Page 275 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 247
'[ 1 l 0 Log-derived salinities in
"
I
I
I
2000 clean sands.
Lab-measured salinities on
X sidewall samples from shales
3000

4000

5000
,i,i.-=
.ff
,.11.-.
6000
:
7000 - R..X
o.
x.X
8000 .==
-- ;X*'*'*"X ~Top of overpressured
"
~
X* .Xx zone
,
9000
Xo:: ~
_ ~e-~ _
10,000 X'~ ~
l I I I
0 30 50 70 90 110
Salinity, ppm x 10 3 NaCI
Fig. 10-9. Salinity trend in an offshore Louisiana well. (Modified after Fertl and Timko, 1970.)

these horizons are approximately 1.8 to 1.9 times hydrostatic. Formation water samples
were obtained from drillstem tests, and their analyses indicate lower salinity values
in the first high pressure horizon compared to the salinity in the shallower section in
the Port Canning Well No. 1. Formation water samples from the Bodra-1 well were
obtained by drillstem testing over a section from 3681 to 4193 m. Fig. 10-10 shows
the relationship between salinity and fluid pressures in the Bodra-1 well. The salinity
data from the drillstem test should not be considered reliable. Open flow drillstem tests
taken over long intervals will contain fluid contributions from many sands present in the
section. Sahay (1999), however, pointed out that the log-derived salinity values indicate
a decreasing trend above the high-pressure zone at 3750 m. The salinity anomaly
synchronizes with the resistivity and density log trends in the Bodra-1 well.
An exploratory well drilled in 1975 in the Kutch Basin encountered high pressures
ranging from 389 to 545 kg/cm 2 beginning at 2600 m down to TD at 4575 m. Sahay