Porosity

Porosity is the percentage of the pore volume in a rock to the bulk volume of that rock.

Geological classification of porosity:

• Primary porosity was formed at the time of deposition of the sediments.
• Secondary porosity was developed after deposition and burial of the formation

Sandstone reservoirs practically all have primary porosity whereas carbonate reservoirs usually have secondary porosity.

Engineering classification of porosity:

• Total (absolute) porosity is the ratio of the total pore volume in a rock to the bulk volume of that rock, regardless of whether or not those pore spaces are interconnected.
• Effective porosity is the ratio of the interconnected pore volume to the bulk volume.
  

Porosity of sandstones reservoirs typically ranges from 8% to 38%, with an average of 18%. About 95% of sandstone porosity is effective porosity.
Carbonates reservoirs have porosities that usually range from 3% to 15%, with an average of about 8%. About 90% of carbonate porosity is effective porosity.

External link

POROSITY BASICS
http://www.spec2000.net/12-phibasics.htm