Porosity

The porosity of a rock is the fraction of the volume of space (pores) between the solid particles of a rock to the total rock volume. This defines the total volume available for hydrocarbon saturation.

The formula for porosity is:

$$\phi = \frac{V_{\text{pore}}}{V_{\text{bulk}}} = \frac{V_{\text{bulk}} - V_{\text{matrix}}}{V_{\text{bulk}}}$$

Where $\phi$ is the porosity, $V_{\text{pore}}$ is the pore volume, $V_{\text{bulk}}$ is the bulk rock volume, and $V_{\text{matrix}}$ is the volume of the solid particles the rock is composed of, also known as the rock matrix. Porosity is a value from $0$ to $1$, or a percentage between $0\%$ and $100\%$. The units for porosity are $pu$, or porosity units.

Some ways of determining porosity include:

• Sonic logs
• Density logs
• Neutron logs
• Resistivity logs

Some types of porosity include:

• Total porosity: the definition above
• Primary porosity: the porosity of a rock resulting from its original deposition
• Secondary porosity: the porosity of a rock resulting from diagenesis