Since the mid-nineteenth century mankind has traveled the world in search of "black gold", as Texans used to call oil. There is no other source in existence that currently satiates our thirst for energy than the black crude sludge that is extracted from the ground in millions of barrels a day. Petroleum products are used primarily for energy production, but are found in everything from plastic baby bottles to computer hardware.
One could consider petroleum the first energy rung on the advanced technological ladder, right above the basic rungs such as wind, fire, water, and in combination, steam energy (Advanced windmills and hydroelectric damns are exactly that, advanced, so they do not count). Some analysts suggest petroleum is the key precursor to modern technological civilization, as we would never have evolved beyond the 19th century without this miracle fuel source. An economic system naturally demands cheaper and simpler solutions first. Petroleum is cheap to extract (until recently) and its energy density and safe storage potential is vastly more efficient than other technologies that require a lot more sophistication to produce and manage.
Unfortunately, petroleum is the byproduct of millions of years of cellular decomposition, and is thus defined as a finite, non-renewable energy source. While it has taken millions of years to build up this most important reserve, it has taken modern civilization just two hundred years to tear it down. It can be analogous to taking years to build a building, while only seconds to destroy it. So we must beg the question, "how much petroleum is left in the ground to extract?".
This is where estimates vary wildly. One must remember that what is left is not completely what is going to be used. Does it make sense to pay 1.2 barrels worth of petroleum to extract just one? Of course not. There will always be "leftover" reserves in the deepest bowels of the earth that are considered "unrecoverable reserves". Current estimates project we are approaching or have already approached a "peak" of production. You need only look at the current state of affairs with world petroleum prices to understand a problem is occurring in the energy industry.
Most estimates place the world petroleum depletion mark sometime in the early to middle of this century. That is not a lot of time when considering the daunting challenges of switching over a two hundred year old infrastructure to more diverse, renewable options. If we don't switch over in time, it's quite literally back to the stone age, and with this rung of the technological ladder depleted, we may not be able to climb back up again.
Fortunately, there is a bright side to our current bleak energy future, and it's a darn good one when you consider the location on the planet (for Americans anyway). Colorado, Utah, and Wyoming have what could be considered the largest petroleum reserve on the planet in the forum of oil shale. Canada has the second largest reserve in the forum of tar sands, and the US also has what could be considered the fourth - all that bituminous coal in the Appalachians and Midwest.
All of these key petroleum reserves hold billions more barrels of petroleum than Saudi Arabia's total reserves (the current largest conventional reserves in the world). The previous problems with these unconventional reserves have been in the extraction and refining (as well as environmental) cost factors that have previously driven away its exploitation. With a barrel of petroleum well over the $100 USD mark today, companies are turning their eyes back to these more complicated solutions.
Let us hope they take advantage of them fast enough. If not, it's back to paper and pen D&D instead of World of Warcraft.