Fossil fuels are formed by the compression of the dead matter of plants and animals within the crust of the Earth. The plants and animals that died due to some calamity millions of years ago got covered by the rocks and soil. The internal heat of the Earth and the pressure due to the weight of the rocks turned them into thick dark crude oil. This process is natural and time consuming. It is impossible to recreate this process by artificial means on a large scale to make more solid fuels.
Interestingly, mining of extra-terrestrial planets, moons and asteroids is an ongoing research and the means to do it are being prototypes in the research laboratories of NASA and other space organisations.
