There are several. Crude, Heavy, and Light oil in my game are set by the Superheating mod https://mods.factorio.com/mod/Superheating (1.02 MJ, 600kJ, and 1.2 MJ respectively), and Petroleum is set by Krastorio 2 (at 900 kJ).
Looking at the code of Gas Boiler, that mod assigns fuel values depending on the Fuel Value of Solid Fuel (so it divides the value of Solid Fuel by a factor based on how much fluid it takes to make the Solid Fuel), which seems like a reasonable measure. In vanilla Solid Fuel is 12 MJ, if it takes 20 butane for 1 Solid Fuel then around 600 kJ seems reasonable for butane (you can tweak it up or down depending on how much gain or loss you want the process to have, taking other products of refineries into account).
I've also taken a look at Methane, and it indeed uses a fuel value of 5 kJ, but that is probably balanced around its specific planet mechanic. In vanilla(ish) circumstances 5 kJ is too little to be able to use industrially. Considering you're using petroleum and coal to make methane, you almost never would want to produce methane just to burn it in a furnace, but it would be nice to just have a value there so that it fits in better with the rest. If 20 petroleum + coal makes 40 methane, a fuel value of 300 to 400 kJ seems reasonable (but it's up to you whether you want to make the process gain or lose energy efficiency).
Looking forward to the further development of the mod!
