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Proposity(%) is equal to pole volume devided by total volume into 100 percent.
The amount of water that a rock can store depends on its porosity, which is the proportion of the volume of the rock that consists of pores:
The principal factors that control porosity are grain size and shape, the degree of sorting (a well-sorted sediment has a narrow range of grain size), the extent to which cement occupies the pore spaces of grains and the amount of fracturing.
The diagram illustrates how porosity varies with the degree of sorting and with the grain shape in unconsolidated sediments (sediments that have not been compacted or cemented).
Unconsolidated sediments with rounded grains of uniform size (i.e. perfectly sorted) are the most porous (Section a). Sediments decrease in porosity as the angularity of the grains increases because the grains can pack more closely together, the bumps of some grains fitting into indentations in others (Section c).
The porosity is also lower if the sediment is poorly sorted, because small grains can occupy the spaces between larger grains (Section b).
Porosity in unconsolidated sediments: Section a is well sorted, having high porosity; Section b is poorly sorted having low porosity; Section c has angular grains and low porosity.
In consolidated rocks: Section d has porosity diminished by cementation; Section e has solution porosity as it has partially dissolved; Section f has fracture porosity.
Sections a-d show vertical sections 1 cm across, Section e and Section f are 1 m across.
It is important to distinguish clearly between porosity and permeability.
Porosity is a measure of how much water can be stored in a rock, whereas permeability is a measure of the properties of a rock which determine how easily water and other fluids can flow through it.
Permeability depends on the extent to which pores are interconnected.
For unconsolidated sediments, the finer-grained silts and clays are less permeable than the coarser sands and gravels, even though they are more porous. There are two major reasons for this:
First, the smaller grain sizes in silt and clay result in a greater surface area of particles relative to volume, so water tends to be held in the pores by surface tension.
Second, the platy and angular shape of clay particles means that they tend to interlock and isolate the spaces between them, which further inhibits the movement of water through the sediment.
Usually, consolidated and cemented sedimentary rocks (and igneous and metamorphic rocks) are not very permeable, but sometimes processes such as solution or fracturing create secondary permeability.
The higher hydraulic conductivities of some of the igneous, metamorphic and consolidated sedimentary rocks are mainly due to secondary permeability caused by fracturing or for limestone, solution.
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