Page 127 - Origin and Prediction of Abnormal Formation Pressures

106 L.A. BURYAKOVSKY, R.D. DJEVANSHIR, G.V. CHILINGAR, H.H. RIEKE III AND J.O. ROBERTSON, JR.

With increasing depth, the pore size of shales progressively decreases according to
the following equation:
dMe -- 4.6e -~ (4-1)

where dMe is the median pore size (in lxm); and H is the depth (in km).
Most of the clay minerals in the Productive Unit of Middle Pliocene age belong to the
montmorillonite and hydromica groups. The kaolinite content varies from 15 to 20%,
chlorite from 5 to 10%, and mixed-layered minerals, from traces up to 5%.
The X-ray analysis showed the variations in clay mineral contents with depth,
with no clear-cut regularity. Montmorillonite is present in large amounts (40% on the
average, reaching 75% in individual samples) throughout the Productive Unit. This
means that there has been no obvious transformation of montmorillonite to hydromica
in these clays, at least down to a depth of 6200 m. Table 4-3 (with regard to the
depth of occurrence) and Tables 4-4 and 4-5 (with regard to the location) present
data on montmorillonite, hydromica and other clay mineral contents in sedimentary
rocks of the Apsheron Archipelago (Neftyanye Kamni-2, Gryazevaya Sopka, Banka
Yuzhnaya-2 and Gyuneshli offshore areas), the South Apsheron Offshore Zone (Bakhar
oil and gas field), the Baku Archipelago (Sangachaly-mor6, Duvanny-mor6, Bulla
Island, Bulla-mor6, Alyaty-mor6, Khamamdag-mor6, Garasu, Sangi-Mugan, and Kamen
Persiyanina offshore areas), and the Lower Kura region (Kyurovdag and Karabagly
onshore areas) (see Buryakovsky et al., 2001).
Oligocene through Miocene shales of the Muradkhanly oil field (Middle Kura
Trough) have been studied onshore. The cores of Chokrak rocks were studied from
a depth of 2825-2830 m: montmorillonite and mixed-layered clay, with chlorite,
hydromica and volcanic ash. Organic matter is represented by skeletons of marine
microorganisms (coccoliths). The rock is fairly loose and unconsolidated. The Maikop
rocks have been studied on cores taken from depths of 3080-3085 m and 3287-3292
m. These rocks of marine origin contain montmorillonite clay with some ash. The ash
(volcanic glass) is often altered to montmorillonite. Broken grains of pyroxenes and
amphiboles with a typical cleavage are locally present. Montmorillonite, chlorite and
mixed-layered clays are widespread.
The observed distribution of clay minerals is due to different sources of clastic mate-
rial brought to the separate portions of sediment accumulation basin, the predominantly
allothigenic origin of clay minerals, and the variations in the rate of sedimentation. The
Russian Platform, the Kilyazi-Krasnovodsk Zone of uplift, and islands, which existed
north of the Apsheron Peninsula and Archipelago, as well as the southeastern slope
of the Greater Caucasus served as the primary source regions for clastic material for
the Apsheron Peninsula and the adjacent Caspian Sea. The more ancient (Mesozoic-
Paleogene) magmatic and sedimentary rocks of the mountain massifs of the Greater and
Lesser Caucasus and Talysh Mountains served as the primary source of sediments for
the Lower Kura region and the Baku Archipelago.
Montmorillonite and hydromica-montmorillonite minerals may be transformed to
hydromicas during diagenesis and catagenesis, as has been described for almost all
major sedimentation basins throughout the world. These changes in clay minerals
during catagenesis are most probable (not simply possible, as in diagenesis), due to the