Fossils on Friday – Oil and Sedimentary Rocks

Welcome back to Fossils on Friday, as we start the first of our experiments, beginning with oil. Crude oil – the thick black tar-like substance, has been used by humans for centuries. It was first properly harnessed by the Chinese (although many other cultures were using it as ‘pitch’ – covering their boats with it – more about that next week!), and they even adapted into a fuel by the 4th century BC.

But where does oil come from? You may well know that it is extracted from oil ‘wells’. In my mind there are two images that crop up in my mind when I think of oil mining – either the middle-eastern wells (with the pump that always reminded me of a giraffe going down to drink), or the closer-to-home North Sea oil rigs, which I used to watch off the Norfolk Coast when I was younger. But what exactly were they looking for? And what is an ‘oil well’?

The onshore oil rig – pumping oil out of the ground
The offshore oil rig – pumping oil out of the sea bed.

Crude Oil is generally accepted to be the remains of plant and animals that have been compressed and chemically altered into their base hydrocarbons, which is essentially the major composition of oil. If that sounds too technical, don’t worry – we’ll go into more and simpler detail in future posts. For now – just think of oil forming by being squeezed out of fossil creatures and plants!

This process happens during or after the fossilisation process. So the oil, which gets extracted, is always found in sedimentary rock. But not all sedimentary rock with oil in is good for digging an oil well. Oil wells can cost millions of pounds to construct, so there has to be enough oil to make it worthwhile. So when you go looking for where to put an oil well, you need to find a place where the oil has seeped down into a large bubble – where you can then suck it up! The oil gets trapped between the very porous rock above, and an impermeable rock or water below.

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A diagram showing where the oil gets trapped under sedimentary layers. Credit – CC Wikimedia.

In order for this to happen, the oil needs to be formed in a very porous rock – which leads us onto our first experiment! Remember our list of equipment from last post? You need to find:

  • Some different rock types. Recommended variety includes a sandstone, a shale (mudstone) and limestone (chalk). See what you can find around your home and out on walks.
  • Some oil. Vegetable will work fine, but engine oil is the best! Use with caution though.
  • An eye dropper, or failing that – a small teaspoon.
  • A notebook
  • A timer
Our complete experiment kit

Start by lining your different rocks up on a tray or plate. Get your timer ready, and fill your eye dropper or spoon with oil. Engine oil works best, as it is extracted from crude oil (it’s more ‘genuine’ to the experiment), but cooking oil will work as well.

Carefully drop some oil onto each one, and start the timer. You are looking for how long it takes for the oil to seep down through the rock. Just like in the big rock formations, oil either gets trapped in-between each rock layer, absorbs it completely, or sinks down through the layers.

Putting the oil onto a piece of shale – in this case, slate from Wales.
Putting oil onto limestone, in this case phosphorus limestone from Morocco
Putting oil onto sandstone, in this case, Old Red Sandstone from Shropshire

If the experiment went right, you should have seen that the slate is not porous at all – the oil just sat there. The limestone (or chalk) was either not very porous, or very very porous, sucking all the oil up completely. And the sandstone, should have been ‘just right’ allowing the oil to slowly trickle down, and form a puddle at the bottom, depending on how much you added in the first place.

After 10 minutes, the oil is still sitting here!
Within a matter of seconds, the oil has become absorbed!
Over a few minutes, the oil seeps through to the other side.

What this shows is the type of rock needed to extract oil from. Limestone or chalk can hold oil – but it is almost impossible to extract – it’s caught up so tightly in the rock. Chalk is sedimentary, but with very tiny particles, closely trapping the oil.

Shale or slate can actually hold masses of oil, and some of the densest oil deposits in the world are found in between the layers of slate. The problem is, that even though there is plenty of oil in the tiny gaps in-between the layers of shale, the rock itself is not porous at all – so the oil can’t be sucked up. There is another way of extracting the oil though – more of that later!

However, sandstone, it both porous enough to let the oil run through, but not too porous to trap it between the particles of sediment. This is perfect for looking for a place to put an oil well, where the oil has run through the sand, and settled in a big bubble, called a reservoir. With all the weight of the rock on top of the reservoir, the oil is under a huge amount of pressure – so all you have to do it drill down to it, and the pressure is enough to force the oil up and out the top of the ground!

Let’s continue our oil investigations next week, as we look at another way to extract oil.