I went to visit my parents a while back, and their blackberries were ripe for picking. I went out to the vines and picked the easiest to get at ones on the outside areas of the growth of the vines. After I picked the easy ones, I went after the harder to get at ones on the inside. These were a wild variety with the sharp thorns, so the going was much slower and tougher to get the same amount of berries that I had easily gotten from the outside edges. There were a few of them in there that it just wasn't worth the effort and getting scraped up to go and get.
The point of that is that it is kind of analogous to how the oil situation is, as I see it. When we first started getting oil, it would practically bubble up out of the ground, or shoot a geyser when we hit the easy spots. Those are mostly gone now, and we are on to the harder to get at stuff. We are probably not going to run out of oil anytime soon, but the effort required to get at what is left will steadily increase.
One way to look at it might be the amount of energy required to get a certain amount of energy out of the ground as oil. The percent of the energy of the oil recovered that is required to gather it will steadily increase. At some point, it will no longer be worth getting at what is left from an energy perspective, but we are not there yet.
Replacing the Joules of energy that we get from fossil fuels is a formidable task to say the least. I am not a doom sayer who says that it will be impossible, but it will not be easy, and will not be done without significant investment in alternatives. Even are oil infrastructure took huge investment and many years too build, and we did that instead of alternatives because it was the easiest. Most of the alternatives do not have such a favorable energy return on the energy invested as do fossil fuels. Solar is an example of this, although it depends to a large extent upon the useful life of the panels. Wind power has made steady gains as the technology improves and the energy required to gather sufficient fossil fuels increases, and nuclear is probably already on a par or better, security and dangerous byproducts notwithstanding, fusion and other as yet undeveloped technologies may or may not pan out, time will tell but they are probably quite a few years off at best. As time goes by, these alternatives will become increasingly favorable when compared against an ever more difficult to recover fossil fuel supply. I do forsee shortages and enforced conservation in our futures, especially if populations continue to increase in an unsustainable manner as this contributes to our energy usage growing in an also unsustainable manner. One way or another those trends will change though.
This of course does not even take into account the often debated effects of putting all of that carbon from fossil fuels into our atmosphere.
