2013: Advent Oil & Gas: Fossil fuels – what’s in a name?

by on December 2, 2013

It’s probably not escaped your notice that this series is about ‘oil and gas’, but you’ll also have heard a lot about ‘fossil fuels’ – they’re even in the title of this article!  What’s the difference?  The answer is somewhat complicated: fossil fuels are often oil or gas, often something else, but most oil and gas is fossil fuel.  Let’s try and untangle it.

A fossil fuel is a fuel that comes from the burial and breakdown of organic matter.  Plants (anything from ocean plankton to large trees) and to a lesser extent animals are sometimes buried in environments without much oxygen – we tend to call these anaerobic or ‘without air’.  In these circumstances, the organic compounds break down and re-condense over thousands to hundreds of millions of years.  The first stages of this produce peat on land and organic ooze underwater.  Increasing pressure and time will turn peat to lignite and then bituminous coal, and turn ooze to a substance called kerogen (‘wax-producing’).  If these are then exposed to moderate heat (about 60–200°C), the compounds break down in a process called pyrolysis (‘separation by fire’) and they produce the highest-grade fossil fuels: anthracite coal, petroleum (mineral oil), and natural gas.

There are still some issues with this definition – for instance, is peat a fossil fuel?  Many groups class it as such, but it is strictly speaking renewable, since a peat bog left undisturbed will accumulate peat at around 1mm per year.  Perhaps we could call it a ‘subfossil fuel’!

What is undeniably true, however, is that the end products (coal, oil, and gas) are fossil fuels in the true sense, and aren’t renewable.  Peat regeneration is slow enough, but that peat, or seabed ooze, must then be buried for a very long time to turn to bituminous coal or kerogen, and then exposed to the right amount of heat for the right amount of time.  In the case of oil and gas it also needs to be trapped in the right sort of rock formation, which I’ll discuss more in a later article.  These requirements are quite specific although fortunately there have been periods in the Earth’s history where they have been favoured: much of our coal comes from the Carboniferous period around 300 million years ago, when dense swampy forests were very common and a lot of buried plant matter was laid down.

Oil and gas, for the purposes of this series, means liquid or gaseous fuels with a reasonably high hydrocarbon content.  Exactly what this means is going to be the subject of a later article in the series (I said it was a complicated answer!) but suffice to say it doesn’t include hydrogen fuel.

Most of our oil and gas is extracted directly from rock formations, so this is fossil fuel, although it generally needs some sort of processing before it can be useful.  That said, we can also get oil and gas from other sources.  For instance, we can produce them from other fossil fuels: before 1968 all of the gas in Britain was produced from heating coal, and this could well be used in future.  More significantly, the popular label of ‘biofuels’ includes oil derived from processed crops and gas derived from bacterial digestion of waste, and these definitely aren’t fossil fuels.

So in the end whether I write about something in this series depends on what that something looks like to the end user, not where it comes from.  This may seem like a fuzzy way of doing things – so please trust me that it makes sense in light of the way the industry is run.

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