As we’ve seen, oil and gas are critical to the existence of civilization. But for all of our historical studies, we still haven’t answered a basic question:
What is it about oil and gas that makes them so energy rich?
Oil and gas are hydrocarbons: chemical compounds consisting solely of hydrogen and carbon atoms. Their energy is stored within the bonds holding these atoms together. When the bonds are broken by an ignition source, vast amounts of energy are released.
Chemical Structure
Although hydrocarbons are only made up of two different types of atoms, they exhibit a huge range of structures. The hydrocarbon molecules most common in oil and gas, the alkanes, are named for the number of carbon atoms they contain.
Some of the molecular names might sound familiar.
Propane
Propane, with three carbon atoms, is used to fuel heaters and portable stoves. If you have a propane grill, the fuel you are burning consists of propane molecules, each containing three carbon atoms.
Methane
Methane, which has one carbon atom, is typically sold as natural gas, and is used to heat homes, power vehicles, and generate electricity.
Octane
Another molecule you might be familiar with is octane. High octane fuel contains a higher percentage of octane molecules than standard gasoline (although the octane number listed on a fuel pump is actually related to a lab system for rating gasoline rather than a percentage). This allows the fuel to be compressed more without igniting, which is a good characteristic for fuel in a gasoline engine.
The longer and more complex a hydrocarbon is, the more viscous (resistant to flow) it is in liquid form. Alkane hydrocarbons with less than five carbon atoms typically occur as a gas; more complex molecules appear as liquids (such as octane) or solids (such as tar).
Even with a constant number of carbon and hydrogen atoms, there are many ways to arrange a hydrocarbon molecule. Each of these ways of organizing the atoms is known as an isomer. The number of possible isomers increases rapidly once there are more than six or so carbon atoms.
Energy Content
The energy in a hydrocarbon is stored in the chemical bonds between hydrogen and carbon molecules. When the bonds are broken by exposure to an ignition source, they release large amounts of energy, starting a chain reaction of bond-breaking. This reaction is called combustion.
But how did the potential energy end up in the hydrocarbon bonds in the first place?
Just like other sources of primary energy, it turns out that the energy contained in hydrocarbons is ultimately derived from a single primary source: the sun.
Thermogenic and Biogenic Production
The hydrocarbons of interest to drilling operators are formed from the remains of ancient microscopic organisms such as algae. Millions of years ago, these organisms used the energy of the sun to create complex organic molecules from carbon and hydrogen through the process of photosynthesis. After the organisms died, they were buried in an anoxic (lacking oxygen) environment, where bacteria were unable to break them down and take advantage of their energy content.
Ultimately, that energy was trapped deep underground for millions of years, where it was naturally converted by heat and pressure into oil and gas. This process is called thermogenic production because the gas is generated by heat.
There is a second process of degradation of organic matter that results in the production of natural gas called biogenic production. Biogenic gas is formed at shallow depths and in low temperatures by anaerobic bacterial decomposition of sedimentary organic matter. In contrast, thermogenic gas is formed at deeper formations by thermal cracking, or bond-breaking, of sedimentary organic matter into hydrocarbon liquids and gas as well as thermal cracking of oil at high temperatures. Biogenic gas is very dry with almost purely methane while thermogenic gas can be dry, or can contain significant concentrations of “wet gas” components such as ethane, propane, butanes and heavier condensates.
This course is interested in thermogenic production of gas. The next section is devoted to the science behind the burial, formation, and final storage of hydrocarbons.
Images: “Benzene, aromatic hydrocarbon” by pupunkkop via iStock; “Small propane tank connected in a cage” by photographereddie via iStock; “Gas Octane Options” by joshuaraineyphotography via iStock; “Algae, Cosmarium turpinii, micrograph” by NNehring via iStock