Page 121 - Origin and Prediction of Abnormal Formation Pressures

SMECTITE-ILLITE TRANSFORMATIONS 101

E
Q
c-
I0 20 30 40 50 60 70 80 90 100 MPa
Akchagyiian i i I ; i I I I I
to Apsheronian 400
Quaternary
Deposits
F•
1200 - Shales
E
.m O
.m r > r ~ Reservoir Rock
.m
t~ 2000 -
(1)
(3.
o.
2800 -
o r
(1)
:3
o"
(!) 3600 -
r
(1)
>
o
:3
"(3 4400 -
o
IX.
5200 -
~
VII Horizon
6000 - . ~ .,,~

Fig. 4-4. Pore fluid pressure gradient, ~ (in MPa/m) in shales and in reservoir rocks in the Baku
Archipelago. (Modified after Buryakovsky et al., 1995, fig. 2, p. 205.)

compaction. This leads to significant underconsolidation (undercompaction) of rocks
and to development of AHFE In this process, abnormal pressures in reservoir rocks are
caused by those in shales and approach each other only in moderately thick beds. The
regionally developed reservoirs have a better pressure distribution than that in shales;
consequently, their pore pressure is usually lower than that in the enclosing shales
(Fig. 4-4).
In the South Caspian Basin, the drilled Pliocene terrigenous section is 6.5 km thick,
with AHFP unevenly distributed, both vertically and laterally. Presence and intensity of
AHFP are determined by lithofacies of the oil- and gas-beating rocks, tectonics (uplifts),
feasibility of underground water discharge and other factors. The highest clay content
(up to 95%) has been observed in the Productive Unit of the Baku Archipelago. An
important regional feature is the very high porosity of argillaceous rocks, much higher
than those at similar depths in other areas of the world (Buryakovsky et al., 1982, 1986,
1995; Dzhevanshir et al., 1986). Porosity of Pliocene shales in Azerbaijan at depths of