The energy from all the fossil fuels ever burned still exists somewhere in the form of heat but it is inaccessible to us. We cannot use this energy any more and it is irrelevant to any argument about how our resources will never be exhausted.
No it isn't irrelevant. This is an argument taken up by our Presidential Science Adviser and his mentor Paul Ehrlich, both limited resources, “Small is Beautiful”, Zero-Population-Growth, Marxists, with Julian Simon. Simon issued a wager to anyone who could identify a diminishing resource as measured by its price. The point, one which Simon used as the title to his book “The Ultimate Resource”, is that humans use their minds to direct their efforts to each improve life for themselves and their families. Holdren and Ehrlich took up Simon's challenge, combining their bets, and chose minerals presumably certain to become more expensive, and nonrenewable resources. Holdren and Ehrlich lost on all five or six of the minerals they bet on, and waited until after Simon died to write a book claiming they were correct after all.