The descriptions above are valid for conventional oil and gas sources. As demand increases, prices soar and new conventional resources become harder to find, production of oil and gas from unconventional sources become more attractive. These unconventional sources include very heavy crudes, oil sands, oil shale, gas and synthetic crude from coal, coal bed methane and biofuels. Estimates for conventional proven producible oil and gas reserves vary somewhat. The current increase in consumption is just under 2 % per year, or 15% – 20% in a decade for different products, even with energy saving efforts. If this trend continues the time to go figures quoted above will be reduced by one third. The following table shows current estimates and consumption:  

Estimates on undiscovered conventional and unconventional sources vary widely as the oil price; economical production cost and discovery are uncertain factors. With continued high oil prices, figures around 1-2 trillion barrels conventional (more gas than oil) and 3 trillion barrels unconventional are often quoted, for a total remaining producible hydrocarbon reserve of about 5 trillion barrels oil equivalent. Within a decade, it is expected that up to a third of oil fuel production may come from unconventional sources.  

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