Page 259 - Geochemistry of Oil Field Waters

RESEARCH STUDIES 243

Bredehoeft et al. (1963) developed a mathematical model to predict the
distribution of ions within a formation. They assumed that a hydrostatic
head differential opetates between the margin and center of a geologic basin,
producing a water movement upward through confining low permeability
beds. If these low permeability beds contain clay membranes to restrict the
passage of ions, the waters on the upflow, or more permeable, side become
more concentrated in dissolved solids. They theorized that this process
produced the concentrated brines found in the Illinois Basin, and that their
model added weight to the membrane theory of brine concentration. A
major drawback to the model is the tremendous pressures that are necessary
to produce a movement of water upward through confining low permeability
beds.
Graf et al. (1965) found that isotopic fractionation occurred when waters
passed through shale micropores in the Illinois, Michigan, Alberta, and Gulf
Coast Basins. Their study did not yield sufficient evidence to estimate the
total fraction of water movement in the basins subsequent to sediment com-
paction. The 6 "0 concentrations in brines did not indicate a direct correla-
tion with ancient oceans.
A study of the 6D and 6l80 in formation waters indicated that the water
was predominantly meteoric, little exchange or fractionation had occurred
to alter the deuterium, but extensive exchange between the water and rock
had altered the oxygen (Clayton et al., 1966). They postulated that forma-
tion waters in the Gulf Coast Basin lost their original connate water because
of sediment compaction and flushing, and that the present water is meteoric
water which came in through outcrops.
This study was good; however, basic studies concerning the fractionation
and exchange of isotopes between water, hydrocarbons, and rocks need to
be made. Results of such studies should enable more positive interpretations.
A simplistic model was derived to determine the amounts of fresh water
and sea water necessary to create the brine compositions now present in the
Illinois and Michigan Basins (Graf et al., 1966). The model assumes: (1)
perfect efficiency of shale ultrafilters; (2) complete bacterial reduction of
sulfate with replacement in solution of equivalent bicarbonate; (3) complete
removal of bicarbonate and equivalent sodium by shale ultrafiltration; and
(4) magnesium reaction with calcium carbonate to form dolomite. The
dolomitization reaction furnished more soluble calcium than is possible for
the Illinois Basin, so another calculation was made assuming complete loss of
magnesium to clay minerals with no return of calcium.
The calculations indicated that less fresh water passed through the rocks
of the Illinois Basin than those of the Michigan Basin. These data conflicted
somewhat with Clayton et al. (1966) in that they argued that the water
molecules now in the Illinois Basin originated as fresh water, while the data
of Graf et al. (1966) indicated that too few volumes of fresh water passed
through the Illinois Basin to alter the brine significantly.
A study of the hydrodynamics of the Illinois Basin indicated that in