Allow me to introduce you to the Diesel Tree, 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.
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)