Page 62 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 45

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1 J

jS

5o00 , /~

,-l-,-
Wl--,
q '!' / 1
.C: l o,ooo ' r
C}. I
(I)
C~ ,o i

15,000 .........

i ,
20,000 ,, i
0 20 40 60
Porosity (~). %

Fig. 2-13. Various compaction models showing the relationship between porosity and depth of burial for
shales and argillaceous sediments. 1 --- Proshlyakov (1960); 2 = Meade (1966); 3 = Athy (1930a); 4 =
Hosoi (1963a,b); 5 = Hedberg (1936); 6 = Dickinson (1953); 7 = Magara (1968); 8 = Weller (1959);
9 = Ham (1966); 10 = Foster and Whalen (1966). (Modified after Rieke and Chilingarian, 1974, fig. 17,
p.42.)

transition from clay to shale likely occurs at about 35% porosity, because the chemical
changes and cementation between the grains impart rigidity to the skeletal structure.
There is also some recrystallization of the clay particles during this stage (Hedberg,
1936).
Recrystallization stage is the third and final stage with porosities less than 10%. The
main compaction mechanism during this stage is recrystallization under high pressures.
Reduction of the pore volume occurs slowly and only with large pressure increments.
The larger crystals may grow at the expense of the smaller ones, and a gradual transition
may occur from a shale to a slate and then to phyllite.