Page 61 - Origin and Prediction of Abnormal Formation Pressures

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

argillaceous sediment since deposition. There is a problem in evaluating the effects of
depositional rates and geologic age in developing a simple sediment compaction model.
Nevertheless, empirical data suggest that the effect of age and depositional rates are
commonly predictable.
Although the effect of temperature on formations is difficult to evaluate, experiments
by Warner (1964, pp. 50-79) suggest that at temperatures less than 200~ temperature
may not have a significant effect (other than in accelerating compaction rates). Most
compaction models utilize clay minerals of an idealized size and shape, which are influ-
enced by mechanical rearrangement during burial. The following theories, presented in
chronological order, are intended to enable the reader to better visualize the interrela-
tionship among pressure, porosity reduction, and interstitial fluid release in argillaceous
sediments. A comparison of the relationship between the porosity and depth of burial is
shown for several regions in Fig. 2-13.

Athy's compaction model

According to Athy (1930a) compaction represents a simple process of squeezing
out the interstitial fluids and thereby reducing the porosity. In relatively pure shales
a definite relationship exists between porosity and depth of burial (Fig. 2-14). After
a sediment has been deposited and buried, the pore volume may be modified by: (1)
deformation and granulation of the mineral grains; (2) cementation; (3) solution; (4)
recrystallization; and (5) squeezing together of the grains. The continued application of
overburden or tectonic stress is the mechanism by which porosity is reduced and bulk
density is increased further. Athy (1930b) pointed out that the amount of compaction
is not directly proportional either to reduction of pore volume or to increase in bulk
density because of the above-mentioned processes.

Hedberg's compaction model

Hedberg (1936) stated that because of the numerous processes involved in com-
paction, it is not possible to express satisfactorily pressure-porosity relationships for
clays and shales throughout the entire depth range by any one simple equation. Hedberg
(1936) determined the porosities of shale core samples taken from Venezuelan wells
from depths of 291 ft to 6175 ft. An analysis of these data, led Hedberg (1936) to
propose a compaction process consisting of three distinct stages.
The first stage consists mainly of the mechanical rearrangement and dewatering of
the clayey mass in the pressure interval from zero to 800 psi. During this period of
dewatering, there is a rapid decrease in porosity for small increments of additional
overburden pressure. Expulsion of free water and mechanical particle rearrangement are
dominant in the porosity range from 90% to 75%. Some adsorbed water is also lost
during this stage.
Between a porosity of 75% and 35%, adsorbed water is expelled from the sediment.
Mechanical deformation of the clay structure occurs below a porosity of 35% where
the clay particles come in closer contact with each other. As a result, there is a greater
resistance to further reduction in porosity. According to Hamilton (1959, p. 1407), the