Mitchell Swartz wrote "a cubic mile of seawater contains the fuel equivalent of all the Earth's oil reserves." That got me to thinking. I ran the numbers, and I believe that is about right, if I have done my arithmetic correctly. I assume this means the fuel is deuterium. I estimate it is about 6 times oil reserves. That is a rough estimate.

1 cubic mile = 4.17E+09 cubic meters = 4.17E+12 kg of water

1 kg of ordinary water is 0.015 at% deuterium. When fused in D-D fusion, this deuterium produces 13,000 MJ

4.17E+12 liters * 13,000 MJ = 5.42E+16 MJ deuterium energy in 1 cubic mile

Oil reserves are ~1.7 trillion barrels (1.65E+12). This is roughly 47 times annual oil consumption. The reserves do not decline much because new oil is discovered, and extraction techniques improve. See the stats and graph here: https://www.worldometers.info/oil/ and Statistica (graph below).

1 barrel of oil (1 boe) produces 5,861 MJ

1.65E+12 boe * 5,861 MJ = 9.67E15 energy in oil reserves

So, 1 cubic mile has about 5.6 times more potential energy than the world's oil reserves.

Total world primary energy consumption is ~638 quads. World energy consumption is also quoted at 587 quads, which I assume means 51 quads of primary energy are lost in energy conversion. That seems low. Anyway, let's go with 638 quads.

1 quad = 1,055,055,852,620 MJ (1.06E12).

So, the deuterium in 1 cubic mile can power the world for 80.5 years, at present rates of primary energy consumption.

Cross-check:

World oil consumption was 35 billion boe per year in 2016 (worldometers) (3.54E10 boe). This produces ~2.08E+14 MJ. That is 31% of 638 quads. Various sources say that oil produces ~31% of the world's primary energy, down from 44% in 1971. So the numbers add up. No doubt these other sources use the same numbers I just did, but anyway, my arithmetic seems right.

Bonus points:

There are 332.5 million cubic miles of water on earth, according to the USGS. So that is enough deuterium to last 26.8 billion years, which is far longer than the sun will last (5 billion years). That is at present rates of consumption. If consumption increases by a factor of a million, mainly with energy production in outer space, fear not! There is far more deuterium on other planets and in the Oort cloud. As Arthur Clarke put it in 1963:

"The heavy hydrogen in the seas can drive all our machines, heat all our cities, for as far ahead as we can imagine. If, as is perfectly possible, we are short of energy two generations from now, it will be through our own incompetence. We will be like Stone age men freezing to death on top of a coal bed."

- Profiles of the Future