Geology: Carbonate Platform

An article i wrote few months ago…

Carbonate Platforms

Remember when we were young, we used to visit exotic islands to have a holiday, having a whale of time on the pristine beach with our friends or family, little that we know that most of the beaches and islands are actually carbonate islands. One of the most astounding examples of carbonate platform is the Bahamas Island and platform on the Great Barrier Reef. So, what is so special about these islands?

These carbonate Island or carbonate platform in scientific term, is unlike a gigantic land that is created from volcanic eruption or decreasing or sea water level to from a piece of land. It is just actually a sedimentary body which possesses topographic relief, and is composed of various sea plants and dead organism (microbes) which induce carbonate precipitation through their metabolism that deposits for a long time. Nevertheless, factors such as existence of life reef building organism, light and water temperature can affect the formation of these islands.

Carbonate platform is proven to be highly effective in storing and retaining vast quantities of gases and liquids for a long time, which explains why it is a perfect place to store carbon dioxide, CO₂ . These gases are compressed and injected underground. At these depths, carbon dioxide behaves as supercritical, sharing densities similar to those liquids and viscosity similar to gases. It moves through pore spaces as though it were a gas and takes in less space, as though it were a liquid. The deeper it goes, the denser it becomes and also less space is requires. Henceforth, it is potentially effective in storing huge amount of gases.  The geological distribution of rocks forms a trap by holding these gases under a layer of impermeable cap rock combine with narrow anticlinal folds of formation.

Carbonate platform’s characteristic also is put in good use in storing liquids minerals. Half or more of the world's petroleum is stored in carbonate reservoir rocks. This is due to the suitable porosity and relative permeability. The feature of sucrose dolomite porosity is important in many oil reservoirs. In these rocks, porosity and permeability have been strongly influenced by composition of carbonate sediment and the rock has been replaced by calcite of sucrose dolomite over a period of erosion and deep water pressurised. Dolomite calcite which happens to be a natural host rock, is a sedimentary mineral carbonate rocks which is capable of storing minerals, in this case oil due to its ordered structure. Cutting edge technology even allows injection of mercury or CO₂ into the cores of the reservoir to increase the pressure to guide the oil to the surface. The method can only be applied when the volume of the mercury or CO₂ can cause enough pressure and force to displace oil from water and overcome its interfacial tension between the rocks in the pores. This method is very favourable in the United States as 50 million metric tonnes of CO₂ are injected annually in the United States into declining oil fields.

As a whole, there is strong incentive to interpret carbonate platform and its ancient carbonate environments with organism communities. Hence, it is imperative for us, human beings to understand their prudent contribution in the storage of increasing amount of carbon dioxide and also its effects on oil-reservoir properties.