Page 220 - Geochemistry of Oil Field Waters
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SEDIMENTARY ROCKS 207
minute for 10,000 years. Compaction water is further considered in a sub-
sequent chapter concerned with the accumulation of petroleum.
The resistance to flow of bound interstitial water is greater than the
resistance to deformation of the sediment framework during the first stages
of compaction. As pressure is increased and sediment compacts, the sedi-
ment framework increases resistance to deformation, which also governs the
deformation rate of the bound-water films and the outflow of the interstitial
water. Rosenqvist (1962) found that bound water has a higher viscosity than
unbound insterstitial water. Permeability decreases with compaction, and
this together with the increased water viscosity leads to additional resistance
to expulsion of bound interstitial water during subsequent compaction.
Porosity decreases during compaction, and at infinite depth it would
become infinitely small. Porosity and permeability are two important factors
in determining the amount of petroleum and/or water that can be recovered
from a given reservoir or aquifer (Pollard and Reichertz, 1952; Caraway and
Gates, 1959). The porosity of an aquifer indicates how much fluid the
aquifer can hold, and the permeability indicates how fluid can move through
the aquifer. If the porosity is high but the permeability is low, the reservoir
may contain large amounts of oil, gas, or water, but they cannot be
recovered unless special techniques are used to increase the permeability.
Sediment diagenesis
Mineralogical and chemical changes occur in the sediments as they com-
pact. The mineralogic composition of recent marine sediments and ancient
marine sedimentary rocks are different, as is the composition of the intersti-
tial water in the recent sediments compared with the waters in ancient
stratigraphic units. Chemical reactions occur between the sediments and
their interstitial water (Chave, 1960). It has been shown that chemical
changes can be measured in recent sediments, e.g., below the sediment-
water interface, changes in pH and Eh result from degradation of sulfate ions
by bacteria (Emery and Rittenberg, 1952).
Numerous chemical inhomogeneities occur within a single core sample of
recent sediments (Degens and Chilingar, 1967). The magnesium concen-
tration in the interstitial w&er decreases slightly with depth, while the con-
centrations of calcium and potassium increase (Siever et al., 1965). The
interstitial waters from continental shelf sediments have higher chloride con-
centrations and higher ratios of Ca/C1, K/Cl, and Rb/C1 than the overlying sea
water; the ratios of Li/Cl and Mg/Cl are about the same as in the overlying
sea water except that the Li/C1 ratios are higher in the innershelf samples
than in the outershelf samples. The Sr/Cl ratio is higher in the overlying sea
water, while the pH and Eh values are lower in the sediments than in the
overlying sea water (Friedman et al., 1968).
A detailed study indicates that virtually no environment exists on or near
the earth’s surface where the pH/Eh conditions are incompatible with