The issue is not one of "running out" so much as it is not having enough to keep our economy running. In this regard, the ramifications of Peak Oil for our civilization are similar to the ramifications of dehydration for the human body. The human body is 70 percent water. The body of a 200 pound man thus holds 140 pounds of water. Because water is so crucial to everything the human body does, the man doesn't need to lose all 140 pounds of water weight before collapsing due to dehydration. A loss of as little as 10-15 pounds of water may be enough to kill him.
In a similar sense, an oil based economy such as ours doesn't need to deplete its entire reserve of oil before it begins to collapse. A shortfall between demand and supply as little as 10 to 15 percent is enough to wholly shatter an oil-dependent economy and reduce its citizenry to poverty.
The effects of even a small drop in production can be devastating. Source For instance, during the 1970s oil shocks, shortfalls in production as small as 5% caused the price of oil to nearly quadruple. Source The same thing happened in California a few years ago with natural gas: a production drop of less than 5% caused prices to skyrocket by 400%.
Fortunately, those price shocks were only temporary.
The coming oil shocks won't be so short lived. They represent the onset of "a new, permanent condition". Source Once the decline gets under way, production will drop (conservatively) by 3% per year, every year. War, terrorism, extreme weather and other "above ground" geopolitical factors will likely push the effective decline rate past 10% per year, thus cutting the total supply by 50% in 7 years. Source
These estimate comes from numerous sources, not the least of which is Vice President Dick Cheney himself. In a 1999 speech he gave while still CEO of Halliburton, Cheney stated:
By some estimates, there will be an average of two-percent annual growth
in global oil demand over the years ahead, along with, conservatively, a
three-percent natural decline in production from existing reserves. That
means by 2010 we'll need an additional 50 million barrels per day. Source