There are two enormous deposits of these viscous heavy oils in the world: one in Venezuela and one in Canada.
Canada’s oil sands resources exist in three major deposits in Alberta: Athabasca, Old Lake, and Peace River. The oil sands are the third largest oil reserves in the world after Venezuela and Saudi Arabia: in 2016 the remaining proven reserves were estimated at 165.4 billion barrels. That’s more than three times the total proven oil reserves located in the USA.
The oil sands underlie about 142,000 km2 of land, but only about 3% of this area, roughly 4,800 km2 can be mined—the remaining oil sands are too deep. Production in 2016 was running at about 2.5 million barrels a day , and is expected to rise to 3 million barrels a day in 2018 .
The oil sands are a naturally occurring mixture of sand, clay or other minerals, water and bitumen–a heavy and extremely viscous oil. The Athabasca deposit, the largest, is mined from the surface. All other bitumen is produced in situ –meaning drilling down to the bitumen, treating it to reduce its viscosity, and then pumping it to the surface.
Surface mining results is very substantial amounts of waste material called tailings. In a real sense, this form of oil sands extraction is much more like a coal mine that an oil drilling platform.
The photo below shows the Aurora oil sands mine in Alberta that excavates bitumen directly from the surface.
The huge size of the oil sands tailings ponds in Alberta exceeds the size of any other tailings ponds or impoundments anywhere on the planet. The largest impoundment—the Mildred Lake settling basin, is reportedly the world’s largest dam in terms of the volume of material used for its construction. In 2017, there were 20 oil sands tailings ponds in northern Alberta holding approximately 1.3 billion m3 of toxic wastewater. The total area of tailings ponds in 2017 was estimated as 220 km2 .
The ponds hold water, clay, sand and residual bitumen. The sand slowly settles and the surface water evaporates, but the layer in between, called the Mature Fine Tailings (MTF), is like a gel. It stays put and resists drying—for decades. It can take more than 30 years for MTF to solidify sufficiently for the land to be restored.
Steam and gravity
The in-situ Steam Assisted Gravity Drainage process was developed in the 1980s at a time when directional drilling technology was becoming employed in north America. In an SAGD installation, two horizontal wells are drilled in the oil sands, one very near the bottom of the formation and the other about five meters above it.
These wells are typically drilled in groups off central pads and they can extend for 800 to 1000 meters in any direction. In each well pair, steam is injected into the upper well and the heated bitumen will start to move and to drain downwards under gravity to the lower well where it is pumped to the surface. SAGD has high production rates and recovers more than 60% of the oil in place. Most major Canadian companies now have SAGD projects in production or under development.
Huff and puff
The cyclic stimulation procedure called “huff-and-puff”, has been in use by Imperial Oil at Cold Lake since the 1980s. The well is put through cycles of steam injection, soak, and production. First steam is injected into a well at a temperature of 300 to 340°C for a period of months. Then the well is allowed to sit to allow heat to soak into the formation. Later, the hot oil is pumped out of the well for a period of months. Once the production rate falls off, the well is put through another cycle of injection soak and production. The method is capable of achieving recovery rates of around 20 to 25%.
There is also a solvent process—where hydrocarbon solvents are injected into the upper well to dilute the bitumen and allow it to flow down to the lower well.
In situ production has the significant advantage that the waste tailing stream is smaller than with surface mining. But in 2016, fully half of the oil sands production was from surface mining—which requires huge tailings ponds.
Apart from the solvent process, the in-situ production processes that allows deeper deposits of oil to be pumped to the surface require steam—which is generated by burning natural gas. The amount of greenhouse gases produced during the extraction and processing of the bitumen is therefore substantially greater than emissions from more conventional oil production. The graph below shows the CO2 emissions associated with the extraction and processing of several types of crude oil produced in North America.
