Page 65 - Origin and Prediction of Abnormal Formation Pressures

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

~ MONTMORILLONrrE BEFORE DIAGENE$1S

AFTER DIAGENESIS TO ILLITE

O ~
"r~
0
0
VOLUME LOST
o.
"o
d, l
r AFTER DIAGENESIS 8= COMPACTION
UNIT LAYER OF CLAY
~ WATER
CLAY PARTICLE
(AS NUMBEREO)
Fig. 2-16. Effect of clay diagenesis on compaction of mudrocks, on assuming that the same number
of particles, crystal aggregates, and unit layers of clay occur in each compaction stage shown. (A) No
effective porosity or permeability; practically all water is bound water. (B) Most bound water becomes free
water; consequently, effective porosity and permeability are greatly increased. (C) Free water squeezed out;
effective porosity, permeability, and original volume are greatly reduced. (Modified after Powers, 1967, fig.
1, p. 1242; in Rieke and Chilingarian, 1974, fig. 57, p. 110. Courtesy of Am. Assoc. Pet. Geol.)

properties of montmorillonite (Powers, 1967, p. 1244). Further increase in overburden
stress alone, resulting from deeper burial of the mud is ineffective in squeezing the
remaining water out of the plastic sediment. At burial depths greater than 1500 to 3000
ft, most of the water exists as water of hydration and is stacked at least four monomolec-
ular layers thick between the unit layers of montmorillonite. Only a small amount of
oriented water occurs between the crystals and particles at depths of about 3000 to 6000
ft (Fig. 2-16A). At burial depths below about 6000 ft, montmorillonite is altered to illite
and the bound water is desorbed and becomes free pore water (Fig. 2-16B).
This causes a decrease in clay-particle size with a corresponding increase in the
porosity and permeability at burial depths of 6000 to 9000 ft. Below a depth of