Page 318 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 287

Based on the field and laboratory experimental data obtained by the writers, the
following conclusions have been reached.
(1) If dehydration of smectite (conversion to illite), with the release of relatively
fresh water, is in part responsible for the overpressured formations and undercompacted
shales, then the undercompacted shales will contain fresher waters (compared to water
in the associated sandstones). Salinity changes (usually freshening of water) have been
used as a warning of impending abnormal pressures while drilling through thick sand-
shale sequences. This could also be due to influx of fresher water from the shales into
sands. Water in shales, both well-compacted and undercompacted, is fresher than that in
associated sandstones.
(2) The salinity of undercompacted shales appears to be higher than the salinity
of associated well-compacted shales. In the literature, comparison is made between
the well-compacted and undercompacted shales of diverse origins. Some investigators
compared interstitial waters in shales having different mineralogy and obtained from
different depths.
(3) In the case of both undercompacted and well-compacted shales, the salinities of
interstitial fluids in shales are lower than those in associated sandstones if all the other
variables remain unchanged.
(4) As compaction fluids move upwards in a thick shale sequence, they become more
saline. Thus the undercompacted shales lower in the sequence may contain fresher water.
(5) The concentration of ions in expelled pore water becomes higher with increasing
sediment-loading rate.
(6) The higher the temperature, the more rapid is the decrease in the ionic concentra-
tion of the expelled pore water up to a certain level (Brown, 1998).
(7) The higher the initial salinity of the pore water, the faster is the rate at which the
concentration of the expelled pore water decreases.
(8) The concentration of high-valence (Mg 2+, Ca 2+) ions may first decrease with
increasing overburden pressure, and then increase to values higher than their initial
concentrations.
(9) In the case of same initial electrolyte concentration of ions in pore water, the
change in concentrations of ions in expelled water from low-cation exchange capacity
clays (e.g., kaolinite) is much lower, at a given overburden pressure, as compared to a
high-cation exchange capacity clay, such as smectite.
(10) There is a need for further research involving laboratory high-temperature and
high-pressure autoclave experiments involving clay compaction (dehydration and con-
version) and the measurement of the resulting D/H and 31SO ratios in the squeezed-out
fluids.
The above conclusions relating to the field and laboratory investigations clarify many
of the observations made on the (1) extracted pore waters from ocean sediments, (2)
oilfield water chemistry, and (3) calculated pore-water chemistry from geophysical log
analyses.
A number of models have been proposed to describe how the chemical changes of
pore-water are brought about by the burial of sediments under gravitational compaction.
Additional modifications can take place, as noted in this chapter, owing to dissolution of
subsurface salt beds and thermohaline convection.