Page 108 - Origin and Prediction of Abnormal Formation Pressures

88 A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON AND E AMINZADEH
Pore Pressure, MPa (psi)

50 (712) 100 (1,423)

1,000
(3,279)

2,000 11"1
(6,557)

3,000
L__ (9,836)
g

4,000
~"(13,115)
a
5,000
(16,393)

6,000
(19,672)
A
Fig. 3-6. Pore pressures in shales (1) and sand reservoirs (2) in oil and gas fields along the
Khamamdag-Sea-Karasu-Sangi-Mugan-Persiyanin Bank trend. A = hydrostatic pressure gradient. (Modi-
fied after Buryakovsky et al., 1986. In Gurevich and Chilingar, 1995, fig. 4, p. 130.)

in the Baku Archipelago and especially in the Kura region are much higher and are
widespread.
Abnormality at shallower depths (about less than 1500 m, i.e., above the Productive
Formation) in the Baku Archipelago is mostly due to vertical migration of fluids from
the deeper formations through deep faults and active and buried mud volcanoes. The
idea of a deep source of abnormality is strongly supported by the fact that abnormal
pressures in the Bulla-Sea field were encountered only in wells situated near longitudinal
deep faults (Khalilov et al., 1988). At the same time, in the much deeper sediments of
the Productive Formation, abnormality decreases near such faults (Yusuf-Zadeh et al.,
1979). A mud volcano in this field is buried below the Akchagyl Formation. Roots of
mud volcanoes in the Baku Archipelago reach Paleogene-Miocene sediments.
Pressures are the highest in the Kura region. Pressure gradients reach 0.0226 MPa/m
in the northeastern slope of the Kyurovdag anticline (Kasumov et al., 1976). The
difference between the measured pressures in sand reservoirs and calculated pressures in
shales reaches up to 40 MPa in both the Kura region and Baku Archipelago (Fig. 3-6).
Calculated abnormalities are rather high even in thin (1 to 2 m) shale beds
(Buryakovsky et al., 1986) and increase with shale bed thickness (Fig. 3-7). The
abnormality level in such beds decreases with time upon production (Table 3-2).