[saulpw]

That's the source, but for chemical reactions you also need a sink. For instance gasoline is not usable as an energy source without an oxidant. Take away the oxygen in Earth's atmosphere (an asteroid has no atmosphere at all) and now gasoline has no potential energy available. Asteroids have no available oxidants (regolith was already fully oxidized long ago).

[asdff]

If it has been oxidized, there's your oxygen source.

[saulpw]

It takes more energy to liberate the oxygen than you could ever get from the fuel. You don't need oxygen atoms, you need the oxidant.