Page 66 - Petroleum Geology

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proportional to thickness when compaction is due to gravity without lateral
spreading. The immense volumes of water expelled will be appreciated when
it is considered that 1 km3 of water-saturated mudstone compacted from
30% porosity to 12.5% porosity expels 0.2 km3 of water. This is equal to the
total pore volume of 1 km3 of sandstone with 20% porosity. This is why there
are difficulties with the concept of connate water (pp. 77-78). The chemistry
of mudstone pore water may differ considerably from that of the displaced
sandstone pore water. By the same token, if this water cannot escape from
the mudstone (or, indeed, from an intercalated sandstone) considerable quan-
tities are retained and buried to greater depths and higher temperatures for a
longer time.
Mudstone compaction, like other sediment compaction, involves chemical
and physicochemical processes as well as the physical, but these are not suf-
ficiently well understood yet for general rules to be formulated. In confining
ourselves to the physical processes, however, we believe that these are suffi-
cient to explain the observed and deduced effects and that it is a valuable -
even essential - simplification.
The matter of mudstone compaction as a function of depth, overburden
load and bulk densities, has been the subject of several studies (Hedberg,
1926, 1936; Athy, 1930a; Dickinson, 1951, 1953). Hedberg’s (1936) and
Athy’s curves (Fig. 3-2) differ from each other in some interesting respects,
but we cannot claim to understand these differences fully.
First, it is essential to understand clearly the nature of such curves. They
are, of course, a plot of present porosity (or wet bulk density, as the case

POROSITY f
7
-
7
-
v
0 7-7 -I--
km ’ ,
/
/

W

5

*LI I XlOOOft
Fig. 3-2. Mudstone compaction curves of Athy (1930) and Hedberg (1936), the latter
generalized.