Reservoir Rock

Porosity and permeability are two key properties of rocks which enable the formation of an oil and gas reservoir (or an underground aquifer). A reservoir will typically consist of an impermeable cap rock above porous and permeable reservoir rock.

This type of geological structure (anticline) is ideal for trapping fluids. (D.Tompkins & J. Watkins, Exploring Earth and Environmental Science, ESWA)

Porosity is a measure of the ratio of the pore spaces (holes) to the volume of the bulk rock and is usually expressed as a percentage. Pumice is an example of a very porous rock, which is why it is often used in gardening. In sedimentary rocks (which are the main rock types in an ocean basin), the porosity will depend on the grain size and shape, the sorting and how well cemented the sediments are. Large rounded grains will not compact together as tightly as small grains, making them more porous. Sandstone is a sedimentary rock which often has quite high porosity.This makes it an ideal rock type for storing fluids in a reservoir or aquifer.

How the make-up of a rock will affect its porosity and permeability (https://www.e-education.psu.edu/earth103/node/719), accessed 07/05/2020

 Permeability is a measure of how easily a fluid can flow through the rock material. This depends on how well connected the pore spaces are. Clay is a sedimentary rock with a high porosity but extremely low permeability. This is due to the shape of the sediments, which are platy so lie on top of another, almost like Lego stacked up. This prevents fluid from filtering through (as shown in the figure above). Clay is often found as a cap rock in a reservoir, as it prevents migration to the surface. Sandstone on the other hand is a rock which usually has relatively high permeability, particularly if the grains are not well cemented together. This means, when a well is drilled, fluids from deep within the reservoir will be able to flow to it. Therefore, less wells are needed to access the oil, gas or water.

In this image the aquifer appears like a huge void filled with water. (https://www.canada.ca/en/environment-climate-change/services/water-overview/sources/groundwater.html), accessed 11/05/2020

Often images of oil and gas reservoirs and the aquifers are misleading. It appears there is a giant void in the ground and the fluids are trapped there.  This is not the case, as fluids are trapped within the pore spaces of rocks and rock materials (sediments). This is very important, otherwise when the fluids were pumped out the rocks above would collapse into the empty void. 

Even so, rapid depletion of a groundwater aquifer can lead to land subsidence.

To conduct your own fun and delicious experiment to investigate porosity and permeability, try this activity.

The written instructions for this activity can be found on the ESWA website.