The next kind of fossil fuel to consider is oil.
Oil, as you know, is what they make gasoline out of.
It's the most convenient fossil fuel.
It's therefore the most precious, the most expensive.
Oil is formed not from land plants like coal.
But mostly from plankton in the ocean, that land on the bottom of the ocean and
can be preserved in the sediments, if the conditions are right.
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.
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 to these source rocks,
it has to be cooked just right to make oil.
There's a depth range in the earth between about
2 and 4 kilometers which is called the oil window.
The temperature gets higher the deeper you get in the earth.
If the source rocks are not buried
to a deep enough level, you'll have immature oil.
Some 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. It's very specific conditions that
allow the formation of liquid oil.
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 trap where it can accumulate.
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.
That's why the inventory of oil on earth
is so much less than the inventory of coal,
because of all of those unlikely things that have to happen,
in order for a carbon molecule to end up as as oil.
Oil has a long chain structures of of carbons.
And each carbon,
because of the number of electrons it has, because of its chemistry,
each carbon has to have four bonds to it.
This is a hexane molecule. 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 2 hydrogens each,
and then 2 bonds there and there on either side.
If you take the average of the carbon and
hydrogens in this molecule it kind of looks like CH2.
Which means that it's more reduced than coal.
This has an oxidation state of, the hydrogen is 2
times plus 1, and so the carbon is, minus 2.
The oxidation state of carbon in coal is zero.
Here, it's minus two, so it's more reduced than coal.
You get more energy per carbon atom
from oil, than you do from coal.
There are two different ways of thinking
about how long the oil will last.
One method is to take the reserves to production ratio.
The units of that, you've always gotta keep
in 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".
Cancelling the units we end up with
units of years and this would be
how much time until the last drop is extracted, that
we now know about, at the present day rate of production.
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 looks like sort of a perpetual
motion machine.
Like there could be oil forever because if we
could discover it as quickly as its being produced.
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 is 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 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 come back down.
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 were looking at the last drop.
Here, we're thinking about where the peak should be,
and if you have demand that's growing exponentially.
Where the shortage is going to happen is at that peak.
Looking at the amount of oil that is thought to
exist on Earth today and the history of that 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.
Peak oil would say that this is the
time when oil is going to start getting more scarce.