Page 133 - Origin and Prediction of Abnormal Formation Pressures

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

kaolinite and chlorite. The rocks have a honeycomb-like texture, which is clearly seen
in oblique sections.
The SEM results indicate that there are both 'forward' and 'reverse' clay-mineral
transformations, which occur simultaneously as the rocks are buried. The cores from
depths of 1400-1800 m show only very slight changes in the clay minerals, although one
can identify damaged sublayers (twisting) at the edges, as well as secondary pores and
cracking in some hydromica grains. There are also microcavities produced by secondary
(diagenetic) processes. Cores from depths greater than 4000 m show more signs of
transformation. Hydromica and montmorillonite predominate, with the montmorillonite
being both primary and secondary. The latter occurs in the interstices between the
hydromica grains and at their edges and cracks. The primary montmorillonite is
disrupted or twisted at the edges and the secondary pores are present.
These Pliocene beds thus show degradation not only of the primary montmorillonite
but also of the hydromicas, which change to montmorillonite. This paradoxical observa-
tion is probably largely responsible for the retention of the same ratio of hydromica to
montmorillonite at depth.
Transformation of clay minerals during catagenesis is a complicated process, pro-
ceeding over a long period of geologic time under the influence of interrelated and
interdependent natural factors. It is extremely difficult to recognize the effect of each
of these factors, i.e., to give a quantitative estimate of their degree of influence. One
of the paths toward the solution of this problem is the utilization of data from the
detailed study of the composition and structural features of clay minerals, with inclusion
of the complex of contemporary methods of mineralogical investigation, performed on
a representative sample of clays from the studied stratigraphic section. The effect of
thermobaric and hydrochemical factors on the postsedimentary (diagenetic and catage-
netic) alteration of Pliocene clays in this region should be studied using the data on
chemical analyses of formation waters, formation temperatures and pore pressures in
clays determined by logging methods.

EFFECT OF THERMOBARIC CONDITIONS
The abnormally low temperatures might be responsible for the absence of clear-cut
clay-mineral transformation. It has been shown by Khitarov and Pugin (1966) and Ma-
gara (1968) that temperature is a basic factor influencing montmorillonite degradation.
Also of interest is the effect of hydromica degradation on the geothermal characteristics.
Inasmuch as the clay hydration is exothermic, there may be elevated gradients in the
depth ranges where the hydromicas are degraded under otherwise equal conditions.
Temperature measurements in deep wells in the South Caspian Basin areas and
onshore of Azerbaijan are of interest. In studied section, the average geothermal
gradient is approximately 16~ and the temperature at a depth of about 6 km does
not exceed 110~ A characteristic feature is that the gradient becomes lower at a depth
of approximately 3-4 km (Table 4-4).
The increased gradient at a depth below approximately 4 km may be related to
hydromica degradation, which releases heat. At this depth, the degradation rate exceeds