Page 65 - Petroleum Geology

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167). Nevertheless, the permeability of sandstones usually remains high enough
not to impede the expulsion of pore water significantly.
Calcarenites may well share some of the compaction features of sands and
sandstones, but solution processes appear to be more important than mechani-
cal compaction in carbonate rocks. McCrossan (1961) found that the dry
density of some Devonian mudstones in Canada increased with CaCO, con-
tent as well as with depth; and the higher the carbonate content, the smaller was
the relative compaction. If stylolites are but a special case of compaction of
carbonates under load, the loss of both fluid and solids (in solution) may
occur.
Compaction of mudstones (“shale” in petroleum jargon), which may con-
tain 50% silt, is a complex process involving irreversible deformation of the
ductile grains and also chemical diagenesis. When mudstone first accumulates
into the stratigraphic record, it has a porosity of about 50% (it is misleading
to think of larger initial porosities in muds because the commonly quoted fig-
ure of 80% is almost certainly not applicable to mudstone in the stratigraphic
record - and muds grade up to dirty water). The early stages of mudstone
compaction probably include significant grain rearrangement. Hedberg (1936),
in a careful study of Tertiary mudstone in some wells in Venezuela, concluded
that there were four overlapping stages of mudstone compaction: first, me-
chanical rearrangement; then dewatering; then mechanical deformation, and
finally recrystallization. These processes can expel vast quantities of pore
water.
Consider the compaction of a unit cube of water-saturated mudstone from
30% porosity 122% porosity under gravity. Porosity is not as convenient
a parameter as void ratio (E) in these computations: E = f/(l - f), the ratio of
pore volume to solids volume. The initial cube with 30% porosity has a void
ratio of 0.43 (Fig. 3-1). When compacted to 12f% porosity, the void ratio be-
comes 0.143. If we make the assumption that the solids volume remains con-
stant at 0.7 units (an assumption that may prove untenable when the diage-
“i
netic history of mudstone is better understood) the volume of liquid retained
is 0.7 X 0.143 = 0.1 units. The bulk volume of the initial cube is therefore
reduced to 0.8 units by the expulsion of 0.2 units of water. These figures are

(solids)

Fig. 3-1. Compaction of unit volume of mudstone from 30% to 12.5% porosity results in
the explusion of 0.2 units of pore water.