Page 81 - Geology and Geochemistry of Oil and Gas

ABNORMALLY-HIGH FORMATION PRESSURE 57
the composition of the initially deposited clays, and the abnormally low
temperatures, i.e., these sediments lie in the early diagenetic zone. In some cases,
absence of potassium ion in the pore water of sediments could explain the absence of
montmorillonite-to-illite transformation.
Of great importance is the study of (1) regularities in the distribution of clay
minerals over the entire section, (2) identification of basic factors influencing the
transformation of montmorillonite to illite, and (3) prediction of catagenetic changes
at greater depths not yet reached by boreholes. It is not always possible to determine
the origin of clay minerals, i.e., whether they are primary or secondary. For example,
Milleau (1968, see Buryakovsky et al., 1989) noted that the montmorillonite formed
at the final stage of illite degradation does not differ from the primary
montmorillonite, as evidenced by the X-ray analysis.
Photomicrographs of fresh broken surfaces of argillaceous rocks of the
Productive Series of the Baku Archipelago (depths of 1400–5200 m) were taken
with scanning electron microscope (SEM) (Buryakovsky et al., 1986, 1988). The
surfaces were examined in sections cut parallel, perpendicular, and oblique to the
bedding. The mineral compositions of these rocks are generally the same throughout
this depth range. The main clay minerals are illite and montmorillonite, with small
amounts of 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 the 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 illite grains. There are also
microcavities formed by diagenetic processes. Cores from depths greater than
4000 m show greater evidence of transformation. Illite and montmorillonite
predominate, with the montmorillonite being of both primary and secondary origin.
The secondary montmorillonite occurs in the interstices between the illite grains, at
their edges, and in cracks. The primary montmorillonite is disrupted or twisted at the
edges and the secondary pores are present.
These Pliocene beds show degradation not only of the primary montmorillonite
but also of the illite, which changes to montmorillonite. Probably, these
transformations are largely responsible for the retention of the same illite-to-
montmorillonite ratio at depth.
Transformation of clay minerals during catagenesis is a complex process,
proceeding over a long period of geologic time under the influence of interrelated
and interdependent factors. It is extremely difficult to determine the effect of various
factors, i.e., to give a quantitative estimate of the intensity of influence of each
one. The solution to this problem probably lies in future investigations. The effect
of subsurface temperature and pressure and hydrochemical factors on the
postsedimentary (diagenetic and catagenetic) alteration of these Pliocene clays
should be studied using the data on chemical analyses of formation waters (e.g.,
availability of potassium ion), formation temperatures, and pore pressures
determined from logs.