As conventional oil becomes less available, it can be replaced with production of liquids from unconventional sources such as tight oil, oil sands, ultra-heavy oils, gas-to-liquid technologies, coal-to-liquid technologies, biofuel technologies, and shale oil. In the 2007 and subsequent International Energy Outlook editions, the word "Oil" was replaced with "Liquids" in the chart of world energy consumption. In 2009 biofuels was included in "Liquids" instead of in "Renewables". The inclusion of natural gas liquids, a bi-product of natural gas extraction, in "Liquids" has been criticized as it is mostly a chemical feedstock which is generally not used as transport fuel. Reserve estimates are based on profitability, which depends on both oil price and cost of production. Hence, unconventional sources such as heavy crude oil, oil sands, and oil shale may be included as new techniques reduce the cost of extraction. With rule changes by the SEC, oil companies can now book them as proven reserves after opening a strip mine or thermal facility for extraction. These unconventional sources are more labor and resource intensive to produce, however, requiring extra energy to refine, resulting in higher production costs and up to three times more greenhouse gas emissions per barrel (or barrel equivalent) on a "well to tank" basis or 10 to 45% more on a "well to wheels" basis, which includes the carbon emitted from combustion of the final product. While the energy used, resources needed, and environmental effects of extracting unconventional sources have traditionally been prohibitively high, major unconventional oil sources being considered for large-scale production are the extra heavy oil in the Orinoco Belt of Venezuela, the Athabasca Oil Sands in the Western Canadian Sedimentary Basin, and the oil shale of the Green River Formation in Colorado, Utah, and Wyoming in the United States. Energy companies such as Syncrude and Suncor have been extracting bitumen for decades but production has increased greatly in recent years with the development of steam-assisted gravity drainage and other extraction technologies. Chuck Masters of the USGS estimates that, "Taken together, these resource occurrences, in the Western Hemisphere, are approximately equal to the Identified Reserves of conventional crude oil accredited to the Middle East." Authorities familiar with the resources believe that the world's ultimate reserves of unconventional oil are several times as large as those of conventional oil and will be highly profitable for companies as a result of higher prices in the 21st century.