Page 21 - Physical Principles of Sedimentary Basin Analysis

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Properties of porous media

2.1 Porosity
Sediments and sedimentary rocks are porous media, and a porous medium is a solid with
holes in it. The holes (pores) are normally connected and a fluid may flow through the pore
space. The passage from one pore to another is through a pore throat, although there is not
always a clear distinction between a pore and a pore throat. The way in which the pores
are connected and the size of the pore throats control how permeable a porous medium is
for fluid flow. The volume of the pore space controls its capacity to store fluid. Figure 2.1a
shows an illustration of a porous medium made of a regular arrangement of spherical grains
of equal size. It is a simple idealization of sediments and sedimentary rocks. A real rock
has a much more complex pore space than the regular packing of spheres, as seen from
the thin section in Figure 2.1b. It consists of grains of a variety of sizes, shapes and min-
erals. The pore space in rocks is also the result of a complex interplay of mechanical and
chemical processes. The porosity is the volume fraction of void space of a porous medium,
expressed as

V p
φ = (2.1)
V t
where V p is the volume of the void space and V t is the total volume (of both solid and
void) of the sample. An alternative way to measure the void space is to relate it to the solid
volume of the rock rather than the total volume. This property, called the void ratio, is

V p φ
e = = . (2.2)
V t − V p 1 − φ
The solid volume of the rock is the difference between the total volume V t and the void
volume V p . Equation (2.2) can also be inverted to give an expression of the porosity as a
function of the void ratio,
e
φ = . (2.3)
e + 1
As we will see later it is often more convenient to work with the void ratio than the
porosity.

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