We posed this question to James Jackson from the University of Cambridge...

Hydrocarbons are all are from dead organic material and you need an astonishing set of circumstances to make oil out of these things and preserve them. First, you've got to concentrate them somewhere where they're not dispersed or oxidised, which means in swamps, marshes, lakes or something like that. Then you've got to heat them up slowly over a long, long period to cook them up to make oil. And then, when they make oil and start to buzz off, you've got to have some way of trapping them. All these things, it turns out, happen on the margins of continents. If you stretch continents and try to pull them apart what happens is they "neck" - and that's what the North Sea is. The North Sea, for example, exists because Scotland and Norway moved apart about a hundred kilometres and so the land stretched and necked; the floor of what is now the North Sea got a bit thinner and it sunk below sea-level.

This caused it to become buried in sediment. Sediments just gets washed in, and all those dead bugs which were there got buried deeper and deeper and gently got cooked for a long time. Eventually, the North Sea stopped stretching. Had it carried on it would have been a plate margin - we would have got the edge of the continent and Scotland would have been over by America somewhere and Norway would still be in Europe. So, the circumstances for making oil are very good on the margins of continents. Especially the margins of oceans like the Atlantic ocean, which is not a plate boundary - there are no earthquakes there.

Now, what happened in Saudi Arabia is that that happened to be on the margin of a huge ocean which separated Asia from the southern continents. So, a hundred million years ago, Africa, India and Arabia were all a long way further south from where they are now and they've all moved north and bashed into Asia. One of those places is [what is now] Saudi Arabia, Iran and Iraq. So what's happened is the margin of that ocean, with the margins of Arabia, and Africa and India, have all just popped up above sea level. So it's not that there is more oil there than anywhere else. There's loads of oil all the way over on the other continental margins but that's underwater. It's hard to get out; it's hard to find and it's hard to suck out. Whereas in Saudi Arabia, it's popped up nicely above sea level and also in Iran and Iraq. So it's actually extremely easy to find.

In essence, it's more that it is conveniently situated than anything else; but, geologically, what you're looking at is like the edge of Ireland, the western side of Ireland, which has just run into something and popped back up above sea level.