Reservoir rocks are rocks capable of storing large quantities of oil and gas. They are characterized by high porosity and permeability. An example of a good reservoir rock is sandstone. Aquifers, which are rocks that produce ground water, have the same properties as reservoir rocks with the exception that they contain water rather than petroleum.
Porosity is the amount of void spaces in a rock and a measure of how much petroleum the rock rock will hold. Permeability is the ease that fluids move through a rock and is determined by the diameter of the channels which connect the pore spaces.
Picture 1 is a scanning electron microscope (SEM) image of unconsolidated sands. Note the void spaces (porosity) produced by the stacking of irregular shaped grains. The remaining three images are photomicrographs of thin-sections, which are produced by grinding a rock till it is thin enough to pass light through it. Picture #2 is the Cambrian Bliss Sandstone. The clear grains are quartz sand and the black material is hematite cement. Because the cement infills the spaces between the sand grains this rock has a very low porosity.
Picture #3 is from an excellent reservoir in the North Sea. This rock was impregnated with blue epoxy so it would be easier to identify the porosity. Some of the sand grains in this rock were dissolved away resulting in the formation of secondary porosity. Picture #4 is of a limestone, a biologic sedimentary rock. Some of the fossil fragments were dissolved to form porosity. Unfortunately because there are no channels which connect the pore spaces , this rock has a low permeability. Even though it has a high porosity because of the lack of permeability it is a poor reservoir.