This is what you do if you're short on oil. Much like the Bergius Process mod by Klonan, but with a much longer processing chain. With Bergius Process you make liquifacted coal and dupe oil. Here, you have several steps:

Making sulfur from petroleum gas,

Making sulfate fertilizer from sulfur and stone (which will be your limiting factor in this production chain),

Making algae from water and sulfate fertilizer,

Making light oil from said algae,

And cracking said oil into gas to restart the cycle.

You'll also notice you can't make heavy oil from algae. This is to make oil still have a use once this chain is set up, at least for express belts and robot frames.

Finally, algae can be used to make nutrient paste, which is essentially an upgraded version of a raw fish.

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