Page 173 - Geochemistry of Oil Field Waters

160 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

Selenium

Selenium is a member of the VI A group of elements and occurs in -2,0,
+4, and +6 valence states, respectively. It is a scarce element with an abun-
dance of about 9 x 10" wt.% of the crust of the earth (Fleischer, 1962).
Large areas of North America are underlain by seleniferous rocks and soils.
These seleniferous rocks are of sedimentary origin and range in age from
Late Paleozoic to Holocene. Selenium is the only known element that can be
absorbed by plants in sufficient amounts to make them lethal when eaten by
animals (Trelease, 1945). Fig. 5.15 illustrates the distribution of seleniferous
vegetation.
Sandstones, shales, and carbonates contain about 0.6, 0.05, and 0.08 ppm,
respectively, of selenium. Sea water contains about 0.004 mg/l of selenium.
A few subsurface oilfield brines from areas where selenium is present in soils
were analyzed at this laboratory, but no selenium was detected in the brines
analyzed. Most brines are present in a petroleum environment under
reducing conditions, and in such an environment, selenium likely is reduced
to the element and precipitated. However, in areas where outcrop water
flows through petroleum-bearing formations, it is possible that selenium in
the form of t)e anion Se03-2 may be present.

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Fig. 5.15. Distribution of seleniferous vegetation in the United States.