The porosity of a rock is the fraction of the volume of space between the solid particles of the rock to the total rock volume. Space includes all pores, cracks, vugs, inter and intra-crystalline spaces. The porosity is conventionally given the symbol f and is expressed either as a fraction varying between 0 and 1, or a percentage varying between 0% and 100%. Sometimes porosity is expressed in ‘porosity units’, which are the same as a percent (i.e., 100 porosity units (pu) = 100%).
However, the fractional form is ALWAYS used in calculations.
Porosity is calculated using the relationship
It should be noted that the porosity does not give any information concerning pore sizes, their distribution, and their degree of connectivity. Thus, rocks of the same porosity can have widely different physical properties. An example of this might be a carbonate rock and a sandstone. Each could have a porosity of 0.2, but carbonate pores are often very unconnected resulting in its permeability being much lower than that of the sandstone. A range of differently defined porosities are recognized and used within the hydrocarbon industry.
For rocks these are:
- Total porosity Defined above.
- Connected porosity The ratio of the connected pore volume to the total volume.
- Effective porosity The same as the connected porosity.
- Primary porosity The porosity of the rock resulting from its original deposition.
- Secondary porosity The porosity resulting from diagenesis.
- Microporosity The porosity resident in small pores (< 2 mm) commonly associated with detrital and authigenic clays
- Intergranular porosity The porosity due to pore volume between the rock grains.
- Intragranular porosity The porosity due to voids within the rock grains.
- Dissolution porosity The porosity resulting from dissolution of rock grains.
- Fracture porosity The porosity resulting from fractures in the rock at all scales.
- Intercrystal porosity Microporosity existing along intercrystalline boundaries usually in carbonate rocks.
- Moldic porosity A type of dissolution porosity in carbonate rocks resulting in molds of original grains or fossil remains.
- Fenestral porosity A holey (‘bird’s-eye’) porosity in carbonate rocks usually associated with algal mats.
- (xiv) Vug porosity Porosity associated with vugs, commonly in carbonate rocks.