74
3 PARTICLES, PORES, AND PERMEABILITY
to packing differences. This trend is probably due to a number of factors that are only
indirectly linked with grain size. Finer sands tend to be more angular and to be able
to support looser packing fabrics, hence they may have a higher porosity than coarser
sands.
Whatever the cause of the relationship, it has been shown empirically that porosity
generally increases with decreasing grain size for unconsolidated sands of uniform grain
size. This relationship is not always true for lithified sandstones, whose porosity often
increases with grain size. This may be because the finer sands have suffered more from
compaction and cementation. Permeability, by contrast, increases with increasing grain
size (Fraser, 1935; Krumbein and Monk, 1942; Pryor, 1973). This is because in finer sediments the throat passages between pores are smaller and the higher capillary attraction
of the walls inhibits fluid flow. This relationship is found in both unconsolidated and
lithified sand.
3.2.3.1.2 Effect of sorting on porosity and permeability
A number of studies of sediments have shown that porosity increases with increased
sorting (Fraser, 1935; Rogers and Head, 1961; Pryor, 1973; Beard and Weyl, 1973).
Krumbein and Monk (1942) and Beard and Weyl (1973) demonstrated that increased
sorting correlates with increased permeabilities. A reason for these relationships is not
hard to find. A well-sorted sand has a high proportion of detrital grains to matrix. A
poorly sorted sand, on the other hand, has a low proportion of detrital grains to matrix.
The finer grains of the matrix block both the pores and throat passages within the
framework, thus inhibiting porosity and permeability, respectively (Fig. 3.23).
Pryor's study (1973) of modern sands from different environments confirmed this relationship for river sands, but showed that beach and dune sands were anomalous in
that their permeability increased with decreased sorting. Figure 3.24 summarizes the
relationships between porosity, permeability, grain size, and sorting for unconsolidated
sands.
3.2.3.1.3 Relationship between grain shape and roundness
on porosity and permeability
Undoubtedly the shape and roundness of grains affect intergranular porosity. Little
work has been done on this problem, probably because of the time needed to measure
these parameters in sufficiently large enough samples to be meaningful.
Fig. 3.23. A well-sorted sediment (right) will have better porosity and permeability than a poorly sorted one
(left). In the latter the space between the framework grains is infilled, thus diminishing porosity. The heterogeneous fabric diminishes permeability by increasing the tortuosity of the pore system.
3 PARTICLES, PORES, AND PERMEABILITY
to packing differences. This trend is probably due to a number of factors that are only
indirectly linked with grain size. Finer sands tend to be more angular and to be able
to support looser packing fabrics, hence they may have a higher porosity than coarser
sands.
Whatever the cause of the relationship, it has been shown empirically that porosity
generally increases with decreasing grain size for unconsolidated sands of uniform grain
size. This relationship is not always true for lithified sandstones, whose porosity often
increases with grain size. This may be because the finer sands have suffered more from
compaction and cementation. Permeability, by contrast, increases with increasing grain
size (Fraser, 1935; Krumbein and Monk, 1942; Pryor, 1973). This is because in finer sediments the throat passages between pores are smaller and the higher capillary attraction
of the walls inhibits fluid flow. This relationship is found in both unconsolidated and
lithified sand.
3.2.3.1.2 Effect of sorting on porosity and permeability
A number of studies of sediments have shown that porosity increases with increased
sorting (Fraser, 1935; Rogers and Head, 1961; Pryor, 1973; Beard and Weyl, 1973).
Krumbein and Monk (1942) and Beard and Weyl (1973) demonstrated that increased
sorting correlates with increased permeabilities. A reason for these relationships is not
hard to find. A well-sorted sand has a high proportion of detrital grains to matrix. A
poorly sorted sand, on the other hand, has a low proportion of detrital grains to matrix.
The finer grains of the matrix block both the pores and throat passages within the
framework, thus inhibiting porosity and permeability, respectively (Fig. 3.23).
Pryor's study (1973) of modern sands from different environments confirmed this relationship for river sands, but showed that beach and dune sands were anomalous in
that their permeability increased with decreased sorting. Figure 3.24 summarizes the
relationships between porosity, permeability, grain size, and sorting for unconsolidated
sands.
3.2.3.1.3 Relationship between grain shape and roundness
on porosity and permeability
Undoubtedly the shape and roundness of grains affect intergranular porosity. Little
work has been done on this problem, probably because of the time needed to measure
these parameters in sufficiently large enough samples to be meaningful.
Fig. 3.23. A well-sorted sediment (right) will have better porosity and permeability than a poorly sorted one
(left). In the latter the space between the framework grains is infilled, thus diminishing porosity. The heterogeneous fabric diminishes permeability by increasing the tortuosity of the pore system.