22 Φεβ 2014 (πριν από 7 χρόνια και 6 μήνες)

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Pore spaces are important because they can hold oil and gas, water for water supplies
and deposits of hydrothermal minerals.

The amount of pore space in a rock is called its

Volume of pore space

Porosity =

x 100

Total volume of rock

Primary porosity
refers to gaps between particles and forms during sedimentation. The
amount of primary porosity depends on the textural features of the sediment i.e.

Secondary porosity
s formed by later processes e.g. by recrystallisation or dissolving of
minerals during diagenesis. (the process of )

Porosity usually with depth of sediment a
s pressure of overlying rocks
compacts the sediment. Clay in particular loses porosity through this process, whilst
sandstone often lose porosity as calcite and silica minerals grow in the pore spaces.

is the measure of the ease with which f
luids can pass through rocks.

Primary permeability
is the rate of fluid flow though pore spaces.

Secondary permeability
is the rate of

flow through secondary pores, cavities and
fractures. Rocks like limestone and granite may be very permeable if they have

jointed by weathering or earth movements.

The permeability of the rock depends on its porosity and also the
of the pore spaces.
If the pores are very small as in clay, the water cannot flow through, although clay may
hold up to 80% water. Cl
ay is therefore
In general non
porous and very
fine grained rocks are impermeable, medium grain
sized poorly sorted sedimentary rocks
are moderately permeable and coarse
grained well
sorted rocks are highly permeable.
These make the best res
ervoir rocks for oil or water.