Porosity and Permeability

Any rock with sufficiently high porosity and permeability may serve as a reservoir rock provided that there is a source of petroleum, a structure, and a tight cap rock. Sediments consist of solid grains and of fluids which for the most part are water but may be oil and gas. Porosity (ϕ) is an expression of the percentage (or fraction) of fluids by volume (Vf) compared to the total rock volume with fluids (Vt), so that ϕ = Vf / Vt.

Porosity is often expressed as a percentage, but in many calculations it is easier to express it as a fraction, for example 0.3 instead of 30% porosity. The void ratio (VR) is the ratio between pore volume (ϕ) and the volume of the grains (1–ϕ).

Void ratio is often used in engineering and it has certain advantages in some mathematical expressions. If we assume that we know the density of the mineral grains, the porosity can be found by measuring the density of a known volume of the sediment. The density of the sediments (ρs) is the sum of the density of the grains, which are mostly minerals ρm, and the density of the fluids (ρf).

Well sorted, rounded sand grains are almost spherical in shape. If we have grains of the same size, which are all quite well rounded and with a high degree of sphericity, we will be able to pack the grains so as to get minimum porosity. Rhombic is the densest packing, resulting in 26% porosity, but this can not be obtained naturally.

Cubic packing, where the grains are packed directly one above another, results in about 48% porosity and this does not occur in nature either. Most well sorted sandstones have a porosity which lies between these two values, typically around 40–42%. Poorly sorted sand may have lower primary porosity and will also compact more at moderate burial depths. Clay-rich sediments have a much greater porosity immediately after deposition, typically 60–80%. This means that immediately following deposition a sand bed is denser than a bed of clay or silt. However, clay and silt lose their porosity more rapidly with burial. Porosity may be classified into different types depending on its origin.

Pore space between the primary sediment grains is often referred to as primary porosity. Intergranular porosity simply means porosity between the grains whereas intragranular porosity means porosity inside the sediment grains. The latter may be cavities in fossils, e.g. foraminifera, gastropods, molluscs, but also partly dissolved feldspar and rock fragments. Pore space formed by dissolution or fracturing of grains is called secondary porosity. Cavities formed by selective solution of sediment grains or fossils are classified as mouldic porosity. A typical example is when dissolution of aragonite fossils like gastropods leaves open pore spaces (moulds).

Particularly in carbonates we may also have porosity on a large scale i.e. as caverns (karst) and in reefs. Pore space produced by fracturing is called fracture porosity. Permeability is an expression of the ease with which fluids flow through a rock.

It will depend on the size of the pore spaces in the rocks, and in particular the connections between the pore spaces. Even thin cracks will contribute greatly to increasing the permeability. Permeability can be measured by letting a liquid or gas flow through a cylindrical rock sample under pressure.

The volume of water which flows through each surface unit in the cross-section A is thus equal to the flux F = Q/A. F can be measured in cm3/cm2/s or in m3/m2/s. This is equal to the Darcy velocity which is m/s. Well-sorted sandstones may have a permeabilities exceeding 1 Darcy and values between 100 and 1,000 mD are considered to be extremely good. Permeabilities of 10–100 mD are also considered to be good values for reservoir rocks. Permeabilities of 1–10 mD are typical of relatively dense sandstones and limestones, so-called tight reservoirs.

There are also examples of rocks with even lower permeabilities being exploited commercially for oil production, for example in the Ekofisk Field where the generally low permeability of a chalk matrix is enhanced by fractures which increase the overall permeability. In the great majority of rocks, the permeability differs according to flow direction. In sedimentary rocks the permeability is much higher parallel to the bedding compared with normal to the bedding. Channel sandstones can also have a marked directional impact on the permeability.