Page 327 - Origin and Prediction of Abnormal Formation Pressures

296 V.A. SEREBRYAKOV, G.V. CHILINGAR AND J.O. ROBERTSON JR.

in the Siberian Platform and their origin due to temperature decrease in geologic
section were analyzed quantitatively. Using the regional situation as an example of the
origin of pressure subnormality, Dobrynin and Serebryakov introduced the concept of
thermodynamic gradient. They took the bottom surface of the permafrost (instead of
the watertable surface) as the reference surface to determine the hydrostatic pressure in
formations below the permafrost.
Dobrynin and Kuznetsov (1993) concentrated on the thermodynamic gradient and its
role in originating the pressure subnormality. They analyzed, from the viewpoint of this
concept, different aspects of pressure subnormality origin, migration and accumulation
of hydrocarbons, and sealing properties of formations with higher pressures than
pressures of the lower formations. These authors also took into account the possible
viscoplastic property of water in narrow pore channels and its influence on the water
flow through formations.

ORIGIN OF ABNORMAL PRESSURES
The origin and characteristics of abnormally low pressure may be related to regional
phenomena, or it can be local. Different origin was ascribed by different scientists. For
example, Berry (1959), Hill et al. (1961) and Breeze (1970) attributed the abnormally
low pressure of the Alberta (Canada), San Juan (New Mexico and Colorado) basins
and the Morrow sands of northern Oklahoma to osmotic-pressure differences. Russell
(1972) in the Appalachian region, and A.G. Durmishyan (pers. commun., 1976) in the
northern Caucasus described the low-pressured reservoirs in well-consolidated rocks,
which have been uplifted and eroded in the geologic past. Barker (1972) ascribed
the subnormal pressure to the removal of overburden, which would cause a drop in
pressure of the pore fluids. Erosional unloading has been suggested to explain certain
abnormally low pressures by Louden (1972) and by Dickey and Cox (1977), although
a quantitative analysis of the process has not yet been presented. Later, Neuzil and
Pollock (1983), using the mass-balance equation (Domenico and Palciauskas, 1979) for
water and grains in a small control volume of saturated porous medium, described the
unloading of saturated elastic rocks caused by decreasing thickness. Thermal effects,
however, were not included. In the opinion of the writers, the thermal effects play the
main role in pore pressure changes. Basically, the influence of temperature on pore
pressure is strongest in regions with more compacted rocks. In undercompacted, plastic
rocks with a high coefficient of compressibility, thermal expansion of fluids can be
compensated by deformation of rock pore spaces. Neuzil and Pollock (1983) noted
the same effect for the overburden removal. They stated that only in rocks of low
permeability is pore pressure likely to be affected by erosion, and if the permeable
unit is effectively isolated by surrounding 'tight' rocks, erosional unloading could cause
pressure lowering. Changes in temperature of rocks could occur in two ways. The first
(regional one), caused by the changes in the temperature at the surface of the Earth in
geologic time, affects usual and unusual fluid filtrations. The second (local one), can be
caused by changes in the temperature of rocks during the significant uplift and erosion,
or subsidence and aggradation. The first phenomenon is a fundamental process in the