Page 269 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 241
Concentration, mg/I x 10,000
0 4 8 12 16 20 24 28
0 0
' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I .... l ' ' '
~-----10,000 mg/I
. "'""........ .... -- 1 0 0 0
"'~
- - 2000
", ~ ...... TDS
- )- ..... :o
/ ...- ....... CI - 3000
4...
1000 _ ,, / 91 ...... .o+
- 4000
.ii~ " ..m -..,n-- HCO~
- 5000
_
:'.g . 6....
E - 6000 .,_.
.ff 2000 .:-: 4 I" ~ o:-.:',,a .ff
~ .....
9
D. "'~ \ " ........ ~...:,:, - 7000 ~
(b
cl - 8000
"~i.~ ~ .t , ...' .......
:
~ , R"
.! ~,..~-~ - 9000
' 9 *.~ ~ ~ a"""~2 Note abrupt change
3000 ! ~ ,-~ o- .... :":::~2~ between normally - l 0,000
t r l I ~ / pressured Hackberry
,',:j / ~ ." sandstone and high
9 I - l 1,000
.,~ ~ / pressured Hackberry
:" ~ ~ 1 :" (Manchester Field)
._ ;W WK ~dChestelSS ~ sandstone - 12,000
9 "
'~ / 7 _N ",. ~'"o.
~
'4" i.~l [:::,.Cl ~ berry SS Field r~ ..... :'.':o~Hackberry SS Field - 13,000
F
4000
~ - 14,000
, s i 11 , Manchester SS le 1111~1 " " I I I I
'~1
Irlldll
Fig. 10-6. Changes in concentration of pore waters within sandstones and shale beds in the Hackberry
and Manchester fields, Louisiana (U.S.A.). The concentrations of HCO~-, SO]-, Ca 2+, Mg 2+, and, K + in
sandstones are less than 10,000 mg/1 and are not shown. (This figure is based on data from Schmidt's
(1975) tables 1 and 2, pp. 332-337. (Modified after Chilingarian et al., 1994, fig. 5-4, p. 116.)
Additionally, sidewall cores from shales in well A-5 were leached and analyzed for
major dissolved constituents.
Schmidt's study showed that the compositions of pore water in shales and those
in sandstones are different. His data on sandstone and shale pore-water chemistry (as
given in his tables 1 and 2, respectively), have been plotted as lumped parameters in
Fig. 10-6. The data reveal changes in the concentration of pore water with depth in
the sandstones and shales. It should be noted that the sandstone pore-water data from
the Manchester Field is only for the high-pressure zone, which lies between 11,200
ft (3400 m) and 12,900 ft (3800 m). The data from the shale sidewall cores were
taken at intervals of every 500 ft (152 m) between depths of 3000 ft (914 m) and
14,000 ft (4257 m) to include normal- and high-pressure zones in the Manchester Field.
There is an abrupt decrease in the concentration of various ions in the high-pressure
Manchester Field. Fig. 10-6 illustrates that there is a significant difference between the