The Chemistry of Petroleum Formation
At its base, petroleum is a fossil fuel, which means it is derived from the remains of organic material. In other words, petroleum results from a number of chemical reactions that occur to material that was once alive. In most cases, liquid petroleum was once zooplankton or algae that settled to the bottom of a sea or lake and was then buried under sediment. The sediment ensured that no oxygen was able to reach the decaying organic matter and this set the stage for the formation of oil.
In most cases, the organic matter goes through several changes that take thousands or millions of years. As sediment continues to pile up and increase pressure on the organic matter, it is first changed into a waxy solid called kerogen. In fact, this material is currently being mined in many “fracking” processes because it can, through chemical conversion, be made into liquid petroleum and natural gas.
Kerogen is formed in a process called diagenesis, the chemical form of which is outlined in the following diagram.
Essentially, heat and pressure break down organic compounds like humin (not human) and various other organic acids, lipids, proteins, and carbohydrates to form long hydrocarbon chains called geopolymers. These geopolymers are the basis of kerogen. Diagenesis is a critical mechanism in the formation of coal and is just the first of several processes necessary to convert solid hydrocarbon to liquid petroleum.
The addition of greater heat is necessary to convert kerogen to liquid or gaseous hydrocarbons and the process takes time. The combination of high temperature and pressure is necessary to carry out the endothermic process known as hydrocarbon pyrolysis. It is sometimes referred to as cracking as well.
Hydrocarbon pyrolysis is irreversible, which means that once a liquid hydrocarbon is formed, it is not converted back into solid form. This is why oil deposits can exist below the surface for millions of years unchanged. Liquid hydrocarbons are really just formed by breaking longer chains. It is a general rule in chemistry that the larger a molecule is, the more likely it is to be solid and the smaller a molecule is, the more likely it is to be a liquid or gas. Long hydrocarbon chains are solid, while medium chains (5 – 25 carbons long) are liquid. Smaller chains (less than 5 carbon atoms), tend to be gases. That is why gasoline at 7 or 8 carbons is a liquid while methane, with only one carbon atom, is a gas.
Versions of cracking are used in industry to create everything from charcoal to carbon fiber to biofuels. The process is often used in oil refineries to breakdown the less valuable heating oil molecules (25 carbons per chain on average) into smaller, more valuable 7 and 8 chain molecules that can be sold as gasoline.
The Importance of Oxygen
Oxygen is critical to many processes and its absence is absolutely critical to the formation of hydrocarbons. When oxygen is present, several things can happen. At the surface, when organic material is first laid down, the presence of oxygen means the presence of bacteria that can quickly consume the decaying material before it has a chance to be buried by sediment. This is why most petroleum deposits were once at the bottom of a sea or lake, often one with very low oxygen content, where sediment had time to accumulate before too much decay could occur in the presence of oxygen.
If oxygen is present, besides derailing the early stages of kerogen formation completely, it can also lead to the formation of acids and other molecules rather than strict hydrocarbon. These are usually detrimental to the formation of hydrocarbon and can even reverse formation that has already occurred.
Finally, levels of oxygen that are not high enough to prevent hydrocarbon formation can still be a problem Low levels of oxygen can lead to the buildup of toxic nitrogen oxide compounds as well as sulfuric and sulfurous acids. All these act as contaminants in petroleum, making it more expensive and difficult to refine.
Natural gas is simply methane and can be associated with oil fields or found in its own deposits. In either case, natural gas can be thought of as the last product of a chain of cracking reactions. Methane is a single carbon atom with four hydrogen atoms. It is the simplest, smallest hydrocarbon and thus cannot be broken down further.
Reservoirs made only of natural gas have occurred in one of two ways. Either the natural gas has leaked from another petroleum deposit that contains other hydrocarbons or all of the hydrocarbons in the deposit have been converted to methane, leaving few if any other hydrocarbons. High temperatures and pressures are necessary for natural gas formation. As a general rule of thumb, the lower the pressure and temperature, the heavier the hydrocarbon will be. Natural gas is only found near the surface if it has escaped from a deeper well.
Methane is also commonly produced by bacteria, making it rather unique among petroleum products (though there are limited instances in which bacteria have been shown to produce things like butane). The bacteria that produce methane are known as methanogens, and can produce methane directly from organic material under anoxic (oxygen free) conditions. These are the bacteria that cause methane production in landfills. Some natural gas may be formed this way during early stages of petroleum formation, but most is likely lost to the atmosphere if there is not a solid layer of sediment to trap it.