlorine, it
can be considered an element of marine origin. The solubility of most boron
compounds, the hydrolytic cleavage of boron salts, and their ability to be
occluded and coprecipitated with other compounds account for the exten-
sive migration of boron. Soluble-complex boron compounds in brines and
connate waters probably are there as a result of the decay of the same plants
and animals that were the source of petroleum.
Shales, sandstones, and carbonates contain about 100, 35, and 20 ppm,
respectively, of boron. Sea water contains about 4.8 mg/l, and subsurface
oilfield water contains from trace amounts to more than 100 mg/l. Fig. 5.13
is a plot of chloride versus boron concentrations of some oilfield brines taken
from some sediments of Tertiary, Cretaceous, and Jurassic age. The plot