Page 104 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 85

The distance of vertical migration can be judged by the lithology and age of rocks
brought by the mud volcanoes to the surface. Large fragments of Cretaceous limestones
and marls are present in the deposits of many mud volcanoes such as Lokbatan,
Otman-Bozdag, etc. (Melik-Pashaev et al., 1983).
Sediments in the South Caspian Basin have been accumulated at a very high rate
of 1.3 km per million years (Djevanshir, 1987). With shales being predominant in
the geologic section, compaction makes a major contribution to the distribution of
formation pressures. Presence of thick, highly permeable sand formations, exposed
by subsequent erosion in some places, and vertical migration of fluids, discharged
from deeper formations to the surface or to the overlying formations allows a rapid
redistribution of pressures. This may be the cause of a specific distribution of pres-
sures, e.g., pore pressures in thick shales noticeably exceed those in the permeable
formations.

Methods used in Azerbaijan to determine abnormal pressures

Authors who have studied abnormal pressures in Azerbaijan distinguished three
different pore pressures: (1) abnormal pore pressures in the permeable formations
(APPF); (2) abnormal pore pressures in shales (APPS); (3) abnormal pore pressures in
thin permeable sand lenses in shales (APTL).
In each case, different measurement methods were used. APPF were mostly measured
by wellbore pressure gauges. APPS were measured indirectly by calculation using
well-logging data. APTL were assumed to be equal to pressures in the surrounding
shales. Calculated values of pressures in shales were compared with pressures exerted
by the weight of the drilling mud column having a density necessary to maintain
wellbore wall stability or that corresponding to the beginning of gas penetration into
the mud. Judging from the texts of the reviewed papers, the static pressure of the
drilling mud column (depth times specific weight) was used. No special estimates of the
pressure evaluation precision were presented.
Deformations of wellbore walls in wells intersecting shales are considered, by almost
all authors, to be a result of abnormal pressure impact.

Distributions of abnormal pressures
Pressure abnormality in the region continuously increases from NE to SW, from Ap-
sheron Peninsula and Apsheron Archipelago toward the Kura Depression (Buryakovsky
et al., 1986). Fig. 3-5 shows changes in the vertical abnormality distribution in this
direction. Table 3-1 illustrates abnormality distribution in the oil and gas fields of
Apsheron and Baku Archipelagos and South Apsheron offshore zone.
According to Durmishian (1972), deeply buried Miocene-Paleogene rocks exhibit
considerable overpressures everywhere except near their outcrops. The upper part of the
Productive Formation of Middle Pliocene age, devoid of oil and gas, has no or very mild
pressure abnormalities. The middle part has widely distributed but mostly moderate
abnormalities. The lower part of the Productive Formation has high abnormal pressures
all over the area. Laterally, as all authors indicate, pressure abnormality increases with
the increase in depth and shale content of rocks.