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I learned in my Industrial chem class that Sour Gas is produced in natural reservoirs  , and a very long time ago (before high O2), elemental Sulfur was made.

Heres what I know: When the reservoir water had an abundance of CaSO4, then some elemental sulfur is made (S8).

My prof said that Sulfate can react with itself to produce elemental sulfur.

What is the balanced equation for such a process? I can't seem to find any combination of compounds where the oxidation states add up.

[Borek]

Interesting question.

I know that sulfates can be reduced by H₂S to elemental sulfur, something like

2SO₄^2- + 6H₂S + 4H⁺ S₈ + 8H₂O

but it is not exactly sulfate reacting with itself. Hydrogen sulfide can be produced by a process (or rather family of processes) known as thermochemical sulfate reduction (TSR). TSR requires some reducing agent, for example hydrocarbons - hydrocarbons react with sulfates producing hydrogen sulfide, which in turn reduces sulfates to sulfur.

Doesn't look like it fits what your prof is suggesting. Could be there are other possible mechanisms, can't say I know much on the subject, I just happened to read not long ago about sources of hydrogen sulfide in crude oil and natural gas.