Page 125 - Origin and Prediction of Abnormal Formation Pressures

104 L.A. BURYAKOVSKY, R.D. DJEVANSHIR, G.V. CHILINGAR, H.H. RIEKE III AND J.O. ROBERTSON, JR.

0.01
/
E /
o 0016 - /
a. '
6 5 4 /

o 0.014
/

0.012

J
J
0.010 - 1 1 1
0 20 40 60 80

Xsh, %
Fig. 4-6. Pore pressure gradient (r/, MPa/m) as a function of clay content (X~h, %). 1-3 -- argillaceous
rocks (1, 2 and 3: lII, IV and V sedimentary rhythms); 4 = aquifers; 5 = oil-bearing sandstones; 6 -
gas-bearing sandstones and siltstones. (Modified after Buryakovsky et al., 1995, fig. 5, p. 207.)

and Lower Kura region with their extraordinarily high porosity, ~sh, owing to rapid
sedimentation and slow compaction.
The formation of abnormal pore pressures in the shales of Azerbaijan has been
experimentally demonstrated by elastic compression of hermetically isolated cores of
Cenozoic shales. Fig. 4-7 shows that pressure in the core rises with the external pressure
and then decreases as the confining pressure decreases, but always remaining higher than
in the case of increasing load, evidently as a result of residual (irreversible) deformation
of the rock.
Abnormally high pressures in the argillaceous sequences may substantially affect
geological processes at depth. It appears that they have played an important part in
folding, clay diapirism, mud volcanism and earthquakes. Models of these phenomena
are described by Coulomb's law and by rheological models of various theoretical bodies.
According to Coulomb's law, resistance to shearing in shales is the first power function
of normal compressive stress. As the abnormal pore pressure in shales increases, the
intergranular stress (effective stress) decreases, down to very low values under certain
conditions. Resistance to shearing determined by friction decreases correspondingly.
This leads to an intergranular sliding and facilitates to a considerable degree the devel-