Page 66 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 49

9000 to 10,000 ft, the water released from the clay is compacted until a new balance
is established corresponding to the water-retaining properties of the illitic alteration
product (Fig. 2-16C).
The relationships among water expulsion, type of clay mineral, and depth of burial
are illustrated in Fig. 2-17, for both expanding and non-expanding clay deposits
(Powers, 1967, p. 1245). The water-escape curves are diagnostic of the porosity,
permeability, and bulk density of compacting argillaceous sediments. Powers stated that
the compaction history of mudrocks depends largely on their original clay composition
and the diagenesis and catagenesis, which they undergo after burial.

Teodorovich and Chernov's compaction model

Teodorovich and Chernov (1968) suggested the following stages in the compaction
of productive Apsheron horizons in Azerbaijan.
(1) The first stage occurs at burial depths of 0 to 8-10 m where there is a rapid
compaction. Porosity in clays decreases from 66% to 40%, whereas that of sandstones-
siltstones decreases from 56% to 40%. Large amounts of water are squeezed out during
this stage (sedimentogenesis and early diagenesis).
(2) During the second stage there was a rapid decrease in the compaction rate in the
intervals from 8-10 m to 1200-1400 m. During this stage, porosities of the shales and
sandstones-siltstones decrease to about 20%.
(3) The third stage (burial to a depth of 1400-6000 m) is characterized by slow
compaction. The absolute porosity of sandstones-siltstones at a depth of 6000 m
decreases to approximately 15-16%, whereas that of shales to 7-8%.

Burst's compaction model

Burst (1969) proposed a compaction model based on a three-stage dehydration
sequence and the transformation of montmorillonite clay to mixed-layer varieties. A
description of this model appears in Chapter 4.

Beall's compaction model

A. Beall (personal communication, 1970), proposed a simple model for consolidation
of clastic muds, based on the data from offshore well core samples, Louisiana, the
JOIDES Deep Sea Drilling Project, and from high-pressure experiments on marine
muds. The initial stage of compaction (down to a depth of approximately 3300 ft)
primarily involves expulsion of fluids by mechanical processes as in the other proposed
theories. Approximately 50% of total consolidation is reached at a very shallow
depth. The average calculated pore throat diameters during the first stage are around
6~.
During the second stage (at depths of 3300 to approx. 8000 ft) about 75% of total
compaction is complete, and pore throat widths in the clays approach 1 *. The fluid
pressures remain hydrostatic. During the third-stage of compaction there is an extremely
slow decrease of porosity with depth, and pore throat diameters are generally less than