Oil, natural gas, and coal have provided abundant energy for our industrial economy, which has driven unprecedented population growth and set the stage for our expansion into space. Settling other planets has the potential of further increasing not only our population, but the populations of other species, far into the future. As I've mentioned, this is one way we can increase our ecological income and “pay” for what we've taken from the biosphere.
There is strong evidence that we have already found and exploited half of the Earth's supply of accessible oil and natural gas. The rest will be progressively more expensive to get, while demand continues to rise. Thus, involuntary reductions in consumption of energy will occur, resulting in a parallel reduction in consumption of other resources. These reductions will paradoxically, and tragically, decrease our ability to generate the income that space travel promises.
To offset the use of oil, an increased role for coal is often proposed, along with nuclear energy and biofuel. Burning coal is a net loss to the biosphere because it adds more pollution, especially in the form of climate changing greenhouse gases; it is also not very useful to the economy because it is not very portable. Nuclear energy also suffers from the portability problem: it is currently only useful for generating electricity. There are other problems with nuclear energy, including its potential use for weapons, difficulty with the safe disposal of waste, and its dependence on the current fossil fuel economy for materials and support. Indeed, this latter problem plagues all of these options, including coal (for building and maintaining power plants). Even the most promising biofuel technologies, while portable, require fossil fuel and material inputs.
The ideal replacement for oil could be used for fueling transportation, generating electricity, providing heat, and of course serving as feedstock for materials. It would need to be much less polluting and easily adaptable to the technologies that currently use oil. The chances of such a super fuel being discovered before usable oil becomes too scarce to sustain our civilization are pretty small right now.