Once upon a time, you could be sure of Shell. If you go down to the investment woods today, the one thing you can be sure of is (another) big surprise. What used to be one of the stock market’s most reliable and predictable income generators suffered another spasm this week with the sale of an asset which it had described as “core” only a matter of months ago. This time it’s the shale oil and gas interests in the Permian basin in the US, one of the most prolific such areas in the world, sold off for $9.5bn.