So gas filled magma chambers convert deep coal deposits, and regular deep pressure geothermal water into methane, oxygen and give off X-rays.
So we can use a Geiger counter to locate gas pockets within the land or sea. There is no time de;lay to polymerise the methane into long chain hydrocarbons. They refill, as we take gas out of them.
They will never run out. This is why natural gas contains a little Helium gas.