Page 255 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 227

pressures and the compaction process is still continuing to squeeze out pore water
(Buryakovsky et al., 1994). All of the South Caspian Basin field evidence indicates that
at the present time, pressure systems are not actively leaking or open. Factors opposing
the withdrawal of fluids from the interlayer space of clays in these sediments is probably
slowing down or halting the smectite transformation into mixed-layer and illitic clays.
In conjunction with the low heat flow and low formation temperatures (105-110~ at 6
km), the compaction process is retarded and these factors contribute to the maintenance
of isolated fluid system. For an isolated system, equilibrium must exist between the
pore-fluid constituents and the surrounding minerals. If the system is geologically leaky,
then the chemical equilibrium will change with time. Thus a thermodynamic viewpoint
is sufficient in explaining the large-scale phenomena.
The emphasis of discussion in this chapter is primarily on the expulsion chemistry
of pore water by compaction. Ancillary processes are briefly discussed, if they have an
effect on the pore-water chemistry. The main thrust of discussion is to clarify and sub-
stantiate what could be important to petroleum exploration and recovery operations. Key
component to such an understanding is the integration of available laboratory-simulated
compaction data with field measured data and modified as evidenced by structural and
thermodynamic imprints.

Evolution of seawater into pore water
Sedimentary basins on the average contain about 20% pore water by volume. This
pore water at depth is hot and saline, and frequently occurs under high pressures. Some
low-salinity waters, however, are often associated with abnormally high fluid pressure
zones. It must be pointed out that interstitial waters are mobile and are the agents by
which chemical constituents are transferred from one place to another. Most of the
dissolved constituents present in the waters trapped during sedimentation are squeezed
out during the initial stages of compaction.
It is of interest to briefly review the historical development time line of the theories
of subsurface fluid origins. Washburne (1914) thought that the pore water contained
in sediments is not just buried seawater, but subsequent studies have shown that pore
waters in marine Tertiary sediments are essentially remnants of seawater entrapped
with the sediments during deposition (Chave, 1960; Manheim, 1976; Sayles, 1979).
Degens et al. (1964) analyzed the oxygen isotope composition of a number of pore
waters ranging in geologic age from the Cambrian to the Tertiary, and reported that
the ~180 values of the highly saline oilfield brines do not deviate appreciably from
the 3180 values of present-day seawater. Later studies by Sayles and Manheim (1975)
have shown that in all but the most slowly deposited sediments, pore water exhibits
changes in its composition during burial. The rate at which pore water is expelled from
argillaceous sediments depends not only on the overburden pressure and the physical
and chemical properties of the contained fluids, but also on the texture, structure, and
mineral composition of the sediments.
Table 10-1 shows chemical changes of pore water held in marine sediments with
respect to the rate of sedimentation and depth of burial. Results from the Deep Sea
Drilling Project showed that biogenic sediments and pelagic clays undergoing a rate of