Page 239 - Geochemistry of Oil Field Waters

RESEARCH STUDIES 223

Some waters are mixtures of the infiltration water and trapped ancient sea
water.. Also, the rocks containing the waters often contain soluble con-
stituents which dissolve in the waters or contain chemicals which will
exchange with chemicals dissolved in the waters causing alterations of the
dissolved constituents.
The amounts of dissolved constituents found in oilfield waters range from
less than 10,000 mg/l to more than 350,000 mg/l. This salinity distribution
is dependent upon several factors including hydraulic gradients, depth of
occurrence, distance from outcrops, mobility of the dissolved chemical
elements, soluble material in the associated rocks, ion exchange reactions,
and clay membrane filtration.
Concentration of sea water can occur by surface evaporation, and there
are at least three independent processes that can cause major changes in
buried, isolated sea water:
(1) Dilution with meteoric or fresher waters which have entered outcrops.
(2) Reactions with minerals in the sediments and sedimentary rocks (the
reactions are often temperature and pressure dependent).
(3) Membrane filtration through clays and shales as a result of pressure
and osmosis.

Playa deposits

Jones et al. (1969) studied the composition of brines in shallow, fine-
grained playa deposits in the Great Basin. The concentrations of dissolved
solids in the water in these sediments often were as much as five times
greater than in the water in the associated lakes. They attributed the concen-
tration processes to capillary evaporation and entrapment of fossil brines
when the salinity of the lake water was greater (lake nearly dry).

Continental Slope drill holes

Manheim and Bischoff (1969) analyzed pore waters from drill holes on
the Continental Slope of the northern Gulf of Mexico. A relationship be-
tween the salinities of the waters and the proximity of diapiric structures
was found. This indicated that salts are leached from salt-bearing sediments
to increase the salinities of the pore waters. In some samples the high
bromide and potassium concentrations suggested that late-stage evaporitic
minerals such as carnallite and polyhalite were leached from salt bodies.
They postulated that molecular diffusion is a major mechanism which in-
fluences the distribution of salt in the pore waters. Similar conclusions have
been made for saline waters in other areas.

Relation to petroleum accumulations

Van Everdingen (1968) suggested that major circulation systems of for-