sources, heavy crudes are often priced at a discount to lighter ones. The increased viscosity and density also makes production more difficult (see reservoir engineering). On the other hand, large quantities of heavy crudes have been discovered in the Americas, including Canada, Venezuela and California. The relatively shallow depth of heavy oil fields (often less than 3000 feet) can contribute to lower production costs; however, these are offset by the difficulties of production and transport that render conventional production methods ineffective. Specialized techniques are being developed for exploration and production of heavy oil. Extraction Production of heavy oil is becoming more common in many countries, with 2008 production led by Canada and Venezuela. Methods for extraction include Cold heavy oil production with sand, steam assisted gravity drainage, steam injection, vapor extraction, Toe-to-Heel Air Injection (THAI), and open-pit mining for extremely sandy and oil-rich deposits. Environmental impact With current production and transportation methods, heavy crudes have a more severe environmental impact than light ones. With more difficult production comes the employment of a variety of enhanced oil recovery techniques, including steam flooding and tighter well spacing, often as close as one well per acre. Heavy crude oils also carry contaminants. For example, Orinoco extra heavy oil contains 4.5% sulfur as well as vanadium and nickel. However, because crude oil is refined before use, generating specific alkanes via cracking and fractional distillation, this comparison is not valid in a practical sense. Heavy crude refining techniques may require more energy input though, so its environmental impact is presently more significant than that of lighter crude if the intended final products are light hydrocarbons (gasoline motor fuels). On the other hand, heavy crude is a better source for road asphalt mixes than light crude. With present technology, the extraction and refining of heavy oils and oil sands generates as much as three times the total CO2 emissions when compared with conventional oil, primarily driven by the extra energy consumption of the extraction process (which may include burning natural gas to heat and pressurize the reservoir to stimulate flow). Current research into better production methods seek to reduce this environmental impact. In a 2009 report, the National Toxics Network, citing data provided by the Carbon Dioxide Information Analysis Center of the government of the United States and the Canadian Association of Petroleum Producers (CAPP), stated that emissions of CO2 per unit of energy produced were