Page 313 - Origin and Prediction of Abnormal Formation Pressures

282 H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.

of dissolved constituents both inside and outside the clay suspension is unity (f = 1).
Furthermore, in Smith's case, the concentration of either negative or positive ions is
expressed as a fraction of pore volume rather than concentration per unit of bulk volume.
An additional assumption is that there is no association between cations and negatively
charged clay particles. A complete association would eliminate the cation-exchange
capacity, such that the concentration of ions, both inside and outside the suspension,
would not change with decreasing porosity. Partial association, therefore, would be
expected to reduce or delay changes in these concentrations. Realizing this, Smith
extended his model to include the case in which the cations are partially associated with
exchange sites on the negatively charged particles.
Smith (1977) compared his experimental data, for the infinitesimal equilibration
case, with equations describing his, Appelo's (1977), and Bolt's (1961) models. The
plotted data show the variation between porosity and salinity of squeezed-out pore water
from smectite clay (Smith, 1977, fig. 6, p. 384). Neither Bolt's nor Smith's equation
gives good agreement with the experimental results. The trends predicted by these two
models deviate from the experimental results with decreasing porosity. At high values
of porosity, the three models (Appelo, Bolt and Smith) all are in good agreement with
the experimental data.

ISOTOPE STUDIES OF INTERSTITIAL FLUIDS
Aside from the ion concentrations, the stable isotopic composition of water is another
parameter to characterize pore waters. Deuterium and oxygen-18 concentrations in
meteoric surface waters vary by about 43 and 5.6%, respectively, and are linearly
related (Degens and Chilingar, 1967). A comparison of the 180/160 and D/H2 ratios
shows that atmospheric precipitations normally follow a Rayleigh process at liquid-
vapor equilibrium. The Raleigh process explains why with higher altitudes and latitudes
fresh waters become progressively lighter, whereas tropical samples show very small
depletions relative to mean ocean water. In contrast to meteoric waters, ocean waters
are isotopically heavy and fall within a narrow range of 1 and 0.1% for deuterium
and oxygen-18, respectively. Evaporation strongly affects the 180/160 and D/H2 ratios
by causing a preferential depletion in the lighter isotopes ~H and 160, which are
concentrated in the vapor phase. The remaining water will be heavier, and the D and 180
contents will show a relative increase.

Geological observations and evaluation

Degens and Chilingar (1967) pointed out that hydrochemical field evidence by
Knetsch et al. (1963) showed that in the Nubian Series aquifer in the western Egyptian
Desert the oxygen isotopes were not fractionated during subsurface transportation over
a distance of 700 miles and at a depth range of 500-2000 ft. The oxygen isotope
ratios of the water samples taken at intervals of 20-100 miles stayed within 1%o,
whereas the chemical composition fluctuated strongly in response to migration and
diagenesis.