Page 135 - Origin and Prediction of Abnormal Formation Pressures

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

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:L

"13
N
t,f)
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o
E
~ O
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I I
2 4
Depth (H), km
Fig. 4-9. Variation of median pore size (dMe, ~tm) in shales of the Baku Archipelago with depth (H, km).
(Modified after Buryakovsky et al., 1995, fig. 8, p. 212.)

The dependence of the montmorillonite content on the pore pressure gradient in
shales is shown in Fig. 4-8B (also see Table 4-5). There is a close correlation
between these two parameters. In the regions of the Baku Archipelago and Lower
Kura region, characterized by intense development of AHFP (pore pressure gradients
in shales of 0.018-0.019 MPa/m), the montmorillonite content in shales in the section
reaches an average of 53%. In regions with moderate development of AHFP (Apsheron
Archipelago and the South Apsheron Offshore Zone), the montmorillonite content
decreases to 17%.
There is no adequate discussion in the literature on the role of pore pressure in shales
on clay-mineral diagenesis and catagenesis. It can be shown theoretically that rising
pressures reduce the dehydration rates. The production of hydromica in clays, therefore,
involves an increase in the free water volume by the release of bound water, which
is denser than free water. A factor opposing this volume increase (such as high pore
pressure in shales) will reduce the dehydration rate.
On the other hand, AHFP can lead to hydromicas degrading to secondary montmoril-
lonite by the absorption of water. Under such conditions, reduced grain size of the clay
minerals favors degradation of hydromica, which occurs in this section, as shown by the
relationship between the pore size and depth (Table 4-3 and Fig. 4-9; pore sizes were
determined from SEM data).
The writers propose the following scheme for the relationship between clay-mineral
transformation and the thermobaric conditions.
In a basin where the subsidence rate is equal to the accumulation rate, the depth
at which diagenetic transformation (desorption of water) begins remains stable and
is largely determined by the geothermal gradient. Inasmuch as the desorbed water
has a greater volume than the interlayer water, abnormally high pressures may de-
velop if the water cannot escape. Under some conditions, the rising pore pressure