Page 241 - Geochemistry of Oil Field Waters

RESEARCH STUDIES 225

Brine classification

A study of the evolution of subsurface brines in Israel by Bentor (1969)
led him to classify brines into four groups. The first group consists of brines
similar to sea water except for an increased concentration of calcium and a
decreased concentration of magnesium, which he attributed to dolomitiza-
tion. The second group was similar to sea water but contained two to three
times higher concentrations of dissolved salts, deficient in sulfate and
magnesium, and enriched in bromide and iodide. The sulfates were lost by
organic reduction, the magnesium was lost by exchange reactions with clays,
and bromide and iodide were added by organic sources. The third group was
a high-salinity calcium chloride-type brine formed by surface evaporation
and later modified in the subsurface by differential ultrafiltration, The
fourth group was a highly saline, calcium chloride type with Ca/Na ratios
greater than one. This group was divided into two subgroups where the first
subgroup is a highly saline and highly differentiated Early Paleozoic brine,
while in the second subgroup they are old Paleozoic brines which were
submitted to an additional cycle of surface concentration by evaporation.

Ion association

Truesdell and Jones (1969) studied ion association in brines and found
that, except for the chloride ion, the major simple ions form ion pairs, while
the minor and trace metals in brines form coordination complexes. Selective
ion electrodes can be used to determine directly the ionic activities of
sodium, potassium, chloride, fluoride, and sulfide in brines. Experimental
data were used to calculate chemical models for ion association and coordi-
nation complexes in brines. These models are useful in explaining the
chemical behavior of brines.

Relation to lithology

Kramer (1969) used factor analysis to study the relationships of the brines
to the type of rock from which they were taken. His results indicated that
the major ions in most brines are sodium, calcium, and chloride; brines are
enriched in calcium and bicarbonate and are deficient in magnesium and
sulfate relative to sea water. The factor groupings did not reflect the lithol-
ogy of the rocks from which the brines were taken, indicating that such a
relationship does not exist or is difficult to detect. The brine analyses used in
the study were primarily macro analyses and did not include pH, minor, or
trace constituents. A study of this type would benefit significantly if the
following conditions were met: (1) use only the best available sampling
methods; (2) use field analysis techniques; (3) use positive lithology identifi-
cation; and (4) use only the best available laboratory methods of analysis.