Page 110 - Origin and Prediction of Abnormal Formation Pressures

90 A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON AND E AMINZADEH

0.020
~..'- (0.087)
6

5
2 4

.~7
r
"0

t9

o~

IX.
0.010 I ,
(0.043) 4 (13) 8 (26)
Thickness of Shales, m (ft)

Fig. 3-7. Pore pressure gradients in shales vs. thickness of shale beds: 1 -- Zhiloy Island; 2 -- Oil Stones; 3 =
Bakhar; 4 = Sangachaly-Sea-Duvannyy-Sea-Bulla Island; 5 = Bulla-Sea; 6 = Khamamdag-Sea-Karasu-
Sangi-Mugan-Persiyanin-Bank. (Modified after Buryakovsky et al., 1986. In Gurevich and Chilingar, 1995,
fig. 5, p. 133.)

The writers analyzed the reviewed data on pressure distribution in Azerbaijan. They
tried to determine (1) whether or not current theories of the pressure abnormality
distribution in Azerbaijan are complete enough, and (2) whether or not the pressure
measurement methods are fully reliable. Although up to now a considerable amount
of research work has been done on abnormal pressures in Azerbaijan, not all aspects
were exhaustively studied. The analysis conducted by the writers suggested some new
areas and directions of possible research work that can be done by the oil companies to
increase drilling and production efficiency in Azerbaijan.
(1) Most authors who studied the Azerbaijan overpressures believe that compaction
of sediments is the totally dominant cause of pressure abnormality and, therefore, there
should be a close relation between pressure and porosity abnormalities. This concept is
not convincing from the viewpoint of an analysis of the nature of geological processes. It
also contradicts the well-known fact that many very shallow fields had initial pressures
higher than the overburden pressure (Anikiev, 1964). This cannot be explained by
compaction only and recognition of a contribution of vertical migration of hydrocarbons
to pressure abnormality in these fields is unavoidable.
(2) Deformation of wellbore walls is considered to be mostly the result of shale
plasticity combined with the impact of pressure abnormality. This may not be completely
true.