But it’s not only greenhouse gases that are emitted from the oil sands sites. A 2016 study led by Environment Canada found that the “evaporation and atmospheric oxidation of low-volatility organic vapors from the mined oil sands material is directly responsible for the majority of the observed secondary organic aerosol mass. The resultant production rates of 45–84 tonnes per day make the oil sands one of the largest sources of anthropogenic secondary organic aerosols in North America. ” 
Oil sands are of course a fossil fuel, and where there’s fossil fuels there’s always mercury. A study in 2014 measured mercury in the spring-time snowpack in the region of the oil sands. The authors concluded:
Total mercury loads were predominantly particulate-bound.. and increased with proximity to major developments. Methylmercury loads increased in a similar fashion…suggesting that oil sands developments are a direct source of methylmercury to local landscapes and water bodies. 
In May 2013, media outlets around Detroit, Michigan, began publishing articles about large piles of petroleum coke stored along the Detroit riverfront. Called petcoke, the coke is a black solid composed mainly of carbon with traces of sulfur, metals, and nonvolatile organic compounds.
The coke piled up in Detroit was a by-product of the nearby Marathon refinery—which was processing heavy crude oils derived in part from Canadian oil sands. The large piles of black coke, which were clearly visible from across the river in Windsor, Ontario, triggered local concerns over the potential impact of the material on human health.
Petcoke is a by-product of the refining of heavy oils and bitumen. High in carbon but low in hydrogen, it is the final product of thermal decomposition in the condensation process in hydrocarbon cracking. It has commercial value because, being mainly carbon, it has a higher heating value than coal, and is marketed as a fuel or as catalyst coke. Many US oil refineries have installed coking equipment in order to take advantage of increased supplies of heavy crude oil from the Athabasca oil sands.
Petcoke is priced at a discount with respect to coal on commercial markets, and so replaces coal in many industrial applications. However, petcoke produces more carbon dioxide than coal when burned. In terms of CO2 emissions, it’s a dirtier fuel than coal.
So to recap. The exploitation of the Alberta oil sands :
– requires huge tailings ponds, including the largest in north America
– generates the highest greenhouse gas emissions per barrel of oil
– is responsible for one of the largest sources of organic aerosols in north America
– leads to widespread local terrestrial methylmercury pollution—a well-known neurotoxin
The massive environmental impact caused by the exploitation and development of the Alberta oil sands is quite unlike anything else on the planet.
Check out these sources:
 See Alberta energy: Facts and statistics, at http://www.energy.alberta.ca/OilSands/791.asp
 See: Alberta oil sands industry quarterly report. Winter 2017. Available at : www.albertacanada.com/business/statistics/oil-sands-quarterly.aspx
 Photo from: Petroleum coke: the coal hiding in the tar sands. Oil change international.
 See the report by the Pembina Institute: Tailings ponds: The worst is yet to come, at http://www.pembina.org/blog/real-ghg-trend-oilsands
See : Environmental impacts of oil sands development in Alberta, http://www.resilience.org/stories/2009-09-22/environmental-impacts-oil-sands-development-alberta. Also : http://calgaryherald.com/business/energy/tailings-ponds-a-critical-part-of-albertas-oilsands-legacy which gives the area of the tailings ponds as 220 km2.
 See the book by the former CEO of Suncor, Rick George: Sun rise: Suncor, the oil sands and the future of energy., Harper Collins 2012.
 See : Oil sands technology. https://sunshineoilsands.com/?page=oil-sands-technology accessed 26 Jan 2018.
 The graph is taken from : The real GHG trend: Oilsands among the most carbon intensive crudes in North America. Accessed at : http://www.pembina.org/blog/real-ghg-trend-oilsands
 See : Oil sands operations as a large source of secondary organic aerosols. https://www.researchgate.net/publication/303509842_Oil_sands_operations_as_a_large_source_of_secondary_organic_aerosols, and also : Alberta’s oilsands industry is a huge source of harmful air pollution, study says. http://www.cbc.ca/news/technology/oilsands-soas-1.3599074
 Kirk J. L.., Muir, D., Gleason A., Wang X., et al. “Atmospheric deposition of mercury and methylmercury to landscapes and waterbodies of the Athabasca oil sands region”, Environmental Science and Technology, 2014. 48(13), pp7374-7383.
 See: Andrews A., and Lattanzio R.K., Petroleum coke: industry and environmental issues, Congressional Research Service, October 2013