Page 299 - Origin and Prediction of Abnormal Formation Pressures

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

Early Stage
of Late stage of
compaction compaction
IL ~JL~
! TM *- !1"

.e

r~o
e
a=
",... Pore pressure
i===,--..=m ~==, .==,
Time
e

Compaction

Not to scale

Fig. 10-27. Schematic showing the interrelationships among the degree of compaction and variations in the
total effective and pore-water pressures with respect to time. (Modified after Chilingarian et al., 1994, fig.
5-22, p. 141.)

temperature (geothermal gradient) as proposed by Burst (1969) and Perry and Hower
(1970) and is independent of the burial rate. According to Brown (1998), however,
the transformation of smectite to illite appears to be very dependent on the burial rate
inasmuch as it takes a considerable amount of time to run to completion.
In conclusion, the experimental results point to the possible creation of a fresh water
zone in the AHPF zones and support the previously discussed laboratory results on
chemistry of pore water squeezed out at various pressures and temperatures.

Experiments involving mixtures of oil and seawater

Discussion on the expulsion of pore fluids from compacting argillaceous sediments
would not be complete, if a brief mention of the role of crude oil is left out. Aoyagi
et al. (1985) performed laboratory experiments to delineate the mechanism of primary
migration of crude oil from the source to reservoir rocks. A sample of Na-smectite clay
(19% by volume) was mixed with crude oil (3%) and seawater (78%). This mixture
was compacted for 25 days under a constant pressure of 14,502 psi (100 MPa) and a
temperature of 140~ (60~ Fig. 10-31 shows that the proportion of oil in the expelled
liquid gradually increases with time, possibly due to decreasing water saturation. The
chemical composition of expelled pore water is presented in Fig. 10-32. The amounts of