8.5 SANDSTONES
357
ganic geochemists. The diagenesis of sandstones has been described in great detail by
Larsen and Chilingar (1962), Folk (1968), Pettijohn et aL (1972), Marshall (1987), Meshri
and Ortoleva (1990), Morse (1994), Giles (1997), and Montanez et al. (1997).
The following account principally concerns those aspects of sandstone diagenesis
that affect porosity and permeability. Before proceeding to the details of sandstone cementation, however, it is necessary to consider the chemistry of the fluids that move
through the pores, and their effect on cementation and solution. The following account
begins by considering sandstone diagenesis from the broad aspect of porosity gradients.
It then proceeds to consider the details of how porosity is diminished by cementation
and enhanced by solution.
8.5.3.2 Porosity Gradients in Sandstones
As a general statement it is true to say that the porosity of sandstones decreases with
depth. The porosity of a sandstone at a given depth may be expressed thus (Selley,
1978):
4) d = ch p - G'D,
where ~b a is the porosity at depth D, qb p is the original porosity at the surface, G is the
porosity gradient, and D is the depth below surface. The original porosity at the surface
will depend on depositional process and provenance (as discussed in Section 3.2.3). The
porosity gradient will be a function of sandstone composition, pore fluid composition
and history, temperature, and time (Selley, 1978). There are now a number of studies of
porosity gradients in sandstone basins, notably by Fuchtbauer (1967), Wolf and Chilingarian (1976), Nagtegaal (1978), Magara (1980), and Schmoker and Gautier (1988), who
have produced an improved formula for calculating porosity as an exponential function
of depth:
ch = a . e bz,
where ~b is the porosity, z is the depth, and a and b are constants representing particular sediment properties. Figure 8.17 shows some examples of sandstone porosity gradients that have been recorded. It is interesting to note that porosity loss is largely linear
with depth. The rapid loss of porosity with shallow burial shown by clays is largely absent. This is because compaction is of less importance in clean sands (Lundegard, 1992).
Intuitively one would expect wackes to lose porosity faster than arenites. This is because they are not as well sorted as arenites and therefore have lower primary porosities. Furthermore, because of their clay content, the wackes may undergo more compaction than clean arenites. Of the wacke sands the chemically stable quartz-wackes
may lose porosity at a lower rate than the chemically unstable greywackes; similarly,
one might expect arkoses to lose porosities faster than quartz-arenites. As Fig. 8.18 illustrates, there is some truth to these generalities, but the story is often more complicated than this, as will be revealed shortly.
Geothermal gradient is another factor that may be expected to affect sandstone
porosity loss, because cementation will proceed faster in hot basins than in cool ones.
357
ganic geochemists. The diagenesis of sandstones has been described in great detail by
Larsen and Chilingar (1962), Folk (1968), Pettijohn et aL (1972), Marshall (1987), Meshri
and Ortoleva (1990), Morse (1994), Giles (1997), and Montanez et al. (1997).
The following account principally concerns those aspects of sandstone diagenesis
that affect porosity and permeability. Before proceeding to the details of sandstone cementation, however, it is necessary to consider the chemistry of the fluids that move
through the pores, and their effect on cementation and solution. The following account
begins by considering sandstone diagenesis from the broad aspect of porosity gradients.
It then proceeds to consider the details of how porosity is diminished by cementation
and enhanced by solution.
8.5.3.2 Porosity Gradients in Sandstones
As a general statement it is true to say that the porosity of sandstones decreases with
depth. The porosity of a sandstone at a given depth may be expressed thus (Selley,
1978):
4) d = ch p - G'D,
where ~b a is the porosity at depth D, qb p is the original porosity at the surface, G is the
porosity gradient, and D is the depth below surface. The original porosity at the surface
will depend on depositional process and provenance (as discussed in Section 3.2.3). The
porosity gradient will be a function of sandstone composition, pore fluid composition
and history, temperature, and time (Selley, 1978). There are now a number of studies of
porosity gradients in sandstone basins, notably by Fuchtbauer (1967), Wolf and Chilingarian (1976), Nagtegaal (1978), Magara (1980), and Schmoker and Gautier (1988), who
have produced an improved formula for calculating porosity as an exponential function
of depth:
ch = a . e bz,
where ~b is the porosity, z is the depth, and a and b are constants representing particular sediment properties. Figure 8.17 shows some examples of sandstone porosity gradients that have been recorded. It is interesting to note that porosity loss is largely linear
with depth. The rapid loss of porosity with shallow burial shown by clays is largely absent. This is because compaction is of less importance in clean sands (Lundegard, 1992).
Intuitively one would expect wackes to lose porosity faster than arenites. This is because they are not as well sorted as arenites and therefore have lower primary porosities. Furthermore, because of their clay content, the wackes may undergo more compaction than clean arenites. Of the wacke sands the chemically stable quartz-wackes
may lose porosity at a lower rate than the chemically unstable greywackes; similarly,
one might expect arkoses to lose porosities faster than quartz-arenites. As Fig. 8.18 illustrates, there is some truth to these generalities, but the story is often more complicated than this, as will be revealed shortly.
Geothermal gradient is another factor that may be expected to affect sandstone
porosity loss, because cementation will proceed faster in hot basins than in cool ones.