Page 79 - Geology and Geochemistry of Oil and Gas

ABNORMALLY-HIGH FORMATION PRESSURE 55
The abnormally-high formation (pore) pressures (AHFP) in reservoirs are known
to be caused by several diverse factors. It appears, however, that the most probable
mechanism of AHFP development in regions with thick sedimentary rocks
(sand–shale sequence) is a rapid sedimentation and gravitational compaction. This
leads to significant underconsolidation (undercompaction) of rocks and to a
development of AHFP. Abnormal pressures in reservoir rocks are often caused by
the influx of water from the overpressured shales. Pressures in sandstones and shales
approach each other only in moderately thick beds. The reservoirs of regional extent
have a better pressure distribution, than those in shales; consequently, their pore
pressure usually is lower than that in the enclosing shales (Fig. 3.13).
In the South Caspian Basin, the drilled Pliocene siliciclastic section is 6.5 km thick,
with unevenly distributed AHFP, both vertically and laterally. Presence and
magnitude of AHFP are determined by studying the lithofacies of the oil- and gas-
bearing rocks, structure of the uplifts, sand/shale thickness ratio, influx of water
from the shales into sands, integrity of caprocks, distribution of faults and fractured
zones, etc.

Fig. 3.13. Relationship between the pore-pressure gradient and relative clay content (after Buryakovsky
et al., 1995, Fig. 5, p. 207). 1-3 — Argillaceous rocks from three different areas; 4 – aquifers, 5 – oil-bearing
sandstones and siltstones, 6 – gas-bearing sandstones and siltstones.