Page 130 - Origin and Prediction of Abnormal Formation Pressures

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

TABLE 4-4
Variation of the geothermal gradient and pore pressure gradient in sedimentary rocks with depth (average
values are shown in parentheses) (after Buryakovsky et al., 1995, table 3, p. 210)

Depth range Pore pressure gradient Geothermal gradient
(m) (MPa/m) (~
1000-2000 0.012-0.020 10-15
(0.016) (12)
2000-3000 0.013-0.021 10-12
(0.017) (11)
30004000 0.014-0.022 8-11
(0.018) (10)
4000-5000 0.015-0.023 15-19
(0.019) (17)
5000-6000 0.015-0.023 21-21
(0.019) (22)
More than 6000 0.016-0.024 15-25
(O.O2O) (2O)

rise in temperature and pressure as the sediments are buried. Consequently, during late
catagenesis, the clay-mineral assemblage consists of two components (hydromica and
chlorite), no matter what was the initial composition.
On the other hand, virtually unaltered montmorillonite has been observed at great
depths and in large amounts (Kheirov, 1979). Kheirov explained the almost unaltered
montmorillonite found at a depth of 6026 m in the Pliocene beds of the Baku
Archipelago as due to specific sedimentation conditions, the composition of the initial
material and the effects of abnormally low temperature, i.e., these sediments lie in
the early diagenetic zone. Possibly a lack of potassium in interstitial solutions has
also played an important role. Of great importance is the study of regularities in
the distribution of clay minerals over the entire section, the identification of basic
factors influencing the transformation of montmorillonite to illite, and the prediction of
catagenetic changes at greater depths not yet reached by boreholes.
It is important to note that the results do not always allow one to judge correctly the
origin of clay minerals, i.e., whether they are primary or secondary throughout the depth
range. For example, Millot (1949) noted that the montmorillonite formed in the final
stage of hydromica degradation does not differ very greatly from true montmorillonite,
the X-ray characteristics being the same.
Of interest are the photomicrographs of freshly broken surfaces of shales and
argillaceous rocks of the Productive Unit of the Baku Archipelago (depths of 1400-
5200 m) recorded with a scanning electron microscope (SEM). The surfaces were
examined in sections parallel, perpendicular and oblique to the bedding. The mineral
compositions of these rocks are on the whole the same throughout the depth range. The
main clay minerals are hydromica and montmorillonite, with subordinate amounts of