Page 64 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 47

1.8 5.4 12.8 20.0 27.2 34.6 41.8 49.1
01 . . . .

"-~~2.0
9
"8
~ 3.0

4.0 / -.

!

5.0
98 196 392 588 784 980 1176
Pressure, N/m 2
Fig. 2-15. Interrelationship among porosity, depth of burial and overburden pressure. N = unit of force
(Newton) = 102 g-force = 105 dyn. (Modified after Weller and Vassoevich, in Kartsev et al., 1969; in Rieke
and Chilingarian, 1974, fig. 18, p. 43.)

overpressured zones. In addition, application of laboratory soil-compression tests to
buried sediments presents some problems.
Weller (1959) proposed a compaction process starting with a mud at the surface
having a porosity between 85% and 45%. As the overburden pressure increases owing
to sedimentation, the interstitial fluids are expelled from the pore space (porosity ranges
from 45% to 10%). As a result, there is rearrangement of the mineral grains and
development of closer packing. Compaction in this stage is related to yielding of the
clay minerals between the more resistant grains. Weller theorized that at about 10%
porosity, the non-clay mineral grains are in contact with each other, and the clays are
being squeezed into the void space. Further compaction (porosity < 10%) requires
deformation and crushing of the grains.

Powers' compaction model

Powers (1967) presented a shale fluid-release theory based on changes in clay
minerals and bulk properties with depth in argillaceous sediments. His theory assumes
that mineralogical transformation of montmorillonite to illite occurs during deep burial,
with the consequent release of large volumes of bound water from montmorillonite
surfaces to interparticle areas where it becomes interstitial water.
In the case of marine montmorillonitic sediments buried to a few hundred feet, a
balance is reached between the water retained in the sediment and the water-retaining