Page 261 - Geochemistry of Oil Field Waters

CONCLUSIONS 245

present in the modified sea water but also observed how it redistributed the
dissolved solids to prQduce salinity variations.
They concluded that formation waters result from mixing of surface
waters with modified marine or nonmarine water in the subsurface rocks,
that exchange of oxygen isotopes between the water and rock caused differ-
ent water types in different basins, and that formation waters that have
passed through shale ultrafilters are more depleted in deuterium.
A study of the Surat Basin showed that most of its hydrocarbon accumu-
lations are associated with quasi-stagnant waters. The salinities of these
quasi-stagnant waters were higher than were the salinities of the waters in the
more dynamic recharge areas. The investigators postulated that these high
salinity waters were formed by membrane filtration because of cross-
formational flow and also that the hydrocarbon accumulations in these
quasi-stagnant areas resulted from release of hydrocarbons mobilized by a
moving water. The hydrocarbons were released because of the higher
salinities of the waters in the quasi-stagnant areas (Hitchon and Hays, 1971).
A study of waters in sedimentary rocks of Neogene age in the northern
Gulf of Mexico Basin was made by Jones (1969). The hydrologic conditions
currently found in these sediments are similar to conditions that previously
occurred in older sedimentary basins. Osmotic flow has a dominant influence
upon the hydrology of normally and abnormally pressured aquifer systems
in the northern Gulf Basin.
Jones (1969) found that many forces such as gravity, sediment diagenesis,
different water salinities, ionic and molecular diffusion, different electrical
potentials of sediments, thermal potentials, pressure, and osmotic membrane
filtration affect the hydrology in this basin.
Fowler (1970) found that salinity variations within the Frio sands in the
Chocolate Bayou field, Brazoria County, Texas, are the result of selective
concentration of ions by shales acting as membranes. In this field, pressures
seem to reflect the flow paths of the waters, and the greatest changes in
pressures are found across shaly sections. Analyses of water samples from
this field over a 28-year period indicate decreasing salinity with production
time caused by dilution of the original brines by waters squeezed from the
shales adjacent to the aquifers.
Chilingarian and Rieke (1969) reviewed the processes which can alter the
chemical composition of formation waters. They concluded that most of the
original water was sea water, and that the concentration process in many
cases results from compaction and membrane filtration rather than evapora-
tion. Their experimental results indicated that solutions squeezed out of
rocks during compaction progressively decrease in dissolved solids concen-
trations with increasing depth.

Conclusions

The origin of oilfield waters is related to many natural processes. Initially,