It must be said: The whole reason it's called "cracking" is because it is breaking the hydrocarbons into smaller molecules, which are lighter, thus heavy->light->gas.

That said I don't know the best term for the reverse.

Alkylation.

https://en.wikipedia.org/wiki/Alkylation_unit

may suggest adding option for tougher/more expensive recipes by mod-setting:
sulfuric acid or flouric acid (angels petrochem extension, i think adds it, can't remember...)
if modsetting for harder acid-requirement recipes is enabled, and a fluid with the correct name exists, it's added as an ingredient, otherwise: just use the normal cracking recipe. ?

That would be great.

I'll see what I can do.

Alkylation is not the reverse of hydrocarbon cracking, it is a further process that refines petroleum into higher octane fuels. There really isn't a process for reversing the refining of oil, who would bother to invent one even if it could be done? That would be like developing a process to turn metals back into raw ore or recombine cheese back into skim milk to produce fresh cow's milk (which definitely couldn't be done). of course this is science fiction but I'd call it recombinance rather than cracking or alkylation. It really doesn't apply, it's a genetic term, but theoretically the morphology works out.

...did you read the link?

"In short, the alky produces a high-quality gasoline blending stock by combining two shorter hydrocarbon molecules into one longer chain gasoline-"

In short, there is an established process to turn short chains into long ones.

Yes, short chains into long ones, but not into the original crude oil, which is stated clearly at the beginning of the second paragraph in the very article you linked. Alkylation produces high density, high octane fuels, or rather fuel components. The process is also obsolete and not used in new refineries, but that isn't relevant in the game setting. I don't know if you don't fully understand the process or are just being defensive, but you are never going to be able to produce heavy or light oil by alkylation. Think of it as a form of pure diesel (it isn't, but close enough for the layman), not suitable for fuel use but when combined with other petroleum mid-stage products leads eventually to diesel fuel and high octane petroleum, among other things. If you were to want a completely accurate simulation of the real process of refining in Factorio, your petroleum subfactory would have to expand about five or tenfold and include a plethora of other processes and chemicals and it would not be conducive to good gameplay. For a mod like this, I would consider introducing one additional petroleum product and then adding functionality to use it in various ways like producing flamer fuel and so on. This would give you a way to redirect excess petroleum (you shouldn't need to, but what the heck...) into such products without causing the eyeballs of petroleum engineers to explode.

The eyeballs of petroleum engineers already explode from how simplified the process is.

And that sulfur is primarily produced by Petroleum gas, of all things.

Well, simplification is one thing and a mod the size of Bobs, Angels and Py put together to really simulate the oil refining and production process accurately is another, but while the former is making a complex process simple for the purpose of making a game that is not too complex for mere mortals (i.e.: non petroleum engineers) and the latter is overboard, the concept of alkylation reversing a one way chemical process is pure fantasy. My goto guy (who is a petroleum engineer) for this kind of information (though doing IT for the petroleum industry for much of my career equipped me with enough knowledge to get by as long as I don't try to actually design or run a refinery in real life) has no problem with Factorio refining as a simplification to make the game as he put it "not insane", although yes the sulphur production makes his brain hurt. Thankfully there is beer, something this game could use and we frequently do while playing.

Sulphur, well, that is simply a by-product of the refining process, although before petroleum fuel became a thing it was the sulphur people wanted and everything else was waste. You don't want it in your fuel. Fortunately there are uses for the stuff, so taking it out doesn't lead to massive landfills of a dangerous chemical. If I were one to actually make mods myself, I would likely make only a fuel changes to the petroleum system, with sulphur becoming a by-product of the two types of cracking and possibly of solid fuel production as well (that one is a bit iffy though and would have to vary depending on what is being used as input.

How about this?

https://mods.factorio.com/mod/Petrol-Condensation-Reload

A rose by any other name...

But having slept on it and hashed it over with my petrol guy, we came to the conclusion that although it's impossible in reality, we're just going to engage in suspension of disbelief for the time being and go with it. It might, however, lead to my (or us) finally pushing ourselves to figure out a way to balance the simplification-for-gaming-purposes with realistic petroleum processes and make a mod of it, more complex than the default model and with a few extra intermediate products but still far from reality.

And hey, thanks for forcing me to think about this more thoroughly, and to Gentil for the mod that prompted it. I'm actually excited to play the game again for the first time in a while.

I'd be interested in a bit more complication in the oil process.