Geoscience Reference
In-Depth Information
Fig. 1.11
Relation
between
porosity
and
permeability
for
various
grain
size
categories
(Chilingar
1963
)
including glacial till, glaciofluvial, and glaciolacustrine sediment forms, are the
principal components of aquifers. The type and origin of sediments affect their
permeability, which in turn controls the water transmission potential of aquifers.
Sedimentary rocks, consisting mostly of sandstones and carbonates, are bodies
of major hydrological significance. Sandstone makes up about 25 % of the sedi-
mentary rocks, originating from environmental depositions (e.g., floodplain, del-
taic, and marine shoreline), which are characterized by cementing materials like
quartz, calcite, and clays in various stages of alteration (Freeze and Cherry
1979
).
The porosity of sandstone in extreme cases can be as low as 1 %, as compared to
sands that reach only as low as 30 %. The relationship between porosity and
permeability for various grain sizes is presented in Fig.
1.11
. It was observed that
an increase in porosity of a few percent generally corresponds to a large increase in
permeability.
Carbonate rocks consist mostly of calcite and dolomite with minor amounts of
clay. The porosity of carbonate rocks ranges from 20 to 50 %, but in contrast to
sandstone, it tends to decrease with depth. Often, carbonate rocks are fractured,
providing a permeability that is much greater than the primary one. In some cases,
initial small-scale fractures in calcite and dolomite are enlarged by dissolution
during groundwater flow, leading to an increase in rock permeability with time.
