As the cheap oil from old mature fields is depleted, and we replace it with expensive new oil from unconventional sources, it forces the overall price of oil up. This is because oil prices are set at the margin, as are the prices of most commodities. The most expensive new barrel essentially sets the price for the lot.
Research by veteran petroleum economist Chris Skrebowski, along with analysts Steven Kopits and Robert Hirsch, details the new costs: $40 - $80 a barrel for a new barrel of production capacity in some OPEC countries; $70 - $90 a barrel for the Canadian tar sands and heavy oil from Venezuela’s Orinoco belt; and $70 - $80 a barrel for deepwater oil. Various sources suggest that a price of at least $80 is needed to sustain U.S. tight oil production.
Those are just the production costs, however. In order to pacify its population during the Arab Spring and pay for significant new infrastructure projects, Saudi Arabia has made enormous financial commitments in the past several years. The kingdom really needs $90 - $100 a barrel now to balance its budget. Other major exporters like Venezuela and Russia have similar budget-driven incentives to keep prices high.
Globally, Skrebowki estimates that it costs $80 - $110 to bring a new barrel of production capacity online. Research from IEA and others shows that the more marginal liquids like Arctic oil, gas-to-liquids, coal-to-liquids, and biofuels are toward the top end of that range.
http://www.smartplanet.com/blog/energy-futurist/the-cost-of-new-oil-supply/468