Page 134 - Origin and Prediction of Abnormal Formation Pressures

SMECTITE-ILLITE TRANSFORMATIONS 1 11
(A) (B)

~,~ 60

~--- 40
~
O
E J 9
c-
O 20
r
,.I,--
I I I [ I t I
~ 14 18 22 26 0.012 0.014 0.016 0.018
Geothermal Gradient (G), ~ Pore Pressure Gradient (qpor), MPa/m

Fig. 4-8. Dependence of montmorillonite content (M, %) on the (A) geothermal gradient and (B) the pore
pressure gradient in shales. (Modified after Buryakovsky et al., 1995, fig. 7, p. 211.)

some limit, which causes hydration to predominate over dehydration. Attention should
be given, therefore, to the effects of temperature on diagenetic and catagenetic processes.
A temperature increase accelerates the process of montmorillonite degradation,
which, in turn, favors its catagenetic transformation into non-swelling minerals (hy-
dromica and chlorite). Consequently, sections with a high geothermal gradient should
be characterized by small montmorillonite content. On the other hand, inasmuch as
a temperature decrease retards the process of montmorillonite degradation, sections
with low geothermal gradient should be characterized by high montmorillonite con-
tent.
Table 4-5 and Fig. 4-8A demonstrate the dependence of montmorillonite content
on the geothermal gradient in shales of the South Caspian Basin. The greatest mont-
morillonite content is found in the shales of the Baku Archipelago and Lower Kura
region, which is characterized by a low geothermal gradient (16~ The Apsheron
Peninsula and the adjacent offshore areas, having a higher geothermal gradient (24.0-
28.5~ are characterized by lower montmorillonite contents.
The low temperature apparently does not favor the transformation of montmorillonite
to hydromica, which reduces the montmorillonite degradation rate. Under otherwise
equal conditions, the transformation should increase with depth, which means that some
additional factors must be influencing the transformation.
Inasmuch as the transformation of montmorillonite into hydromica proceeds with the
removal of interlayer water, conditions at which desorbed water leaves the pore space
without hindrance will be favorable for the development of this process. Every factor
opposing the withdrawal of fluids from the interlayer space of clays, therefore, may lead
to slowing down or cessation of the reaction of transformation of montmorillonite into
hydromica or chlorite. Possibly, such a factor is the abnormally high pressure, which
occurs virtually throughout the section. The pressure gradients in the pores of shales
at 1000-6000 m are based on more than 2000 determinations and range from 0.012 to
0.024 MPa/m, with a mean of 0.018 MPa/m (see Fig. 4-4 and Table 4-4).