10. Ferdows
> Country: Iran
> Est. Total Resource: 31 billion barrels
> Est. Remaining: 31 billion barrels
> Discovery Date: 2003

Iran’s Ferdows field is located about 50 miles offshore of the country’s Bushehr province. Together with two other fields in the group, Mound and Zagheh, the three are believed to hold 38.5 billion barrels of oil. Ferdows also contains trillions of cubic feet of natural gas, and Iran has signed an agreement with a Malaysian company to develop a natural gas liquefaction plant that could be in operation by 2014. U.S.-led sanctions against Iran have made it difficult for the country to develop its substantial oil reserves and its even more substantial natural gas reserves.

9. Carioca-Sugar Loaf
> Country: Brazil
> Est. Total Resource: 33 billion barrels
> Est. Remaining: 33 billion barrels
> Discovery Date: 2007

Carioca-Sugar Loaf is one of three supergiant discoveries in the Santos Basin offshore of Brazil. At an estimated reservoir size of 33 billion barrels, Carioca-Sugar Loaf is four to five times the size of the other two fields in the basin — Tupi and Jupiter — each of which is believed to hold 5-8 billion barrels of total resource. Brazil’s state-controlled oil company, Petrobras, recently announced that it would spend a staggering $225 billion to develop its Santos Basin reserves — and even that might not be enough. The reservoirs are located in water that is more than 5,000 feet deep and below 25,000 feet or more of sea floor. These figures represent a serious technological challenge. There is no firm data yet on how much of the oil in place could ultimately be recovered.

8. Cantarell
> Country: Mexico
> Est. Total Resource: 35 billion barrels
> Est. Remaining: 4 billion barrels
> Discovery Date: 1976

Cantarell is the largest conventional oil field ever discovered in the western hemisphere. Some attribute the field’s formation to the same asteroid impact that is believed to have caused the extinction of the dinosaurs. Production from Cantarell has fallen dramatically since 2003, when production peaked at 2.1 million barrels/day. The field produced about 500,000 barrels/day at the end of 2010 and is expected to drop below 400,000 barrels/day at the end of this year. Like the number five field on our list, Burgan, Cantarell is not expected to be productive beyond the end of the decade.

7. Kashagan
> Country: Kazakhstan
> Est. Total Resource: 38 billion barrels
> Est. Remaining: 7-9 billion barrels
> Discovery Date: 2000

Kashagan’s discovery in the northern Caspian Sea in 2000 was the largest new conventional field found since Prudhoe Bay in 1968. Development of Kashagan has been delayed by high costs and serious environmental concerns. The original budget called for development costs of $57 billion, but the figure was raised in 2007 to $136 billion because the technical challenges of extracting the oil are significant. First, the reservoir fluid contains a high concentration of a very corrosive compound. Not only does it destroy the equipment, but if allowed to escape, it could devastate the unique ecosystem of the area. Second, the field is located in relatively shallow waters that freeze in the winter. Third, reservoir pressures are very high, which combined with the high levels of toxic gas, creates an expensive safety issue. Production is expected to start in 2012, but this date could well be pushed out again.

6. Bolivar Coastal
> Country: Venezuela
> Est. Total Resource: 44 billion barrels
> Est. Remaining: 14 billion barrels
> Discovery Date: 1917

The Bolivar Coastal field is really a complex of large and small reservoirs around the northern and eastern edges of Venezuela’s Lake Maracaibo. The largest reservoir is Tia Juana, where the total resource was originally put at 15 billion barrels. The entire Maracaibo Basin is thought to hold up to 44 billion recoverable barrels, of which about 30 billion have been extracted through 2006. Of the fields on this list, the Bolivar Coastal has been in production the longest.