butter for some bitumen at room temperature, making extraction difficult. These heavy crude oils have a density (specific gravity) approaching or even exceeding that of water. As a result of their high viscosity, they cannot be produced by conventional methods, transported without heating or dilution with lighter hydrocarbons, or refined by older oil refineries without major modifications. Such heavy crude oils often contain high concentrations of sulfur and heavy metals, particularly nickel and vanadium, which interfere with refining processes, although lighter crude oils can also suffer from sulfur and heavy metal contamination. These properties present significant environmental challenges to the growth of heavy oil production and use. Canada's Athabasca oil sands and Venezuela's Orinoco heavy oil belt are the best known example of this kind of unconventional reserve. In 2003 the estimated reserves were . Heavy oil sands and bituminous sands occur worldwide. The two most important deposits are the Athabasca Oil Sands in Alberta, Canada and the Orinoco heavy oil belt in Venezuela. The hydrocarbon content of these deposits is either crude bitumen or extra-heavy crude oil, the former of which is often upgraded to synthetic crude (syncrude) and the latter of which the Venezuelan fuel Orimulsion is based. The Venezuelan extra heavy oil deposits differ from the Canadian bituminous sands in that they flow more readily at Venezuela's higher reservoir temperatures and could be produced by conventional techniques, but the recovery rates would be less than the unconventional Canadian techniques (about 8% versus up to 90% for surface mining and 60% for steam assisted gravity drainage). In 2011, Alberta's total proven oil reserves were 170.2 billion barrels representing 11 percent of the total global oil reserves (1,523 billion barrels) and 99% of Canada's oil reserves. By 2011 Alberta was supplying 15% of the United States crude oil imports, exporting about of crude oil. The 2006 projections for 2015, were about . At that rate, the Athabasca oil sands reserves would last less than 160 years. About 80 percent of Alberta's bituminous deposits can be extracted using in-situ methods such as steam assisted gravity drainage and 20 percent by surface mining methods. The Northern Alberta oil sands in Athabasca, Cold Lake and Peace River areas contain an estimated 2 trillion barrels (initial volume in place) of crude bitumen and extra-heavy oil of which 9 percent was considered recoverable using technology available in 2013. It is estimated by oil companies