Geology Reference
In-Depth Information
pressure is identified at practically all prospects in the region in the sec-
tion interval of Upper Cretaceous through Pliocene. As the thickness
of clay series increases, the absolute value of overpressure sometimes
approaches the geostatic load. This requires the application of weighted
drilling fluids (density 1.8-2.3 g/cm
3
in the central portion of the West
Kuban trough and in the Kerch-Taman area) (Durmishyan, Muradian and
Rachinsky, 1976; Kotov, Yermolayev and Fedotova, 1973; Kucheruk and
Ozerny, Mogilevskaya, 1981; Ozerny, 1985). Many factors indicate their
mostly syngenetic origin. It is caused by restricted pore fluid outflow from
consolidated paleo-undercompacted plastic clay sequences, by their elastic
compression at neotectonic events and by the effect of mineral dehydration
water during catagenesis. These factors are: spatial omnipresence, regional
AHPP development regardless of the structural conditions, its control
of the common disharmonious folding, of present-day mud volcanism
(Kerch-Taman zone) and diapirism, intensity, type and geologic environ-
ment of its manifestations in wells in the process of drilling.
Abnormal formation pressure in reservoirs also develops in all tectonic
zones and section intervals (Table 3.6, Figure 3.10).
Table 3.6 and Figure 3.10 show that maximum abnormal formation
pressure in all waterhead complexes is recorded in the Kerch-Taman zone
where the section is mostly a clay facies (95% and up), and lens-shaped
reservoir development is sporadic. In the West Kuban trough (clay content
below 85%) AHFP distribution is more differentiated over the area; in the
areas where the complexes' thickness is decreased and reservoir beds are
relatively common, the abnormality values are usually lower.
The scarcity of actual data about the
Mesozoic
sediments currently pre-
vents the generation of a unique picture of AHFP spatial distribution and
origin. Tentatively, however, it may be suggested that AHFP there was
initially mostly syngenetic and was caused to a significant extent by the
decrease in reservoir pore space due to its secondary cementation and by
the effect of the difference in heat expansion factors of fluids and rocks
at great depths (temperature). This judgment may be based on a rather
significant pressure excess and abnormality factor, generally low porosity
and permeability of water-saturated rocks (Bedcher and Solovyev, 1956;
Dobrynin and Serebryakov, 1978; Kotov, Yermolayev and Fedotova, 1973)
and high formation temperatures (over 120
°
С), etc.
On the other hand, the lengthy existence and hence the preservation
of high abnormal formation pressure suggests the permanent compensa-
tion of their relaxation. Quantitatively it is hardly possible to have been
a result of only specified syngenetic processes. This in turn suggests also
the probability of partial operation at the current stage both of epigenetic
