Page 260 - Geochemistry of Oil Field Waters

244 ORIGIN OF OILFIELD WATERS

recent times, before pumpage, the differences in vertical head in the deep
aquifers were insufficient to cause upward flow through shale, resulting in
ultrafiltration (Bond, 1972). In fact the head differentials were barely suf-
ficient to enable upward flow through an open conduit.
Berry (1969) outlined the relative factors that influence membrane filtra-
tion in geologic environments. The membrane properties of shales are caused
by the electrfcal properties of their clays and organic materials. Clays
predominantly are cation exchangers with singly charged SiO- and
AlOSi-% sites and minor anion exchanges with replaceable OH- ions.
Divalent cations are adsorbed in preference to monovalent cations and
sodium is hyperfiltratcd with respect to lithium and strontium with respect
to calcium, because of preferential adsorption of ions with ionic potentials
most similar to the ionic potential of the exchange site. The selectivity of
hyperfiltration for the halogens is C1 > Br > I > F because of their substitu-
tion for OH in the clays. Thus, in waters concentrated by this process the
Ca/Na, Na/Li, Sr/Ca, Cl/Br, Br/I, and I/F ratios should increase. These ratio
increases have been found in some brine systems, but by no means in all
systems.
Billings et al. (1969) found five types of formation waters in the Western
Canada Sedimentary Basin and postulated the origin of two of the types.
One type of water was formed by selective membrane filtration which pro-
duced waters containing high concentrations of dissolved solids. A second
type was a mixture of membrane-concentrated formation water and bitterns
formed after the precipitation of halite but before the precipitation of
sylvite. They theorized that the alkalies were filtered selectively by clay-
shale membranes, producing a concentrated brine, and that the relative con-
centration pattern is Rb > K > Na > Li. This pattern is the reverse of what
occurs by ion exchange but is similar to the surface mobilities of cations
along clay surfaces.
A detailed study of the Western Canada Sedimentary Basin, including a
determination of the rock volume and pore volume (Hitchon, 1968), the
effect of topography upon the fluid flow (Hitchon, 1969a), and the effect of
geology upon the fluid flow (Hitchon, 1969b), strongly suggested that
thermal, electro-osmotic, and chemico-osmotic forces are operating within
the basin to affect the fluid energy gradients. Pressure differentials of about
98 kg/cm*along with salinity differences of 200,000 mg/l between forma-
tions in close proximity were found which suggest that chemico-osmotic
forces are occurring.
Hitchon and Friedman (1969) used chemical analyses and stable-isotope
analyses for hydrogen and oxygen for surface waters, shallow ground waters,
and deep ground waters in a study of the origin of formation waters in the
Western Canada Sedimentary Basin. They postulated that surface waters have
mixed with diagenetically altered sea water to form the formation waters.
Using mass balance data for the deuterium and dissolved solid contents of
the formation waters, they calculated not only how much fresh water is