Page 62 - Petroleum Geology

CHAPTER 3

COMPACTION OF SEDIMENT AND SEDIMENTARY ROCKS,
AND ITS CONSEQUENCES

SUMMARY

(1) Compaction is a diagenetic process that begins on burial and may con-
tinue during burial to depths of 9 km (30,000 ft) or more. Compaction in-
creases the bulk density of a rock, increases its competence, and reduces poro-
sity.
(2) Sands compact with relatively little loss of porosity or permeability,
but other diagenetic processes may considerably reduce porosity and perme-
ability with authigenic minerals in the pore spaces.
Mudstones compact with serious and permanent loss of porosity and perme-
ability.
Carbonates compact to varying degrees depending on the proportion of
plastic material. Most appear to compact by solution processes rather than
by mechanical compaction.
(3) Compaction can proceed only with the compression and expulsion of
a commensurate proportion of the pore fluids, which move to positions of
smaller energy. If mudstones alternate with more permeable beds, the more
compactible mudstones expel fluids both upwards and downwards to the
more permeable beds. The downward energy gradient in the lower part of
the mudstone makes the mudstone a perfect barrier to the upward migration
of fluids. Lateral migration takes place in the relatively permeable, intercalated
beds.
(4) Pore-fluid pressures in mudstones cannot be measured, but they are in-
ferred to be greater than normal hydrostatic in a compacting mudstone be-
cause this is a necessary condition for flow. In a thick mudstone loaded faster
than the corresponding rate at which the fluids can be expelled, compaction
is retarded and the mudstone retains the mechanical properties it had at shal-
lower depth; and the pore pressures are correspondingly elevated above the
normal hydrostatic. The pore fluids are bearing part of the overburden load.

COMPACTION OF SEDIMENT AND SEDIMENTARY ROCKS
Compaction under the gravity load of overlying sediments is a fundamental
geological process that is necessarily an important topic of petroleum geology.
It affects stratigraphic relationships and thicknesses, pore-water movement