Page 273 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 245

Chloride concentration, mg/I x 1000
0 30 60 90 120 150
| 9 9 I 9 9 I m ! I ' ! I ' ' I
0 ..
Well compacted shales ( 9 value}
,,
: i . ---4-== Well compacted shales (max. value]
: i : ..... ~"" Undercompacted shales ( 9 value)
: i : "'~" Undercompacted shales (max. value)
i ! -- al~- Associated sandstones [min. value)
..
i p ]~ ~. : X O ~ Associated sandstones (max. value)
3000 -..!.~-.--- ............. ..L." ..................................... .......... /;.~ .......
,
: i ",~i i , ",,! : !
i~ .[ 0 ! l: I : :
" ~r : \ - 1 : :
:{ . : ,,,.0 : x I ........ i :
::i i .-?" i ,,'" i .......... ................ i
-
i .I, "; D"" i ~'" i Data off scale
;
.
X.
"
:
.
.
.
A:." 6000 ---!.-l-. '---I~ ............ ............................. ........................ ~!" .............................. ..... r ~'-"~"i""
jl
:
"
":"
""
~
*
"
.
~
"

{3. ~ "'~ _.-~
9 i\ l
:- . ]+ : ..It .,-" +'"""+'"" "<'"-
9000 ...... :--~--./ ........................................... ....... ~....:.-.-:.T.~ ..... i.,:-.:i ...................... 9 .....
i
9 :~ i ..... .--' ,.-? i .........................
7
)~ -: ........ ! ,. ~ i :.
~'" : : ....r i i i
I ~ ~,~ ....... .:." .... + + + +
.
12,000 . I....~.;,.- ........ ~....: ............................. ~. ............................. .............................. : .............................. i...
i~ ~ :i i ""-... ! i ! :
t
~
il~ ~.,. i '"-+._. i i i
i ,, .... ." .......... -i ................ ! i ::
:
:: .... ............ ..................
............................. i ............................. + ............................. i ............................. ........................... i..-
I ,ooo -i
I I I I I I I I ~ I a , I n ~ I
Fig. 10-7. Change in the magnitudes of the maximum and minimum chlorinity values of undercompacted
and well-compacted shales and associated sandstones. (Modified after Chilingarian et al., 1994, fig. 5-5, p.
120.)
under-compacted (53 samples) shales over a depth interval of 2500-7000 ft from an
Indonesian oilfield containing overpressured formations. The chlorinity was expressed
in terms of the amount of chlorine per gram of shale or ppm of interstitial solution.
Salinity of the interstitial solutions in undercompacted shales, which are associated with
overpressured sands, were found to be higher than those in well-compacted ones. Shale
porosities were determined from sonic logs. In both cases, salinity of the interstitial
solutions in shales was found to be much lower than that in the associated sands. Similar
salinity trend in an offshore Louisiana well is presented in Fig. 10-9.
Bengal and Kutch basins, India
Abnormally high pressures occur below the Miocene-Pliocene unconformity in the
Port Canning and Bodra areas of the onshore portion of the Bengal Basin at depths
of 3800 m and 3730 m, respectively (Sahay, 1999). Calculated formation pressures for