Page 102 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 83

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Fig. 3-3. Distribution of mud volcanoes in Azerbaijan. (Modified after Melik-Pashaev et al., 1983. Editors
of the map: A.A. Ali-Zadeh, E.M. Shekinskiy and A.A. Yakubov.) 1 = faults separating large structural
elements; 2 = faults important for mud volcanism; 3 = smaller faults; 4 = large, periodically erupting mud
volcanoes; 5 = buried mud volcanoes inactive for 100 years and more; 6 -- all other mud volcanoes; 7 = oil
and gas fields. (In Gurevich and Chilingar, 1995, fig. 1, p. 126.)

fragments of shale fall down from the borehole walls (Melik-Pashaev et al., 1983).
Instability of some shale is caused by its lithology. For example, Lower Pliocene
(Pontian) shales have gypsum inclusions. They easily soak in water and separate into
small laminae that fall down from the borehole walls.
Faults are numerous in the area. In the Apsheron zone nearly all anticlinal structures
are cut by faults. Longitudinal faults, that have amplitudes of several hundred to 2000
m (6557 ft), are especially important. Such faults contribute strongly to the formation
of mud volcano channels. Yakubov et al. (1971) showed that longitudinal faults in the
southwestern Caucasus cut not only the Pliocene formations but also the Miocene-
Paleogene and Mesozoic ones. This provides a possibility for mud volcanoes to have
deep roots into the Mesozoic formations.
Active and buried mud volcanoes are widespread over the territory of the South
Caspian Basin. Active mud volcanoes are well known in Azerbaijan in the Apsheron,
Kobystan and Kura regions, and in the Apsheron and Baku Archipelagos. Mud volca-
noes are situated on the axes of anticlinal structures (Fig. 3-3), but not necessarily on
the anticlinal crests (Melik-Pashaev et al., 1983). Yakubov et al. (1971) described more
than 220 mud volcanoes and their numerous gryphons and salses in the Azerbaijan. In
SW Kobystan alone, there are more than 650 active gryphons and salses that emanate an
average of 500 m 3 of gas per day each. Many mud volcanoes are buried and their fluids
cannot reach the Earth's surface.