Page 155 - Geochemistry of Oil Field Waters

CALCIUM 143
c

Normal evaporite curve
500

M
M
P
r
20
10
1,000 I 0,000 lO0,OoO
MAGNESIUM, mg/l
Fig. 5.8. Comparison of the magnesium concentrations of some Pennsylvanian (P) and
Mississippian (M) age formation waters from Oklahoma with an evaporating sea water.

crystal lattice of alternate ions of calcium and magnesium. The large differ-
ences in the ionic radii of Ca (0.99 A) and Mg (0.65 A) are the reason for this
diadochy.
Magnesium ions in aqueous solution have a large attraction for water
molecules and probably are surrounded by six water molecules in octahedral
arrangement. This may account for the paucity of magnesium in soils,
because the small cation becomes large by hydration. Sodium has a similar
reaction, but potassium, which does not, is readily adsorbed by soil colloids.
Shales, sandstones, and carbonates contain 15,000, 7,000, and 47,000
ppm of magnesium, respectively (Mason, 1966). Subsurface brines contain
from less than 100 mg/l to more than 30,000 mg/l; however, many subsur-
face brines are depleted in magnesium if compared to a sea water evaporite
sequence, (Table 5.11). Sea water contains about 1,300 mg/l. Fig. 5.7 is a
plot of chloride versus magnesium for some subsurface brines taken from
Tertiary, Cretaceous, and Jurassic age sediments. The position of the normal
evaporite curve indicates that all of these waters were depleted in magnesium
with respect to this curve (Collins, 1970). Fig. 5.8 is a plot showing similar
depletion of some subsurface brines taken from some sediments of Pennsyl-
vanian and Mississippian age.

Calcium

The abundance of calcium in the crust of the earth is about 3.55 wt.%
(Fleischer, 1962), making it the most abundant of the alkaline earth metals,