Page 254 - Origin and Prediction of Abnormal Formation Pressures

226 H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.

RELATIVE SCALE SIZE RESERVOIR PROPERTY PICTORIAL DESCRIPTION GEOLOGIC ENTITY

MICROSCOPIC J.l. m PORE SPACE FRAMEWORK
/ +

PORE
MESOSCOPIC m m NETWORK
DISTRIBUTION
I l l ! /~i.) ]

MACROSCOPIC m PORE )I"I ' I "I. 9 .Ira-W"
SYSTEM /

MEGASCOPIC m )o km LITHOHYDRAULIC
UNIT
~.!+[_li/l

REGIONAL DEPOSITIONAL
GIGASCOPIC > km FLOW / MIGRATION AND
PATTERNS DEFORMATIONAL
SYSTEMS
Fig. 10-1. Scaling description of geologic entities with respect to sediment properties. (Modified after
Chilingarian et al., 1992, fig. 1-2, p.7.)

discussion about mechanisms taking place. Fig. 10-1 presents a schematic of the scaling
phenomena as applied to a sedimentary environment.
How long does it take for an over-pressured system to dissipate? What are some
of the other phenomena impacting the chemistry of pore waters and do they mask the
whole fundamental process of migration and retention by gravitational compaction?
Giles (1997) is certainly correct in stating that certain conditions will open avenues of
migration for some of the components and not to others, depending on the conditions
within the sedimentary system. Buryakovsky et al. (1994) discussed the relation of the
abnormally high formation pressures in the South Caspian Basin to undercompaction of
the very thick (up to 25 kin) accumulation of Quaternary-Pliocene sediments and the
retardation of the smectite to illite conversion process in shales at depths down to 6 km.
The origin of abnormally high pressure in argillaceous sequences is often attributed to
smectite dehydration as it is altered to illite. Field data from the Baku Archipelago shale
sequence, however, show that smectite remains practically unaltered down to a depth of
6 km. Also in Azerbaijan, the undercompacted character of the Cenozoic shales implies
that their sealing properties are determined mainly by their abnormally high formation