Classification of Porosity

Porosity can be classified in two ways: by the shape and connection of the pores (a morphological view), and by when the pores formed (time of deposition). This article explains both classifications and why each type of pore matters for producing oil and gas. If you are new to the topic, start with what porosity is.

Classification by Pore Connection

From a morphological point of view, pores are classified as:

1. Catenary (interconnected) pores

This type of pore has more than one throat connected to other pores. Hydrocarbons are relatively easy to extract from such pores.

2. Cul-de-sac (dead-end) pores

This type of pore has only one throat connecting it to other pores. It may yield some hydrocarbon by expansion as reservoir pressure drops.

3. Closed (isolated) pores

This type of pore is closed. It has no throat and cannot connect with other pores, so it cannot yield hydrocarbons in the normal production process.

Classification by Time of Deposition

Reservoir pores in sedimentary rocks are found as two general types, based on their time of formation: (1) primary, also called intergranular or depositional porosity, and (2) secondary, also called intermediate or post-depositional porosity. Each type has subdivisions.

Primary Porosity

Primary porosity is divided into two types: intergranular (interparticle) porosity, which occurs between the grains of a sediment, and intragranular (intraparticle) porosity, which occurs inside the grains.

• Intergranular porosity is more typical of sandstones. It is also found in newly deposited lime sand, but is seldom preserved there because cementation destroys it. Pore spaces are connected to each other by throat passages. Unless there is extensive later cementation, such reservoirs generally have both good interconnected porosity and good permeability, and their effective porosity is close to total porosity.
• Intragranular porosity is more typical of newly deposited skeletal lime sands. It is unusual for these pores to be preserved. They are generally filled by cement during early burial, but in some cases the cement is leached out and the original intraparticle pore is left.

Secondary Porosity

Secondary porosity forms within a reservoir after deposition. The major types are:

• Fenestral porosity: develops where there is a gap in the rock framework larger than the normal grain-supported pore spaces. It is characteristic of lagoonal pelmicrites, where dehydration caused the laminae to shrink and buckle. It is less frequently encountered.
• Intercrystalline porosity: occurs between crystals and is found in several important oil and gas fields. In recrystallized limestones it is negligible, but crystalline dolomites often have high intercrystalline porosity.
• Solution porosity (moldic and vuggy): forms where grains or fossils are dissolved (moldic) or where larger cavities are dissolved (vuggy).
• Fracture porosity: open fractures in the rock. It adds little to total storage but can greatly improve flow.

Porosity of core samples is measured in the laboratory; see laboratory measurement of porosity. How porosity relates to flow is covered in relation between porosity and permeability.

Frequently Asked Questions

What is the difference between primary and secondary porosity?

Primary porosity exists when the sediment is deposited. Secondary porosity is created later, for example by dissolution, recrystallization or fracturing.

Which pore type is best for production?

Interconnected (catenary) pores, because fluids can move through several throats.

Why is intercrystalline porosity important?

Crystalline dolomites often have high intercrystalline porosity, and this is the type found in several important oil and gas fields.

Key Takeaways

• Pores are classified by connection (catenary, cul-de-sac, closed) and by time of formation (primary, secondary).
• Interconnected pores produce most easily; closed pores do not produce.
• Primary porosity includes intergranular and intragranular; secondary includes fenestral, intercrystalline, solution and fracture porosity.

See also: total vs effective porosity.