Page 63 - Petroleum Geology

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and salinities, migration of petroleum and probably also of base metals. All
sediments and sedimentary rocks compact to some extent, and the processes
lead to changes in some of their properties. Most obviously, they become
more dense, less porous and less permeable; and the rock unit becomes thinner.
The mechanical strength of a rock increases with compaction, and it becomes
less drillable. Compaction normally increases with depth due to the increase
in load, the increase in the duration of loading, and the increase in the tem-
perature of the material loaded.
Compaction is a diagenetic process that begins with, or very soon after,
sediment accumulation. In early stages, the material has virtually no cohesive
strength, and the grains can be rearranged mechanically by shocks due to
earthquakes and environmental shocks due to heavy surf and thunderstorms.
This rearrangement changes the grain packing from unstable towards stable
packing - in a well-sorted sand, for example, from about 45% porosity to-
wards about 30% porosity. Subsequent burial and diagenesis of a chemical
nature may reduce the porosity further and give to the material a consider-
able cohesive strength. These trends are observed qualitatively when drilling
boreholes. At shallow depth, the hole is drilled very quickly, but is suffers
wash-outs and has a very irregular geometry usually much larger than the dia-
meter of the bit that drilled the hole. At greater depths, the amount varying
from province to province, the sediments acquire a cohesive strength: the
penetration rate decreases and the hole takes on a regular shape not much
larger than the bit that drilled it. Cores can be recovered intact.
Porosity and bulk density of a sedimentary rock are related:

pbw = fpw ' -f)Ps = ps -f(ps -pw) (3.la)
where p bw is the bulk wet mass density of the rock that contains pore fluids
of mass density pw and solids of mass density ps, with fractional porosity f.
There are practical advantages in expressing this in terms of weight densities
because the mean weight density multiplied by depth gives the pressure in
that material at that depth:
Ybw = 7s - f(r, - Yw). (3.lb)
Equations 3.1 show that as the porosity changes from 1 (no solids) to 0 (no
fluids) the bulk wet densities change linearly from that of the fluid to that of
the solids (Table 3-1). The practical advantage of thinking of these parameters
in this way is that as the grain size decreases, porosity becomes increasingly
difficult to measure, but bulk weight density easier.
Porosity of a sedimentary rock cannot be reduced without commensurate
compression or expulsion of pore water. Pore water can only be expelled if it
is free to move away. The rate of compaction is therefore related not only to
the rate of subsidence and loading, but also to the permeabilities of the com-
pacting rocks and the permezbilities of all the rocks affected by the conse-
quent water movement. Lithologies therefore also play a part.