The Earths crust has a lot of minerals. Most minerals are also recyclable so we won't run out of minerals any time soon. Mining minerals on the Moon or from asteroids is within reach so I've fine with using them for exports.
Nuclear power for electric generation makes sense once a "safe" technology is developed and I am told that there are very promising designs on the drawing board. So I am not too worried about running out of electricity.
The only way to replace natural gas that would make any sense is to harvest the methane at the bottom of the ocean. There is lots of methane in the gas giant planets but getting methane from say Jupiter is not going to be easy.
Oil Fields Are Refilling... Naturally - Sometimes Rapidly There Are More Oil Seeps Than All The Tankers On Earth
Deep underwater, and deeper underground, scientists see surprising hints that gas and oil deposits can be replenished, filling up again, sometimes rapidly.