The boom that helped make the U.S. the world's largest oil producer could be ending. Oil prices are down amid weak demand, and investors no longer seem willing to write the industry a blank check.

(Image credit: Spencer Platt)

The agency says the so-called Wolfcamp shale in Texas contains 20 billion barrels of oil and 16 trillion cubic feet of natural gas that are recoverable by hydraulic fracturing and other means.

Mexico has opened up its oil and gas fields to foreign investors. But they're slow to enter, as low oil prices, drug violence and other challenges trump the lure of a vast and undeveloped shale bed.

(Image credit: Victor Ruiz)

Venezuela's leader was in Qatar this week seeking billions to shore up his flagging economy. Oil prices have fallen by nearly 50 percent since last year and are now near a six-year low.