Page 101 - Origin and Prediction of Abnormal Formation Pressures

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

permeability and porosity. The writers approached the pressure distribution data in the
South Caspian Basin using this concept.
Beginning with the geological setting of the region, the South Caspian Basin was
formed mostly during the Early and Middle Pliocene. The total thickness of sedimentary
rocks reaches 22,000 m (72,131 ft) in the deepest part of the basement surface. Major
oil, condensate and gas reserves are present in the shale-sand sequences (Productive
Formation) of Middle Miocene. The thickness of this formation is up to 4000-4500 m
(13,115-14,754 ft) (Ali-Zadeh et al., 1985). Basic anticlinal structural elements were
developed during the Middle Pliocene when intense tectonic activity took place. The
next peak of tectonic activity, which occurred during the second half of the Quaternary,
completed tectonic development of regional and local geological structures, formed
most faults, and strongly enhanced the mud volcano activity. Tectonically most active
areas were the Apsheron Peninsula, Apsheron Archipelago and Sub-Kura region (see
Buryakovsky et al., 2001).
The Mesozoic-Miocene tectonic stage has produced mostly sub-latitudinally oriented
structures, whereas structures of the Oligocene-Quaternary stage are sub-longitudinally
oriented.
Mesozoic to Oligocene-Miocene deposits are represented mostly by flysch and
finer marine molasses. For Pliocene to Quaternary rocks, coarser, mostly continental,
molasses are typical. Marine formations constitute 80 to 90% of the whole sedimentary
section of the region.
Owing to the very rapid Oligocene-Quaternary sedimentation (that began in Pontian
time) and thick shales with low thermal conductivity, the geothermal gradient in the
Azerbaijan part of the South Caspian Basin is as low as 16-18~ This helps to
preserve the sealing properties of the shales. For example, formation temperatures in
the Baku Archipelago fields are 110 to 115~ with a high content of montmorillonite
at depths of about 6000 m. The least compacted shales are encountered in the Middle
Pliocene Productive Formation in the Lower Kura Depression and Baku Archipelago
(Kheirov et al., 1990). Porosities of shales in a vertical geologic section vary appreciably
due to variations in lithology: it is actually impossible to find lithologically identical
rocks even within the same horizon (Kheirov et al., 1990). Although the density of rocks
increases with depth, poorly consolidated highly permeable sands can be encountered at
depths of 5 to 6 km and more.
It is believed that despite some consolidation, shales retain their plasticity and good
sealing property even at depths of more than 5-6 km (Mekhtiev et al., 1988) owing
to their mineral composition. The montmorillonite content varies widely laterally and
in vertical sections, but mostly remains within the range of 20% to 60% (Buryakovsky
and Djevanshir, 1985; Asadov et al., 1988; Bunyatov and lmanov, 1989; Kheirov et al.,
1990). X-ray analysis and electron micrographs (SEM) of shales show definitely that the
rather stable content of montmorillonite in the Baku Archipelago fields is caused by the
dominance of secondary montmorillonite formation over its destruction down to depths
of at least 6200 m (20,328 ft) (Buryakovsky et al., 1986).
Very intensive Quaternary tectonic stresses and movements greatly affected the shale
physical properties: many shales, strongly deformed by tectonic and diapir movements,
are not cohesive enough and, thus, are very unstable in the boreholes, i.e., large