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Sir Bill Gammell named his oil company Cairn because he liked the imagery of building stones gradually to create something monumental.

It was an odd choice, in that oil exploration doesn't seem gradual.

You have to do lots of preparation, but in the end, there's a brief point at which you either get lucky or you don't.

There are some very substantial shirts to be lost in the process, and Ernst & Young's oil and gas index this morning shows that the risk appetite in current economic circumstances is putting people off the sector.

Likewise, BP has learned that even when you get lucky, other risks can still be lurking deep underground and under water.

But today's agreement to sell a 51% share in Cairn India, the company it spun off on the Mumbai stock exchange, shows there are big, big returns for those willing to take the risk.

The price being paid by Vedanta Resources is £5.4bn, as that Indian metals company diversifies into the energy sector - or perhaps it's hedging against future energy price fluctuations, when its metals business is so exposed to them.

The champagne will be flowing at Cairn's Lothian Road headquarters.

But spare a thought for the people at Shell, who sold Cairn the Rajasthani drilling rights for an oil industry pittance.

A substantial share of the sale is to be handed back to the Cairn shareholders who took that risk, while some will be put into continuing exploration activities, notably in Greenland.

And the risks continue. Don't forget that the drilling going on off the west coast of Greenland this summer is reckoned to have only a 10% chance of success.