Is natural gas inexhaustable

Yes.  While we're using the natural gas that was created by the decomposition
of dead dinosaurs, we're also manufacturing more of it, by the decomposition
of potato peels and dirty diapers in land fills, and of leaves, trees, and dead
wildlife everywhere on Earth. 

The new stuff will be ready to extract and use in a million years or so.

That's the problem ... not that natural gas is exhaustible, but that we're using it
much much faster than it can be regenerated.  So on the time-scale of our
requirements, it's a limited resource, and it's very possible to run out of it.

It's exactly the same as spending money faster than you earn it.
And so is oil.

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