Page 314 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 283

Degens et al. (1964) reported on the analyses of the oxygen isotope composition of
pore waters ranging in age from the Cambrian to the Tertiary. The results presented
by them show that the 3180 values in the highly saline pore waters do not deviate
appreciably from the 3180 of present-day ocean water (~ = %0 deviation relative to the
Chicago Belemnite standard). Deviations from this mean value in pore waters into the
negative range of ~lSo, from Cambrian-Ordovician, Devonian, and Pennsylvanian age
formations in Oklahoma, U.S.A., are always well correlated to a decrease in salinity.
This was explained by Degens and Chilingar (1967) as the effect of dilution with
meteoric waters during migration of the pore water, or by later-stage infiltrations as a
result of uplift, denudation, or other geological phenomena.
This similarity between the isotope characteristics of the pore water and present
seawater leads to the conclusion that the concentration of the inorganic salts has not
been accomplished by syngenetic evaporation. Possible explanations are that it is an
effect of compaction or ion-filtration by charged net clay membranes, or both. Coplen
and Hanshaw (1973) found that when water is forced through a compacted Na-smectite
disc it was depleted in D by 2.5% and in lSO relative to the water left behind the
disc. The enrichment in D of residual waters closely follows a Rayleigh distillation
curve, which results in the enrichment in deuterium in the residual pore water if a
large proportion of the water has been transmitted through the clay membrane. Any
slight deviations into the positive range of ~lSo values in the samples could be caused
by original evaporation in a surface environment, or by isotope equilibration with the
surrounding mineral matter for millions of years.

Isotope studies of shales in the Gulf Coast
Yeh and Savin (1977) measured the 180/160 ratios of size-separated clays from Gulf
Coast shales buried at depths ranging from 1000 to 18,400 ft in three wells. Their O-iso-
tope results showed that the sediments are not isotopically equilibrated systems even
for those buried at depths where the temperatures are around 338~ (170~ The finer
fractions of both clay minerals and quartz are richer in lSo than the coarser fractions.
The values of ISo for the <0.1 ~m fraction fell in the range of +22.45%o (1371 ft) to
+ 17.66%o (5580 ft), whereas the calculated pore-water values changed with temperature
from about -1.8%o (35~ to + 10.9%o (170~ Oxygen isotope disequilibrium among
the clay fraction became less as the temperature increased (Yeh and Savin, 1977). They
noted that the variation of the calculated (not measured) O-isotope ratio of the pore
water with depth in a single well indicates a lack of communication between the water
at different depths. Calculated pore-water O-isotope ratios from O-isotope fractionations
between coexisting fine-grained quartz and clay indicate that the diagenetic reaction
of the clay minerals probably proceeded under conditions of a leaky pressure system.
Degens and Chilingar (1967) commented that compaction and filtration by charged-net
clay membranes should not noticeably influence the O-isotope ratios of waters, but will
be influenced by diagenesis. One of Yeh and Savins' conclusions is that any upward
movement of pore water in the compacting sequence occurred after chemical reaction
and O-isotope exchange of silicate minerals in the overlaying rocks had ceased. This
is in contradiction with the results of the previously discussed laboratory compaction
experiments of dehydration taking place before conversion.