Allow me to introduce you to the ***[Diesel Tree](https://en.wikipedia.org/wiki/Copaifera_langsdorffii)***, a tropical plant that naturally produces some petroleum-like precursors (turpenes). It's not exactly high yield, but it *is* a fuel source that ***literally grows on trees***.

[![copaiba sao paulo][1]][1]

Public Domain - By Mauroguanandi

Seeing as there's no "hard science" tag, and we're talking about people who can ***terraform whole planets***, I'd recommend a genetically modified version that has both a higher oil yield and a wider climactic range (*seeing as it can't be "the tropics" everywhere*). If you need a reason to have the trees pre-apocalypse, well even today, there's a lot of non-fuel things made from petrochemicals, so it'd be useful to have a low-maintenance self-sustaining source (*especially on a frontier world*)

**N.B.** if you want a ***purely*** organic solution, you're going to have to come up with a work-around for the fact that the world of today is chewing through our fossil fuels many, ***many***, orders of magnitude faster than they were laid down to begin with. You'll probably have to accept a limited, albeit renewable, supply of fuel until civilization is able to build some solar panels and electrify everything.


  [1]: https://i.sstatic.net/XWeNNFGc.png