Page 119 - Origin and Prediction of Abnormal Formation Pressures

SMECTITE-ILLITE TRANSFORMATIONS 99
%

20 40 60 20 40 60 20 40 60 20 40 60 20 40 60
,, i i i i i ; I 1 1
II 9 c ;l .. 9 " Cc)
2000 ' 'ca o Cd) Ce)
oo
o 9 ea
,o
oe 9
E 3000 ee 1"
oo I!. !.
1
9 oeO
..c: 4000 oQ
NI,,,,- 9 eo 9 9 e 9 9 ~Q
o. 9 e e,
9 9 m o e 9 9 . 9 e 9
o 9 eee 9 !: :o
5000 ,. o ~
.;Io , 9 eJ o 9 :8 ~
9 9 " "I:', "'i|" : !t" oe
.,I
9 Q
I oe
6000 -0 9 , II, 9 9149 et 9 oQ .::,|o 18
e,, 9 9 9 9 9 9 9 QOqS "~ e 9 9
Fig. 4-2. Contents of various clay minerals in the Productive Unit of the Baku Archipelago: (a) mont-
morillonite, (b) hydromica, (c) kaolinite, (d) chlorite, and (e) mixed-layered minerals. (Modified after
Buryakovsky et al., 1995, fig. 4, p. 206.)
Fig. 4-1. On the other hand, field data in the South Caspian region, as discussed in
this chapter, shows that a practically unaltered montmorillonite is present in the Baku
Archipelago deposits at depths down to 6 km, i.e., throughout the entire drilled section
(Fig. 4-2). This suggests a subordinate role of montmorillonite dehydration in the total
process of AHFP development.

BURST'S COMPACTION MODEL

A compaction model based on a three-stage dehydration sequence and the transfor-
mation of montmorillonite clay to mixed-layer varieties was proposed by Burst (1969).
The initial dehydration stage is essentially completed in the first few thousand feet
of burial as the interstitial water content is reduced to approximately 30% (20-25%
interlayer water and 5-10% residual pore water) (Fig. 4-3). During the second stage,
the argillaceous sediment is in a state of quasi-equilibrium as it continues to absorb
geothermal heat. Pressure is relatively ineffective as a dehydrating agent because of
the increased density of the interlayer water packet. As soon as the heat accumulation
is sufficient to mobilize the interlayer water, one of the two remaining interlayers of
bound water (statistically averaged) is discharged into the bulk system. Burst (1969,
p. 80) stated that the amount of water in movement should constitute 10-15% of
the compacted bulk volume. During the third stage, the final water increment, having
approximately capillary water density, gradually is forced out of the clay mineral lattice
and voids as sediment temperature increases. Burst's dehydration-compaction model
was discussed by Rieke and Chilingarian (1974).