Ask the average person what an oil or gas reservoir looks like and there's a good chance that they'll tell you it's a underground cavern filled with oil and gas. Reservoir actually looks like this. It consists of one or more layers of sedimentary rocks which themselves can be made up of thinner beds of sediment that are partially saturated with oil and or gas.
The maximum amount of oil, and or gas, a reservoir can hold is determined by its porosity, which the fraction of open space or pore space in the rock. The quality of a reservoir is also determined by its permeability. This is a measure of the connectivity between the pore spaces and thus reflects the ease or difficulty of extracting oil and or gas from the reservoir.
A desirable reservoir rock then is one that has lots of pore space or high porosity and allows for oil and or gas to easily pass through it and thus has high permeability.
Here are examples of three common types of reservoir rocks. The sandstone, made of sand grains. The limestone made up of shell material. And shale made up of silt and clay, which are very fine grain sediments. The shale also contains organic matter, which is why it's black.
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