Classification of Porosity
Pores are classified based on their morphological viewpoint as:
Catenary or inter connected pore: This type of pore has more than one throat connected with other pores and extraction of hydrocarbon is relatively easy from such pore, as shown
Cul-de-sac or connected or dead end: This type of pore has one throat connected with bother pores. It may yield some of the hydrocarbon by expansion as reservoir pressure drops as shown
Closed or isolated pore: This type of pore is closed. It does not have throat and cannot connect with other pore. It is unable to yield hydrocarbons in normal process as shown
Classification of Porosity based on their time of deposition
Reservoir Pores are found as two distinct general types in sedimentary rocks based on their time of formation. These are: (1) Primary, or Intergranular or Depositional Porosity and (2) Secondary, or Intermediate or Post-depositional Porosity.
Each type of the pore has subdivisions, which can be summarized in Table 1 below:
Primary Porosity
Primary porosity is divisible into two types: intergranular or interparticle porosity, which occurs between the grains of a sediment and intragranular or intraparticle porosity
Intergranular porosity is more typical of sandstones. It is also generally found within newly-deposited lime sand. However, in lime sands it is seldom preserved because of porosity loss by cementation
With intergranular porosity, the pore spaces are connected, one to another, by throat passages. Unless there is extensive later cementation, reservoirs with intergranular porosity generally have both good interconnected porosity and good permeability. Effective porosity in these reservoirs is equivalent to total porosity.
Intragranular porosity is more typical of newly-deposited skeletal lime sands.
It is unusual for such pores to be preserved. They are generally infilled during early burial by cementation but, in some cases, the cement may be leached out to leave the original intraparticle pore.
Secondary Porosity
Secondary porosity is porosity formed within a reservoir after deposition. The major types of secondary porosity are:
Fenestral;
Intercrystalline
Solution (moldic and vuggy);
Fracture
Fenestral porosity is developed where there is a gap in the rock framework larger than the normal grain-supported pore spaces. Fenestral porosity is characteristic of lagoonal pelmicrites in which dehydration has caused shrinkage and buckling of the laminae. This type of porosity is less frequently encountered.
Intercrystalline porosity occurs between crystals and is the type of porosity found in several important oil and gas fields. In recrystallized limestones, intercrystalline porosity is negligible. However, crystalline dolomites often possess high intercrystalline porosity.
