Page 28 - Origin and Prediction of Abnormal Formation Pressures

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS 11

bioturbation creates a well-oxidized depositional environment. The early burial state is
recognized as a reducing zone where anaerobic bacteria are dominant. Rieke (1972)
presented a discussion of the transformation of clay minerals from field observation and
laboratory experiments.
During sedimentation, montmorillonite clay adsorbs water into its three-dimensional
lattice structure that is later released into the pores of the surrounding porous media
during compaction and burial. The transformation of montmorillonite clay to illite
occurs between 80 ~ and 120~ releasing an amount of water equal to one half of its
volume (Powers, 1967). This infusion of water leads to further undercompaction in the
geopressured zone. When the fluid pressure exceeds the lithostatic pressure, the faults
act as valves for discharge of fluids upward into the hydro-pressured aquifers overlying
the zone. As the formation pressure declines, the valves close until the pressure once
more exceeds the lithostatic pressure (Jones, 1975; Bebout, 1976).
Various investigators have shown that during compaction accompanied by deep
burial, a diagenetic conversion of montmorillonite to illite, and also kaolinite to chlorite,
occurs with increasing depth as the subsurface temperature increases. Progressive
modification of the structure of montmorillonite with its eventual disappearance was
observed with increasing burial depth in the Wilcox Formation of the Gulf Coast (USA).
Burst (1969) proposed that the disappearance of montmorillonite in the sediments
was caused by conversion to illite as Mg 2+ cation was substituted in the silicate
lattice structure for A13+ ion, accompanied by fixation of the interlayer potassium.
Powers (1959) and Weaver (1961) have also reported on the lack of non-interlayered
montmorillonite in deeply buried sediments. The gradual change of montmorillonite
clay to illite and kaolinite to chlorite has been investigated by many authors: Fuchtbauer
and Goldschmidt (1963), Dunoyer de Segonzac (1964), Perry and Hower (1970), Van
Moort (1971) and others.
The fact remains, however, that smectite-to-illite transformation during diagenesis
and catagenesis does not occur in many overpressured environments (see Chapter 4).

Osmosis as a factor for generation of abnormal pressure

Osmotic pressure occurs when two solutions having different ionic concentrations
are separated by a semipermeable membrane that will allow the solvent to pass through
by diffusion from the more dilute side to the more concentrated side of the membrane.
The osmotic flow will continue until the chemical potential of diffusion is equal on both
sides of the membrane; thus, the pressure increase occurs if the solvent moving into the
more concentrated solution enters a closed compartment (Fig. 1-6).
McKelvey and Milne (1962) measured the osmotic pressure of 1 N sodium chloride
solution versus distilled water across plugs (0.26-0.51 cm in thickness) of bentonite.
The pressure was 695 psi (4.8 KPa): 95% of the theoretical value.
Probably the natural clay/shale beds will act only as imperfect semipermeable
membranes because of the presence of fractures and large pores, which may be too large
or too weakly charged to restrict the movement of salt. Thus, the generated osmotic
pressure will be less than the theoretical one on the basis of salinity differences across
them.