US oil and gas companies use barrel of oil equivalents (BOE) to make their energy accounting easier. Since oil is a liquid and gas is a gas, the volumes don't add up right. And their energy content per volume is also quite different. So they convert the gas to BOE. According to the US IRS, 1 BOE is equal to 5.8x10^6 BTUs. The Wikipedia article linked above converts gas and coal to BBOE, which is billion BOE.
If I were to tell you that world energy reserves were:
oil - 1371 billion barrels
gas - 6381 trillion cubic feet
coal - 997748 million short tons
that wouldn't do you much good, would it? I guess we could come up with some bullshit equation that would convert barrels of oil to short tons of coal, but there's no reason, since apparently someone has done all the work of converting coal and gas to barrels. If you want to work out the BTUs for yourself, that's fine, but if you look at the BBOE, it gives you the same picture of proportion and scale.