Coral reefs demonstrate primary porosity in carbonates.
Carbonates are minerals that include the carbonate ion, a compound of carbon and oxygen. They are biological materials and deposit close to their original organism’s habitat. Carbonates are mollusk shells, animal and fish bones and coral skeletons. These sediments contain nearly half of the world oil and gas reserves and more than half of its water. Porosity is a measure of spaces, or pores, in a volume of rock. Carbonate rocks are very susceptible to textural change and their porosity changes over time.
Primary porosity in carbonates occurs during the initial deposition of carbonate sediments usually in warm, shallow seas along continental shelves. These sediments derive from the physical and chemical breakdown of microorganism skeletons. The voids between the deposited particles create the rock’s porosity. Coral reefs grow in shallow warm seas and have the chemical composition of calcium carbonate. The spaces between branches of coral reefs are another form of primary porosity.
The deposition of carbonate sediment gradually buries older deposits. As the volume of sediment grows, rainwater replaces the original seawater between the grains of the carbonate deposits. Rainwater is mildly acidic because it dissolves atmospheric gases such as carbon dioxide and nitrous oxides. These mild acids dissolve some of the carbonate sediments and cement them together. This process is dissolution and it reduces the space between grains, creating secondary porosity.
Another stage of secondary porosity occurs when the carbonate sediment comes into contact with magnesium-rich water. The chemical reaction produces a carbonate of magnesium and calcium popularly known as “dolomite.” Magnesium is present in arid coastlines such as the Persian Gulf. Over time, dolomite forms crystals that increase the space between the grains of the original sediment and increase the deposit’s overall porosity.
Carbonate deposits are brittle substances. They do not bend easily in response to Earth movements but fracture and break. These fractures may range between tiny breaks invisible to the naked eye, to wide crevasses. Fractures create another version of secondary porosity. Most of Iran’s oil and gas reserves are present in such carbonate fractures.