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 is formed by later processes e.g. by recrystallisation or dissolving of
minerals during diagenesis. (the process of
)
.
Porosity usually
with depth of sediment as 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 fluids 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. Clay 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 reservoir rocks for oil or water.