Page 75 - Petroleum Geology

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f2 retained is +0.14, and one metre thickness will be reduced to 0.86 m
when the porosity of 15.9% is reached.
The retention of this relatively large volume of water in the mudstone gives
to the mudstone the properties of bulk density, effective viscosity or com-
petence, and all the related properties that a normally compacted mudstone
would have had at a much shallower depth - the depth at which a normally
compacted mudstone would have a porosity of 27.3%.

INTERSTITIAL WATER PRESSURES

It is a matter of common observation, amply confirmed by water wells,
and boreholes drilled for petroleum, that the porous and permeable sediments
and sedimentary rocks at shallow depths in the continents and the continen-
tal shelves are completely saturated with water (locally, also oil or gas), and
that the pressures in this water are closely approximated by:
P=YwZ (3.11)

where yw is the weight density of the water, and z is the depth of measure-
ment below the surface (consistent units being used throughout)*.
Equation 3.11 implies that if the water is free to rise in the borehole, it
will reach the surface. It is, however, a generalization because the weight
density varies with the salinity of the water, its temperature, and its depth
(pressure); and because the level z = 0 should strictly be the water table, not
the ground surface. The weight density of water, which determines the pres-
sure gradient, varies from the superficial value of 9.8 kPa/m (0.1 kgf cm-2
m-l; 0.433 psilft) when it is fresh near the surface, to about 10.6 kPa/m
(0.11 kgf cm-2 m-'; 0.47 psi/ft) when it has 130,000 ppm total dissolved
solids.
Pressures that conform to weight densities within these limits in eq. 3.11
are known as normal hydrostatic pressures; and the gradients that conform
to this range of weight densities, but not necessarily to eq. 3.11, are known
as hydrostatic gradients (Fig. 3-9).
The existence of normal hydrostatic pressures indicates that the pore
spaces containing the water are connected to the surface, however tortuously.
The grains of sediment may be regarded as contained within a water reservoir
wit.h as much validity as the more usual statement that the rocks contain
water. The sediments of many environments, of course, accumulated in water.
It is also a matter of observation (though not so common) that water pres-
sures are found in the subsurface that do not correspond with normal hydro-

* It is conventional to measure z negative downwards below a datum such as sea level, but
positive downwards from the surface of the ground or rig floor.