conventional oil
How it's produced: Drilling in the ground
Where it's found: Middle East, Russia, United States, elsewhere
Average production cost per barrel: $9
Greenhouse gas (ghg) emissions from production: 5 grams of carbon equivalent per megajoule
Potential output: 2,162 billion barrels
Dirty secret: 77% is controlled by state-run companies, so Big Oil is turning to unconventional sources to survive.

enhanced oil recovery
How it's produced: CO₂ is injected into old oil wells to squeeze out the last 30 to 60%.
Where it's found: United States, Middle East
Average production cost per barrel: $16
ghg emissions from production: 67% more than conventional oil
Potential output: 1,011 billion barrels
Dirty secret: Could have an environmental upside if oil companies figure out how to sequester CO₂ in old wells—and guarantee it won't leak out again.

tar sands and heavy oil
How it's produced: Ore is extracted and processed into synthetic crude.
Where it's found: Canada, Venezuela
Average production cost per barrel: $23
ghg emissions from production: 151% more than conventional oil
Potential output: 1,535 billion barrels
Dirty secret: Each barrel of oil produced leaves behind two of toxic waste. Refining tar sands oil produces as much as 80% more CO₂ than conventional refining.

gas-to-liquid synfuel
How it's produced: Natural gas is mixed with oxygen, purified, and processed into transportation fuels.
Where it's found: Russia, Iran, Qatar
Average production cost per barrel: $26
ghg emissions from production: 66% more than conventional oil
Potential output: 3,597 billion barrels
Dirty secret: South Africa perfected the technology in response to apartheid-era trade embargoes.

liquefied coal
How it's produced: Coal is heated and pressurized to create a gas, which is converted to fuel using the gas-to-liquid technique.
Where it's found: The largest coal reserves are in the United States, Russia, China, India, and South Africa.
Average production cost per barrel: $35
ghg emissions from production: 393% more than conventional oil
Potential output: 8,892 billion barrels
Dirty secret: Mercury is a byproduct. Last year, Senator Barack Obama cosponsored a bill that would have provided the industry with federal loans for as much as $20 million. Enviros accused him of pandering to Illinois coal interests.

oil shale
How it's produced: Oil-containing rock is mined, crushed, cooked, and injected with hydrogen.
Where it's found: Colorado, Utah, Wyoming
Average production cost per barrel: $57
ghg emissions from production: 530% more than conventional oil
Potential output: 1,451 billion barrels
Dirty secret: In 2007, former secretary of the interior Gale Norton joined Shell's oil shale team.