Skip to content
Join our Newsletter

It may be crude, but it’s not simple

Todd Whitcombe Speaking of Science Crude oil is not a single compound. Petroleum -- which literally means "rock oil" -- is a complex mixture of hydrocarbon compounds with a few other species thrown in.

Todd Whitcombe

Speaking of Science

Crude oil is not a single compound. Petroleum -- which literally means "rock oil" -- is a complex mixture of hydrocarbon compounds with a few other species thrown in. It is thousands of different chemical compounds mixed together in a complex blend.

These chemical compounds range in weight and size, from simple small molecules such as methane to complex heavy weights with 40 or more carbon atoms. Methane is a single carbon bound to four hydrogen atoms and represents the most reduced form of carbon that we can find. It is also a gas at ordinary temperatures and pressures.

The heavyweight compounds found in crude oil can consist of long chains or multiple rings, hydrogen-rich or hydrogen-poor, complex compounds. They are a far cry from methane in both size and shape, but they are also reduced forms of carbon. Typically, though, they are solids or viscous liquids at room temperatures and pressures.

Industrially, we need to process these materials before they are useful to us. A refinery takes the crude oil and distills it, separating out the components into broad categories based on their boiling point or volatility. For example, the component of crude oil that we call gasoline is typically collected at temperatures ranging from 20 C to 200 C.

A refinery is also capable of converting different fractions of crude oil into one another. Heavy material can be broken down into smaller fragments, most often to make gasoline. Lighter fractions can be combined towards the same end.

The breakdown of crude oil is dominated by gasoline and home heating oil production. About 70 per cent of a barrel is used for these purposes while 20 per cent is an assortment of other types of fuels, waxes, and oils for lubrication. Around 10 per cent finds its way into the production of 80 per cent of all consumer goods. Everything from car fenders to plastic grocery bags to pharmaceutical compounds are made from crude oil and many are digestible.

Crude oil is a complex mixture of chemical compounds used in a wide variety of applications.

But it is not particularly toxic. I know that with pictures of dead and dying animals from the Gulf of Mexico fresh in our collective memory this might sound a little bit "wrong". After all, wasn't it the oil that was killing them?

The answer to that is one of "yes and no". When oil coats a sea bird, the oil interferes with the animal's ability to swim or fly and that is the problem. The animal gets mired in the mess and asphyxiates when it inhales too much oil. It is not poisoned but physically assaulted.

In terms of the overall effects of spill, the difference is a bit like splitting hairs. After all, dead is dead. And massive kills of sea life or, in the case of a pipeline rupture, of plants and animals is never a good thing.

However, the Gulf of Mexico disaster has had some interesting side effects for micro-organisms, many of which are quite happy living on a diet of crude hydrocarbons. Indeed, many microbial organisms thrive on just such a diet.

Bacteria, feeding off the heavy hydrocarbons leaked from the Deepwater Horizon rig, appear to have been initially stimulated to grow in the constant stream of methane, a study in Science reveals. Close to the wellhead, where the plume is fresher and more concentrated in light hydrocarbons such as methane, ethane, and propane, colonies of light hydrocarbon eating bacteria are thriving.

Further away, heavy hydrocarbon eaters dominate as the oil is fractionated in the deep ocean environment. Indeed, this microbial clean up is why scientists have been reporting that the lingering effects of the spill might not be as bad as they were originally projected to be.

Indeed, John W. Finley, a professor of food science at Louisiana State University said: "There are no chemicals of concern turning up in seafood," adding that shrimp and finfish quickly metabolize aromatic hydrocarbons such as those found in crude oil.

That doesn't mean that everything is "just fine" in the Gulf. Monitoring will need to be carried out for years to ensure that accumulations of chemical compounds do not surface in the food chain but there is some hope for the region.

Of course, a spill on land would be a different matter because the colonies of oil eating microbes are simply not present. They live where there is little oxygen. This is why a tanker spill or a pipeline break is so much more devastating as there are simply not enough bacteria to feast on the abundance of compounds from the chemical soup that is crude oil.