Page 69 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 51

1 ,~. According to Beall, NaC1 filtration could probably take place during the third stage,
resulting in the expulsion of progressively less saline fluids to associated permeable
sands, if the latter are present. In Beall's model, overburden pressure between 8000
and 12,500 psi would be required to initiate NaC1 filtration in marine muds. In the
absence of permeable sands, the excess fluid pressure may be generated during the third
stage.

Overton and Zanier's compaction model
Overton and Zanier (1970) proposed a similar model to that of A. Beall with four
zones having different water types:
(1) depths less than 3000 ft m fresh water;
(2) depths of 3000-10,000 ft m depending on the temperature, exponentially increasing
salinities;
(3) depths greater than 10,000 ft- decreasing salinities to the depth of greatest pressure
gradients;
(4) depths greater than 15,000 ft ~ increasing salinities with decreasing water fractions;
physicochemical changes in shales occur in this indistinct zone.
Overton and Zanier (1970) noted that for the Gulf Coast (USA), sands and shales
are difficult to distinguish on SP (self-potential electric log) curves at depths less than
3000 ft, due to similarity of the waters in them. Water expelled from this interval is
loose (free) water, which constitutes 30% to 70% of the rocks. At a critical compaction
depth (depth around and usually less than 3200 ft), shales and sands become readily
distinguishable on the SP curve. In zone 2, fresher water is held in the more-ordered
or crystalline layer next to the clay, whereas saline water is forced into an equilibrium
position in an outer layer (large pores in the shale and in the nearest sand). As the
crystallinity of water increases, ions are expelled into a less-ordered or more fluid
layer. Below a depth of 10,000 ft, shales remineralize (Overton and Zanier, 1970)
and associated sandstones contain fresher waters. The beginning of zone 3 is readily
apparent on the SP curve. The water freshening is probably due to the expulsion of (1)
the last layers of dense, fresh water from shales into sands, and/or (2) water of hydration
resulting from montmorillonite-to-illite alteration.
For further details on compaction of argillaceous sediments, see Rieke and Chilingar-
ian (!974).

CREATION AND MAINTENANCE OF ABNORMAL PRESSURES
Hanshaw and Bredehoeft (1968, p. 1117) suggested a hydrologic model in which
there is a constant flux of water flowing from the compacting sediments. They examined
the rate of fluid production as a result of mineral dehydration and conversion from a
quantitative viewpoint. The creation of excess pore pressure in the formation and its
maintenance with time is a boundary value problem expressed as:
OZh ' Ss Oh'
= (2-45)
Oz 2 K Ot