Page 103 - Origin and Prediction of Abnormal Formation Pressures

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

#92 #75 #41 #79 #63
Sea level
A t \ ,,i~
Q
N2aP

~~~~--" ~---. N2ak

5,01~9"m ,. 4,7 !0 m ~ 4,80 VII
(16,426 ft) 5,260 m (15,475 ft) (15,754 ft)
~. ~ j(17,246 ft) "~ ~ ~" VIII

2
!/I

Fig. 3-4. Bulla Island geologic section: 1 = mud volcano breccia; 2 -- faults. (Modified after Melik-Pashaev
et al., 1983. In Gurevich and Chilingar, 1995, fig. 2, p. 127.)

Activity of mud volcanoes clearly shows the scale of the vertical upward fluid
(gas and water mostly) migration. Faults constitute another important avenue for vertical
fluid migration (Fig. 3-4). Oil and gas generation and vertical upward migration, tectonic
movements forming faults, and deformation of plastic shales forming diapirs and mud
volcanoes have been considered to be part of an integral process as early as 1934 by
I.M. Gubkin (in Melik-Pashaev et al., 1983). This process still continues now. Tectonic
movements cause strong earthquakes in the region. Mud volcanoes erupt periodically,
transferring large volumes of water and gas to the surface and atmosphere, and also to
the subsurface strata. Melik-Pashaev et al. (1983), for instance, believe that the abnormal
pressure in the Bakhar oil field formations is partially caused by mud volcano activity:
a big subsea mud volcano nearby erupts fiercely at time intervals of 1-2 years to 16-17
years.
One of the evidences of the upward fluid migration can be seen in the Oil Stones
field. The oil composition in this field is more or less uniform with a slight tendency
of increasing density toward the oil-water contact. But, at the pool's edge, a light oil
appears which is quite alien for this group of fields (Samedov, 1959).