porosity and permeability

• The pore space in rock can be filled with fluids. 
• Rock type effects porosity - igneous tend to be low porosity. 
• Sorting degree effects porosity, well sorted are high porosity and poor sorting causes a low porosity due to finer grains filling in the gaps. 
• Level of diagenesis can affect porosity, loose and unconsolidated leads to high porosity and compacted sediments have a low porosity. 
• Porosity reduces during burial because grains are compacted together and water is squeezed out, cement also precipitates between the grains to fill the pores. 
• Rounded grains create a high porosity, angular grains create a low porosity because they fit together better. 
• Grain size does not affect porosity. 
• Rocks have a secondary porosity due to joints, fractures and faults - particularly in limestone. 
• Permeability is the capacity of a rock to allow fluids to pass through. 
• Permeability is controlled by:

  • Connectivity of pores
  • Grain size 
  • Coarse - high permeability due to less resistance from friction. 
  • Number and size of joints and fractures. 
• Effective porosity is the volume of connected pores in a unit volume of rock. Clay has a 50% porosity but a 0% effective porosity.