That would also work, in theory. It would also future proof the currency against developments that could make oil worthless, though I don't see that happening anytime soon.

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.

Joules are not equal. The sea is full of them. You have to look at entropy, too. A Joule of electricity would be a more practical measure.

Because it wouldn't be portable. Imagine paying for a restaurant dinner with 2 barrels of oil you had to haul over there.