POROSITY AND PERMEABILITY

trextemperMechanics

Feb 22, 2014 (3 years and 5 months ago)

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POROSITY AND PERMEABILITY

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
porosity.






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
i
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.


Permeability
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

been
jointed by weathering or earth movements.


The permeability of the rock depends on its porosity and also the
size
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
impermeable.
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.