If you want to run a fuel plant, you will need a +2 volatile deposit by default.
You can reduce this the following ways:
- Industrial Planning skill - reduce requirement by -1
- Any AI core in fuel prod. - reduce requirement by -1
- Alpha core in mining - reduce requirement by -1
- Use 2x story point on mining - reduce requirement by -1
This is all cumulative.
So if you take the IP skill alone, you can run fuel production with a +1 deposit.
IP + story points takes you down to +/-0
If you combine any 3 of the above, you can use any volatiles.
Gas Giants are special - they are the most reliable place for volatiles.
They always have some volatiles, and have a good chance to have the most (+2).
They are also the only place you can use something you can find to increase volatiles by a further +3. (This is the only way to fully use
another thing that can be found.)
If you don't want to use a Giant, both Cryovolcanic and Icy worlds can get the maximum amount of volatiles.
Toxic worlds also have a small chance to generate good volatiles deposits.
Also, the 'class' of a planet is largely irrelevant.
All it is is an aggregation of how many different conditions the planet has and at what level.
You can see class V planets that are hellish balls of lava just because they have lots of minerals and some other stuff.
Don't worry about 'class'.
If you don't care about that, Tundra Worlds are habitable and can have volatiles.
Every type of habitable planet (except Desert) can have volatiles.
You don't want volatiles on hab worlds though - it stops you from using something that you'll want to use.