Page 73 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 55

gypsum bed, and hydraulic conductivity of the confining layer. The gypsum-dehydration
mechanism in compacting sediments will produce high fluid pressures only if all the
above variables are within certain definable limits.
This is also true for the montmorillonite-dehydration model: Hanshaw and Brede-
hoeft (1968, p. 1117) assumed that if each cubic centimeter of sediment contains 2 g of
montmorillonite, then dehydration of montmorillonite will produce 0.33 g/cm 3 of H20.
Enthalpy data from Sudo et al. (1967) indicate that 178 cal/cm 3 is required to release
the interlayer water. Employing the following assumed values of 10 -6 cal cm -2 s -1 for
a heat flow rate and 1.6 x 10 -9 cm/s as a burial rate, 6.3 x 101~ s would be required to
increase the depth by 1 cm. In that time, 630 cal/cm 2 would be available from the usual
flow of heat found in the Earth. There would be more than enough heat available for
the dehydration reaction to proceed (Hanshaw and Bredehoeft, 1968). Interlayer water
would be released at the rate of 5.1 x 10 -1~ cm/s. In the phase transition of gypsum, the
reaction went from a solid to a solid plus water; however, in the dehydration of mont-
morillonite the dense water expands. Not all of the water is moved from the reaction
site. If one assumes that all interlayer water has a density of 1.4 g/cm 3 (Martin, 1962),
expansion, upon release to water having a normal density of 1 g/cm 3, will result in an
increase in specific volume (reciprocal of density) of 28.5%. Hanshaw and Bredehoeft
(1968, p. 1117) calculated that the total flow (qo) upward or downward will be equal to
0.285 x 0.5 x 5.1 x 10 -1~ or 7.3 x 10 -11 cm/s. In this case, a conductivity of 10 -12
cm/s and about 106 years would be required to approach lithostatic pressure on the
fluid. In an actively subsiding basin such as the Gulf Coast Basin, this mechanism could
provide a significant increase in pore pressure if the amount of montmorillonite is high
and the permeability is low.

MECHANISMS GENERATING ABNORMAL FORMATION PRESSURES
The mechanisms responsible for generating abnormal pressures can be classified into
three categories (see Tables 2-1 and 2-2), as indicated previously.
(1) Changes in rock pore volume: (a) vertical loading (undercompaction); (b) lateral
tectonic loading; (c) secondary cementation.
(2) Changes in the volume of interstitial fluids: (a) temperature change; (b) mineral
transformations; (c) hydrocarbon generation; (d) thermogenic decomposition of
hydrocarbons; (e) migration of fluids (mainly gas).
(3) Changes in fluid pressure (hydraulic head) and movement of fluids: (a) osmosis;
(b) fluid pressure head; (c) oilfield operations; (d) permafrost environment; (e)
differences in specific weights (e.g., between gas and oil).

Undercompaction

Undercompaction of sediments can occur during rapid sedimentation and burial of
sediments containing a large quantity of clay minerals (Rubey and Hubbert, 1959;
Wilson et al., 1977). Thus, the complete expulsion of water from the sediments does not
occur, and sediments are left as a loosely bound system of swollen clay particles with