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The next kind of fossil fuel to consider is oil.
Oil is what they make gasoline out of.
It's the most convenient fossil fuel.
It's therefore the most precious, the most expensive.
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Oil is formed not from land plants like coal, but
mostly from a plankton in the ocean that land on the bottom of the ocean and
can be preserve in the sediments, if the conditions are right.
So, actually most sediments at the bottom of the ocean don't have enough
organic carbon in them to really make much oil.
Oil comes from very special what are called oil source rocks.
So the entire oil field in the Middle East
comes from source rocks that are just a few feet thick.
Very special rocks, very special conditions are required to concentrate
organic carbon in the sediments to a high enough degree that you can make oil.
Then, after the carbon is deposited in these source rocks,
it has to be cooked just right to make oil.
There's a depth range in the Earth between about two and
four kilometers which is called the oil window.
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So the temperature gets higher the deeper you get in the Earth.
So if the source rocks are not buried to a deep enough level you'll have
immature oil, some sort of tar sand, something like that, or
if it gets buried too deeply out the bottom of the oil window
the carbon will be converted to natural gas.
And so, it's very specific conditions that allow the formation of liquid oil.
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And then to get it out, most of the oil that's extracted is
stuff that has migrated in the sediment column and
found some sort of a trap, where it can accumulate.
So probably, most of the oil that was ever formed on Earth
just leaked back to the surface of the Earth and was gone forever.
But some tiny fraction of it gets trapped in these traps,
which are amenable to harvesting.
So that's why inventory of oil on Earth is so much less than the inventory of coal,
because of all those unlikely things that have to happen in order for
a carbon molecule to end up as oil.
So oil has got long chain structures of carbons,
and each carbon, because of the number of electrons it has,
because of it's chemistry, each carbon has to have four bonds to it.
And so, for a simple, this is a hexane molecule.
So these carbons on the end have to have three
hydrogens in order for this carbon here to have four bonds.
Or these ones in the middle have two hydrogens each and
then two bonds there to carbon to either side.
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Which means that it's more reduced than coal.
So this has an oxidation state of, the hydrogen is two times
plus one and so the carbon is minus two.
So the oxidation state of carbon in coal is zero.
Here it's minus two.
So it's more reduced than coal.
So you get more energy per carbon molecule
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Now there are two different ways of thinking about how long the oil will last.
One method, is to take the reserves to production ratio.
So the units of that, you always have to keep track of the units,
they will tell you what's going on, could be gigatons of carbon for
reserves and then gigatons of carbon per year for the rate of production.
That's what they in the industry call extracting
oil from the ground is production.
So, cancelling the units we end up with units of years and
this would be time of how much time until the last drop
is extracted that we now know about at the present day rate of production.
So if you look at this plot of the historical evolution of
this ratio through time, what you'll see is that there's 40 years of oil left,
and 40 years ago there was also 40 years of oil left.
And it sort of looks like sort of a perpetual motion machine,
like there could be oil forever because if we could
discover it as quickly as it's being produced.
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Another way to think about oil though, Is using an idea known as peak oil.
And this is based on the observation that the rate
of extracting oil from some oil field like the state of Texas, for
example, tends to follow what's called a Gaussian or a bell curve.
This is an empirical relation, this is an observation.
Gee, it kind of looks like this,
it could look different, the rate of extraction could look different.
But what tends to happen is you have sort of a ramping up period
as the infrastructure is getting put into place.
And then the rate of growth tends to sort of taper off, and
then it starts to sort of come back down.
So what this means is that,
the implication of this is that the peak comes when the oil is
only half gone as opposed to over here we are looking at the last drop.
Here, we are thinking about where the peak should be and
if you have demand that is growing sort of exponentially.
Where the shortage is going to happen is sort of at that peak.
So looking at the amount of oil that is thought to exist on Earth today,
and the history of the oil extraction rate so far, we can use this theory
to come up with some estimate of when the peak rate of oil extraction will be.
And we're right about close to the peak now.
The traditional fossil fuel inventories of oil are about 200 gigatons.
And we've already extracted about 100 gigatons of that.
So, we're about halfway there.
So, peak oil would say that this is the time,
when oil is going to start getting more scarce.
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David ArcherProfessor
