Sub-Surf Rocks!


Source Rock

Oil and natural gas originate from this type of rock, which was rich in organic material that on burial did not fully decompose. The application of heat and pressure converts the organic material into hydrocarbons.
Reservoir Rock

Hydrocarbons tend to move up and away from the source rock, a process called migration. When large quantities of hydrocarbons accumulate in the pore spaces of a particular rock, then that has become a reservoir rock. When a reservoir rock is magnified, the tiny pores that contain trapped oil droplets can be seen.

Cap Rock

For hydrocarbons to accumulate in a reservoir rock, there must be some kind of barrier to prevent the oil and gas from migrating right up to the surface. A cap rock is an impermeable layer, through which hydrocarbons cannot pass.

Structural Trap

Hydrocarbons will migrate right up to the surface if they are not prevented. The place where oil or gas accumulates underground is called a trap. Oil is often found in a specific structural geological arrangement, such as an anticline or block faulting, and these are called structural traps. Geophysicists are often looking at seismic profiles in order to identify structural traps.


Permeability is the interconnectivity of the pore spaces which allows hydrocarbons and other fluids to flow through the rock. If there is no permeability, then the a rock cannot be a reservoir as there is no way to extract the oil or gas.

Measuring Permeability

The Darcy is a standard unit of measure of permeability. One darcy describes the permeability of a porous medium through which the passage of one cubic centimetre of fluid having one centipoise of viscosity flowing in one second under a pressure differential of one atmosphere where the porous medium has a cross-sectional area of one square centimetre and a length of one centimetre. A millidarcy (mD) is one thousandth of a darcy and is a commonly used unit for reservoir rocks.

Permeability (mD) Sandstone reservoir quality
<10 Poor
10-100 Fair
100-10,000 Good


Porosity is the small spaces within a rock layer that can hold fluids such as oil or water, a reservoir needs to hold large quantities of hydrocarbons.

Measuring Porosity

Porosity is measured as the percentage of pore volume or void space, or that volume within a rock that can contain fluids.

Porosity (%) Sandstone reservoir quality
<10 Poor
10-20 Fair
20-35 Good