Page 221 - Geochemistry of Oil Field Waters

208 ORIGIN OF OILFIELD WATERS

organic life (Baas Becking et al., 1960). Because COz is the main byproduct
of organic oxidation and the building material of plant and much bacterial
life, it plays a dominant role. Carbon dioxide dissolves in water, producing
the bicarbonate ion and a free hydrogen ion. The concentration of the
hydrogen ion is lo-’ equiv./l (pH 7) at 2OoC in pure water, but when
saturated with COz it rises to lo-’ (pH 5). This reaction moves to the right
with increasing temperature in a closed system. In the presence of organic
constituents, the equilibrium is modified, and the pH range can extend from
2 to 12.
The ionic potentials of the constituents involved in diagenesis are impor-
tant (Cloke, 1966). Those that stay in true ionic solution up to rather high
pH levels are Na+, K+, Mg+’, Fe+’, Mn+2, Ca+’, Sr+’, Ba+’, etc.; they are
the soluble cations, and their ionic potentials range from 0 to 3, where the
ionic potential is the ratio between the ionic charge and the ionic radius.
Constituents that are precipitated by hydrolysis are those with ionic poten-
tials from 3 to 12 and include such ions as AP3, Fe+3, SP4, and
Constituents that form soluble complex ions and usually go into true ionic
solution include B+ 3, V4, N+ ’, P+’ , S6, and Mn+’ ; their ionic potentials
are over 12. In general, the hydroxides of the soluble cations possess ionic
bonds; therefore, they are soluble, the hydrolysates or those precipitated by
hydrolysis form hydroxyl bonds, and the soluble complex ions have hydro-
gen bonds.
Organisms that consume oxygen cause a lowering of the redox potential,
and in buried sediments it is the aerobic bacteria that attract the organic
constituents and remove the free oxygen from the interstitial water. Sedi-
ments laid down in a shoreline environment often differ in degree of oxida-
tion from those laid down in a deep-sea environment (Pirson, 1968). For
example, the Eh of the shoreline sediments may range from -50 to 0 mV
while the Eh of deep-sea sediments may range from -150 to -100 mV. The
aerobic bacteria die when the free oxygen is totally consumed, and the
anaerobic bacteria attack the sulfate ion which is the second most important
anion in the sea water. During this attack, the sulfate is reduced to sulfite
and then to sulfide. Also the Eh drops to -600 mV (Fig.7.2). Sulfide is
liberated and CaC03 precipitates as the pH rises above 8.5 (Dapples, 1959).
Sulfur has two stable isotopes, * S and S, with a mass differential of 6%.
The isotopes are fractionated during the change of S04-2 to Sv2, and SZ
is enriched in the more energetic 32S isotope. The average ratio of 3zS/34S
in normal sea water sulfate is about 21.76 (Ault, 1959). The sulfate isotopes
are useful in interpreting ancient diagenetic stages.
Reactions occur during sediment diagenesis and affect the composition of
the interstitial water. Calcite is precipitated if the pH is high, or it is dis-
solved if the pH is low. Dolomitization occurs as follows:

2CaC03 + MgClz * CaMg(C03)2 + CaClz