Page 126 - Origin and Prediction of Abnormal Formation Pressures

SMECTITE-ILLITE TRANSFORMATIONS 105

0
D_ ~176

_
50
.g

!._
30

I:1.
(J,) lO
0
I I I I
I 0 30 50 70 90
External Pressure MPa
Fig. 4-7. Experimentally determined relationship between the pore pressure (Ppor, MPa) in an argillaceous
rock core and the external (total overburden, confining) pressure (or, MPa). (Modified after Buryakovsky et
al., 1995, fig. 6, p. 207.)

opment of shearing. In such instances, plastic argillaceous sequences can become quite
mobile under high shale pore pressure and are displaced. Depending on the geological
environment and duration, this process may lead to the development of folds, diapirs,
mud volcanoes, and earthquakes. In Azerbaijan, such geologic set-up is quite typical of
thick Paleogene to Miocene argillaceous sequences with extremely high, quasi-geostatic
values of AHFP, with shale pore pressure gradients of 0.020-0.023 MPa/m.
The undercompacted character of Cenozoic shales in Azerbaijan implies that their
sealing properties are determined mainly by their AHFP and still continuing squeezing
out of pore water. The progressively rising capillary pressures, as the pore channel
diameters decrease, determine the sealing properties of argillaceous rocks.

CLAY-MINERAL TRANSFORMATION
Pliocene shales and argillaceous rocks were studied at depths of 1400 to 6000
m at various locations of the Apsheron Peninsula and Apsheron Archipelago, South
Apsheron Offshore Zone, Baku Archipelago and Lower Kura region (Buryakovsky and
Dzhevanshir, 1985, 1986; Buryakovsky et al., 1988).
The pelitic fraction of argillaceous rocks, i.e., the fraction with a particle size of less
than 0.01 mm, accounts for 51-83% (average of 69%) of the total rock mass. The sand
fraction of argillaceous rocks accounts for 1.5% on the average; the content of the silt
fraction in these rocks ranges from 11 to 50% (average of 21%); and the average content
of the carbonate cement is 10%. At a depth of 2000-6000 m, the porosity of normally
consolidated argillaceous rocks fluctuates within the 12-3% range, whereas the porosity
range for the unconsolidated rocks is 28-10%. The corresponding permeability ranges
are (7.5-0.36) x 10 -7 mD and (230-6.2) x 10 -7 mD.