This was important before the Information Age, but now there is no reason to have a lot of physical currency when you can just have ones and zeros. It's easier to use pure energy, the item that embodies scarcity, than something like gold that is much less useful. So it would make much more sense to use oil as the backing currency, or even pure electrical power.
"I know we paid you 25 grams an hour for your last six-month consulting contract, but if you want us to renew it, we just can't afford to pay more than 23..."
If we could not find more gold we could still have economic growth, it's just that the value of gold would rise relative to the goods it could buy. (ie prices would fall)
You can read more here: http://www.mises.org/mmmp/mmmp5.asp
The price of oil goes up and down too much. People wouldn't want to loan or save a currency that changes value so much. The beauty with gold is the price would be stable, as to date no one's come up with a way of getting lots of it quickly and easily.
People still keep searching for and mining gold. That was silly two thousand years ago, but they have the excuse that they didn't know better. I read that current methods used for mining gold in some countries (e.g. Argentina) are especially hazardous for public health and the environment of local populations.
At least right now, it makes a lot more sense to trade in an energy containing commodity than pure energy because it's easier and more familiar to people.