Page 80 - Geology and Geochemistry of Oil and Gas

56 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

An important regional feature of AHFP is a very high porosity of argillaceous
rocks (undercompaction), much higher than those at similar depths in the other
areas of the world (Buryakovsky et al., 1982; Dzhevanshir et al., 1986). Porosity of
Pliocene shales in the South Caspian Basin at depths of 4.0–5.5 km is several times
higher than in the consolidated shales present in other regions at the same depth.
Such a difference is the effect of young geologic age, relative contents of clay and
sand, formation temperature, and other factors. The abnormally-high porosity of
shales is primarily the result of slower rate of compaction compared to the
subsidence rate, due to the slow pore water removal from the compacting
argillaceous rocks during rapid sedimentation.
It should be noted that AHFP in argillaceous sequences is often attributed to the
montmorillonite dehydration as it is altered to illite (hydromica). As shown in Table 3.2,
practically unaltered montmorillonite is present in the deposits of the South Caspian
Basin at depths down to 6 km, i.e., throughout the entire drilled section. That indicates
a subordinate role of montmorillonite dehydration in the total process of AHFP
development in the region.
Montmorillonite and illite–montmorillonite (mixed layered) minerals may be
transformed to illite during the diagenesis and catagenesis, as 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
an increase in temperature and pressure as the sediments are buried. Consequently,
during late catagenesis, the clay–mineral assemblage consists of two components
(illite and chlorite), regardless of the initial composition. This is true in many parts of
world.
On the other hand, virtually unaltered montmorillonite has been observed in
Azerbaijan, at great depths and in large amounts (Kheirov, 1979). Kerimov
explained the almost unaltered montmorillonite found at a depth of 6026 m in the
Pliocene beds of the Baku Archipelago as due to specific sedimentation conditions,

TABLE 3.2
Clay-mineral composition in the Absheron Archipelago Rocks (average values are presented in the
denominator)

Depth range (m) Content of clay minerals (%)
Montmorillonite Illite Kaolinite Chlorite Mixed-layered
1000–2000 10245 35265 15220 5210 Traces
32 44 18 6
2000–3000 35270 20240 0215 0210 Traces
45 35 13 7
3000–4000 15250 30260 5220 5215 025
36 42 14 7 1
4000–5000 15270 10260 0220 0210 0230
40 38 12 6 4
5000–6000 5265 20265 0230 0215 0215
39 39 16 5 2
46000 5270 20260 10225 0210 0225
36 38 16 4 7