POROSITY

Ø Ratio of the volume of space to the total volume of a rock.

Ø Porosity of a rock is a measure of its ability to hold a fluid.

Ø Porosity is expressed as a percentage of the total rock which is taken up by pore space.

       Total porosity

Ø  the ratio of the entire pore space in a rock to its bulk volume.

       Effective porosity

Ø percent of bulk volume occupied by interconnected pores spaces.

Ø A total porosity less the fraction of the pore space occupied by shale or clay

Ø In very clean sands, total porosity is equal to effective porosity.

    Primary  porosity

Ø porosity of the rock that formed at the time of its deposition.

Ø  primary porosity of a sediment or rock consists of the spaces between the grains.

Ø Primary porosity decrease due compaction and packing of grains.

Ø Intergranular pores of clastics or carbonates.

Ø Primary porosity is less than one percent in crystalline rocks like granite.

 more than 55% in some soils.

      Secondary porosity

Ø develops after deposition of the rock.

Ø Vugular spaces in carbonate rocks created by the chemical process of leaching.

Ø  fracture spaces formed due to stress distortion in reservoirs rocks.

Photo from [http://cmex.ihmc.us/cmex/data/catalog/ModernGroundwater/PorosityVoids.html]

Sent in by Ekwe Umahi Makolia, TalkOil9ja.