Page 257 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 229

0 0
I 1,000
II~ ~"~~.~ <:'o: 1 ~~176176
fim ::::-1
,,oooilt} ~---~. ;; ;:" 1 ~~176176

9 2,000 1'~. ~5. ~--.~ ' 1 ~176176176
~
tl~ ~, "~.*~, --'~m 1 '~176176
a II~ "%, ~..'..+ % 1 +~176176
"
tl~
""~.~,
1 ~~176176
[
~,ooo ~ ~~ "~s *:(.ii ~ % 11~176176176
I~x ~.-" " .................. ..--" "--.1'"~176176
.
II 1 '~~176176
4,000 LI 13,000
0 1,000 2,000 3,000 4,000
Concentration, meq/I

Fig. 10-2. Changes in concentration of pore waters with depth (based on data obtained from Collins, 1975,
table VIII, p. 8). A lumped-parameter analysis having a 10-point moving average of 110 data points from 10
formations in 8 sedimentary basins in the U.S.A. (Modified after Chilingarian et al., 1994, fig. 5-1, p. 109.)

on the craton. Sediments in the latter basins have been, for the most part, lithified and
indurated into rocks. Some of the processes affecting pore-water chemical composition
in 'mature' basins consist of dilution by fresh-water recharge, possible concentration by
membrane filtration, late diagenetic mineral transformation reactions, and dissolution of
evaporites (Rittenhouse, 1967; Hitchon et al., 1971; Hanor, 1987a; Aharon et al., 1992).
Locally, the influx of mantle volatiles has further caused postdepositional changes in the
pore-water chemistry (Oxburgh et al., 1986). The pore waters having salinities less than
that of seawater often indicate the presence of a meteoric component. As an example,
the pore waters in the Illinois Basin (craton interior basin) have been modified by
fresh-water influx (Nesbitt, 1985). Table 10-2 lists many of the mechanisms that have
been proposed to account for the origin of subsurface pore waters.

Reliability of water sampling

Most petroleum geologists and engineers have ignored the properties of pore fluids
associated with the less permeable shales in compacting basins. One reason for this
oversight is that the shale permeabilities are so low that the exposed shale intervals in
oil wells rarely produced pore water in measurable quantities. Another reason is that
such studies always had a low priority until the later 1960s when abnormally high fluid
pressures were intensely investigated by the petroleum industry. The water produced